Trouple with Tip calculator

Whenver a user enters a number of users to be less than one or the bill amount to be less than zero , I want their to be an alert box saying “please enter positive values”. The ony thing is , my tip calculator calculates a value, even when It enters number of people to be less than zero or the meal amount to be less than zero.

Anyone have trouble accessing my link?

For my Tip calculator, I don’t want my the word dollars to show yet. I only want the words “dollars” to show after I clicked the “Calculate” Button. When I write the line of code "’ ‘;’ ';", the words dollars do not disappear.