如何在我的JavaScript和HTML代码中删除文本框中的NaN？

I'm trying to make a triangle Hypotenuse calculator. First you put one leg, then the other leg, then you will get the Hypotenuse. But, if you fill in the second box first, It will say NaN. I know its not that important, but is there a way to get rid of it so it says "0" until both boxes are filled? And here is the code:I'm trying to make a triangle Hypotenuse calcul