I need to check whether input is a number in an IF statement

I'm tasked with building a house of "X"es, with the user entering the width and height of a house. One of the conditions is to check whether what they entered are numbers instead of characters. In case it is a character or something else, I'm supposed to write an error. How do I create a condition that checks the int?

I'm sure it's simple, since it's a homework for complete beginners...