Tip Calculator App

Hi,

Good one on this challenge, it looks pretty good and wells very well, but there's some things that are missing;

• As I said, the functionality is good, it works no matter sequence you enter your numbers, but...
• I can enter the negative sign(-), not sure how you skipped that but I am able to enter a bill of -$20 and I shouldn't be able to.
• You should do the same thing you did on the other inputs on the custom input because on it I can enter a multiple number of periods.
• To make the functionality even better you should not call the function that does the calculation if any of the fields are empty, which is why it sometimes result to $NaN, or sometimes does the calculation without the number of people.