     private void Form1_KeyPress(object sender, KeyPressEventArgs e)
        {
            char ch;
            ch = e.KeyChar;
            if (ch == '=')
            {
                this.E_Click(sender, e);
                return;
            }
            if ( (ch == '+' || e.KeyChar == '*' || ch == '-' || ch == '/') )
            {
               
               Calculate(ch);
                lastop = true; 
            }
            else if (last == 'O')
                textBox1.Text = "";
            if ((int)ch >= 48 && ((int)ch <= 57))
            {
               last = ' ';
               textBox1.Text = textBox1.Text + ch;
               lastop = false;
            }
         }
