def getNonNegNum(first,last):

   '''first and last are ints >= 0, where last >= first.
      Input from the user an int between first and last, inclusive.
      if the entered value is valid, return it.  Otherwise,
      print an error message, and Return -1.
      '''

   msg = "Enter an int between " + str(first)
   msg += " and " + str(last) + " "   

   try:
      # This will cause a ValueError exception if the value
      # cannot be converted into an int
      ans = int(input(msg))

      # Throw an exception if the value is not in range (this is a trick)
      if (ans < first or ans > last): 5/0

      #This will be executed only if no exception is raised
      return ans

   except ValueError:
       print("The value must be an int")
       return -1
   except ZeroDivisionError:
      print("The number must be between",first,"and",last)
      return -1

def main():
   first = 10
   last = 20
   # test cases that must be tried:
   #   general positive case: 15 
   #   a non-int, like jjjj
   #   a non-int num, like 4.2
   #   a negative number, say -45
   #   the value indicating an error, -1 (very important!)
   #   a smaller number, 5
   #   a larger number, 25
   #   numbers just outside the boundaries: 9, 21   
   #   numbers on the boundaries:  10, 20

   ans = getNonNegNum(first,last)
   while ans == -1:
      ans = getNonNegNum(first,last)
   print("Finally, we have a valid answer:",ans)

main()