ProgressBar to stop when button clicked

I have written code for Progress Bar : Belowprogress is the code but progress bar has to stop when clicking Stop button can anyone help me in this please :

private void btn_StartLengthyTask_Click(object sender, RoutedEventArgs e)
{
pb_LengthyTaskProgress.Value = InitialSleepTime;
pb_LengthyTaskProgress.Maximum = 100;
Task.Run(() =>
{
for (int i = 0; i < 101; i++)
{
this.Dispatcher.Invoke(() => //Use Dispather to Update UI Immediately
{
pb_LengthyTaskProgress.Value = i;
lbl_CountDownTimer.Text = i.ToString();
});
Thread.Sleep(ThreadSleepTime) ; // here i have to mention (or) clause for cancel button also.
}
});
}