Just enter your first number in the first box on top

Just enter your second number in the second box in the middle

Then click on the button that says calculate

Notice that If you enter a number less than 0, the calculator will prompt you to enter a valid number.

It is because the calculator was made for multiplication of whole numbers. Therefore, the calculator assumes that you will enter a whole number, not integers, which include negative numbers or number less than 0.

Furthermore, if it says to enter a valid number, it will clear the box that contains a value less than 0

Lastly, "Enter a valid number" may mean that you have entered only 1 number.