private void DivButton_Click(object sender, EventArgs e) => FormCalc.Open(FormCalc.Mode.Div);
private void MinusButton_Click(object sender, EventArgs e) => FormCalc.Open(FormCalc.Mode.Minus);
private void MultiButton_Click(object sender, EventArgs e) => FormCalc.Open(FormCalc.Mode.Multi);
private void AddButton_Click(object sender, EventArgs e) => FormCalc.Open(FormCalc.Mode.Add);