WPF IProgress CancallationToken

<Window x:Class="WpfApp180.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp180"
        mc:Ignorable="d" WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <StackPanel Orientation="Horizontal" Margin="6">
            <TextBlock Text="From:"/>
            <TextBox Margin="10,2,2,2" Width="120"
                     MaxLength="10" x:Name="_from"/>
            <TextBlock Text="To:" Margin="20,0,0,0"/>
            <TextBox Margin="10,2,2,2" Width="120"
                     MaxLength="10" x:Name="_to"/>
        </StackPanel>
        <StackPanel Orientation="Horizontal" Grid.Row="1" Margin="6">
            <Button Content="Calculate" Padding="4" x:Name="calc" Margin="4" Click="calc_Click"/>
            <Button Content="Cancel" Padding="4" Margin="4"
                    x:Name="cancel" IsEnabled="False" Click="cancel_Click"/>
        </StackPanel>
        <ProgressBar x:Name="progressBar" Grid.Row="2" Margin="4"
                     Height="30"/>
        <TextBlock x:Name="result" Grid.Row="3" Margin="6"
                   HorizontalAlignment="Center" FontSize="20"/>
        <TextBlock x:Name="progressTb" Grid.Row="4" Margin="6" 
                   HorizontalAlignment="Center" FontSize="20"/>
    </Grid>
</Window>


using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp180
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        CancellationTokenSource cts;
        public MainWindow()
        {
            InitializeComponent();
        }

        async Task<int> CountPrimesAsync(int first,int last,CancellationToken ct,
            IProgress<double> progress)
        {
            var task = Task.Run(() =>
            {
                int total = 0;
                int range = last - first + 1;
                int pCount = 0;
                for(int i=first;i<=last;i++)
                {
                    ct.ThrowIfCancellationRequested();
                    bool isPrime = true;
                    int limit = (int)Math.Sqrt(i);
                    for(int j=2;j<=limit;j++)
                    {
                        if(i%j==0)
                        {
                            isPrime = false;
                            break;
                        }
                    }
                    if(isPrime) 
                    {
                        total++;
                    }
                    if(++pCount%1000==0)
                    {
                        double progValue = pCount * 100 / range;
                        progress.Report(progValue);
                        Dispatcher.BeginInvoke(new Action(() =>
                        {
                            progressTb.Text=$"{progValue.ToString()}%";
                        }));
                    }
                }
                return total;
            });
            return await task;
        }

        private async void calc_Click(object sender, RoutedEventArgs e)
        {
            int first = int.Parse(_from.Text);
            int last=int.Parse(_to.Text);
            cts = new CancellationTokenSource();
            calc.IsEnabled = false;
            cancel.IsEnabled = true;
            result.Text = "Calculating...";
            var prog = new Progress<double>(x => progressBar.Value = x);
            try
            {
                int count = await CountPrimesAsync(first, last, cts.Token, prog);
                result.Text = "Total Primes:" + count;
            }
            catch (OperationCanceledException ex)
            {
                result.Text = "Operation cancelled!";
            }
            finally
            {
                cts.Dispose();
                calc.IsEnabled = true;
                cancel.IsEnabled = false;
            }
        }

        private void cancel_Click(object sender, RoutedEventArgs e)
        {
            cts.Cancel();
            cts = null;
        }
    }
}