First we decompiled the program using dnSpy, yielding:

using System;
using System.CodeDom.Compiler;
using System.ComponentModel;
using System.Diagnostics;
using System.Threading;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Markup;

namespace FlagCalculatorApp
{
        // Token: 0x02000003 RID: 3
        public class MainWindow : Window, IComponentConnector
        {
                // Token: 0x06000004 RID: 4 RVA: 0x0000207E File Offset: 0x0000027E
                public MainWindow()
                {
                        this.InitializeComponent();
                }

                // Token: 0x06000005 RID: 5 RVA: 0x0000208C File Offset: 0x0000028C
                private void Calc(ProgressBar progress, TextBox flag, Label info)
                {
                        MainWindow.<>c__DisplayClass1_0 CS$<>8__locals1 = new MainWindow.<>c__DisplayClass1_0();
                        CS$<>8__locals1.progress = progress;
                        CS$<>8__locals1.flag = flag;
                        CS$<>8__locals1.info = info;
                        CS$<>8__locals1.<>4__this = this;
                        CS$<>8__locals1.val = 0u;
                        uint i;
                        uint j;
                        for (i = 0u; i < 12852275u; i = j + 1u)
                        {
                                if (i % 1000u == 0u)
                                {
                                        base.Dispatcher.Invoke(delegate()
                                        {
                                                CS$<>8__locals1.progress.Value = 1u + i * 99u / 12852275u;
                                        });
                                }
                                CS$<>8__locals1.val = this.Calc5(this.Calc3(this.Calc6(i, 13u), CS$<>8__locals1.val), i);
                                j = i;
                        }
                        base.Dispatcher.Invoke(delegate()
                        {
                                CS$<>8__locals1.progress.Value = 100.0;
                                CS$<>8__locals1.flag.Text = "pactf{dot" + CS$<>8__locals1.val + "N3T}";
                                CS$<>8__locals1.info.Content = "flag calculated!";
                                CS$<>8__locals1.<>4__this.button.IsEnabled = true;
                        });
                }

                // Token: 0x06000006 RID: 6 RVA: 0x00002168 File Offset: 0x00000368
                private void Button_Click(object sender, RoutedEventArgs e)
                {
                        this.button.IsEnabled = false;
                        this.flag.Text = "";
                        this.info.Content = "calculating flag...";
                        this.progress.Value = 1.0;
                        new Thread(delegate()
                        {
                                this.Calc(this.progress, this.flag, this.info);
                        }).Start();
                }

                // Token: 0x06000007 RID: 7 RVA: 0x000021CB File Offset: 0x000003CB
                private uint Calc1(uint a)
                {
                        return a + 1337u - 1336u;
                }

                // Token: 0x06000008 RID: 8 RVA: 0x000021DA File Offset: 0x000003DA
                private uint Calc2(uint a)
                {
                        return a * 9u - (8u * a + 200u) + 199u;
                }

                // Token: 0x06000009 RID: 9 RVA: 0x000021F0 File Offset: 0x000003F0
                private uint Calc3(uint a, uint b)
                {
                        uint num = 0u;
                        for (uint num2 = 0u; num2 < a; num2 += 1u)
                        {
                                num = this.Calc1(num);
                        }
                        for (uint num3 = 0u; num3 < b; num3 += 1u)
                        {
                                num = this.Calc1(num);
                        }
                        return num;
                }

                // Token: 0x0600000A RID: 10 RVA: 0x00002228 File Offset: 0x00000428
                private uint Calc4(uint a, uint b)
                {
                        uint num = this.Calc3(a, 0u);
                        int num2 = 0;
                        while ((long)num2 < (long)((ulong)b))
                        {
                                num = this.Calc2(num);
                                num2++;
                        }
                        return num;
                }

                // Token: 0x0600000B RID: 11 RVA: 0x00002258 File Offset: 0x00000458
                private uint Calc5(uint a, uint b)
                {
                        uint num = 0u;
                        int num2 = 0;
                        while ((long)num2 < (long)((ulong)b))
                        {
                                num = this.Calc3(num, a);
                                num2++;
                        }
                        return num;
                }

                // Token: 0x0600000C RID: 12 RVA: 0x00002280 File Offset: 0x00000480
                private uint Calc6(uint a, uint b)
                {
                        uint num = a;
                        int num2 = 0;
                        while (num >= b)
                        {
                                num = this.Calc4(num, b);
                                num2++;
                        }
                        return num;
                }

                // Token: 0x0600000D RID: 13 RVA: 0x000022A8 File Offset: 0x000004A8
                [DebuggerNonUserCode]
                [GeneratedCode("PresentationBuildTasks", "4.0.0.0")]
                public void InitializeComponent()
                {
                        if (this._contentLoaded)
                        {
                                return;
                        }
                        this._contentLoaded = true;
                        Uri resourceLocator = new Uri("/FlagCalculatorApp;component/mainwindow.xaml", UriKind.Relative);
                        Application.LoadComponent(this, resourceLocator);
                }

                // Token: 0x0600000E RID: 14 RVA: 0x000022D8 File Offset: 0x000004D8
                [DebuggerNonUserCode]
                [GeneratedCode("PresentationBuildTasks", "4.0.0.0")]
                [EditorBrowsable(EditorBrowsableState.Never)]
                void IComponentConnector.Connect(int connectionId, object target)
                {
                        switch (connectionId)
                        {
                        case 1:
                                this.flag = (TextBox)target;
                                return;
                        case 2:
                                this.label = (Label)target;
                                return;
                        case 3:
                                this.button = (Button)target;
                                this.button.Click += this.Button_Click;
                                return;
                        case 4:
                                this.progress = (ProgressBar)target;
                                return;
                        case 5:
                                this.info = (Label)target;
                                return;
                        case 6:
                                this.image = (Image)target;
                                return;
                        default:
                                this._contentLoaded = true;
                                return;
                        }
                }

                // Token: 0x04000001 RID: 1
                internal TextBox flag;

                // Token: 0x04000002 RID: 2
                internal Label label;

                // Token: 0x04000003 RID: 3
                internal Button button;

                // Token: 0x04000004 RID: 4
                internal ProgressBar progress;

                // Token: 0x04000005 RID: 5
                internal Label info;

                // Token: 0x04000006 RID: 6
                internal Image image;

                // Token: 0x04000007 RID: 7
                private bool _contentLoaded;
        }
}

I made a guess that not much was being done, and that the program was only slow due to c# being slow. And rewrote it into C.

#include <stdio.h>
#include <stdlib.h>

unsigned int calc1(unsigned int num) {
    return num + 1337u - 1336u;
}

unsigned calc2(unsigned a) {
    return a * 9u - (8u * a + 200u) + 199u;
}

unsigned calc3(unsigned int a, unsigned int b) {
    unsigned int num = 0;
    for (unsigned int num2 = 0; num2 < a; num2++) {
        num = calc1(num);
    }
    for (unsigned int num3 = 0; num3 < b; num3++) {
        num = calc1(num);
    }
    return num;
}

unsigned int calc4(unsigned int a, unsigned int b) {
    unsigned int num = calc3(a, 0);
    int num2 = 0;
    while ((long)num2 < (long)((unsigned long)b)) {
        num = calc2(num);
        num2++;
    }
    return num;
}

unsigned int calc5(unsigned int a, unsigned int b) {
    unsigned int num = 0;
    int num2 = 0;

    while ((long)num2 < (long)((unsigned long)b)) {
        num = calc3(num, a);
        num2++;
    }
    return num;
}

unsigned int calc6(unsigned int a, unsigned int b) {
    unsigned int num = a;
    int num2 = 0;

    while (num >= b) {
        num = calc4(num, b);
        num2++;
    }

    return num;
}

int main() {
    unsigned int val = 0;
    unsigned int i, j;

    for (i = 0; i < 12852275; i = j + 1) {
        val = calc5(calc3(calc6(i, 13), val), i);
        j = i;
    }

    printf("pactf{dot%uN3T}\n", val);
}

The result is: pactf{dot4019346342N3T}