气灭操作紧急停止后重新启动如何倒计时

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int lefttime;
private void button1_Click(object sender, EventArgs e)
{
int minute;
try
{
minute = int.Parse(this.textBox1.Text);
}
catch (System.Exception ex)
{
this.label1.Text = "输入错误";
return;
}
lefttime = minute * 60;
this.timer1.Interval = 1000;
this.timer1.Enabled=true;
this.timer1.Start();
}

private void timer1_Tick(object sender, EventArgs e)
{
this.label1.Text = "剩余时间" + lefttime.ToString()+"秒";
lefttime--;
}