C# - failed parse exception?

I am writing a program in C#, and I want to catch exceptions caused by converting "" (null) to int. What is the exception's name?

EDIT: I'm not sure I can show the full code... But I'm sure you don't need the full code, so:

int num1 = Int32.Parse(number1.Text);
int num2 = Int32.Parse(number2.Text);