Design a program to calculate sales commission using functions.
In this program, the main function first calls a function called "GetData", which gets the user's input about the sale amount and returns it to the main function. This function should also perform input validation to ensure that the user's input is positive.
Then, the main function calls the second function called "CalculateCommission", which takes the sale amount as a parameter, calculates the sales commission as 5% of the sale amount, and returns the result to the main function.
Finally, the main function calls the third function called "ShowResult" to display the sales commission.