multiplying number in a textbox to the quantity value

multiplying number in a textbox to the quantity value

I have a question, first is, I have this textbox named Amount, then a
numericupdown named quantity1. How do i multiply the textbox which is
double to the value of the numericupdown?
double amt;
double ans;
amt = Double.Parse(Amount.Text);
//I have another textbox called total.
ans = amt * (quantity1.Value) //how do i do this? it has error.