# 2.13:  Modify the previous program to test the user input to make sure it is a usable integer.
# (Hint:  as we did earlier, use str isdigit()).   If the input is not all digits, you can use
# exit('error message') to exit the program.

# Sample program runs:

# how many times should I greet you?  three
# error:  please enter an integer     # i.e., exit('error: please...')

# how many times should I greet you?  3
# Happy birthday to you!
# Happy birthday to you!
# Happy birthday to you!

# how many times should I greet you?  2
# Happy birthday to you!
# Happy birthday to you!

