int i;
        int tk;
        string c;
        private void Button1_Click(object sender, EventArgs e)
        {
            i = 300;
            c ="5:00";
            
            label1.Text = c;
            timer1.Interval = 1000;
            timer1.Enabled = true;
            timer1.Start();
        }

        private void Timer1_Tick(object sender, EventArgs e)
        {
            
            tk = --i;
            TimeSpan span = TimeSpan.FromMinutes(tk);
            string label = span.ToString(@"hh\:mm");
            label1.Text = label.ToString();
            if (i < 0)
                timer1.Stop();
        }