def canBeInt(s):
   ''' s is a string.  Return True if each char of all but the 
       last char of s is in '0123456789', and the last character is
       also in that string or is '\n.'  Otherwise, return False.'''
   numbers = "0123456789"
   
   returnVal = True

   # s[:-1] is s without the last character; we will cover this soon
   for c in s[:-1]:
      if not c in numbers:
         returnVal = False
   returnVal = returnVal and (s[-1] in numbers + "\n")
   return returnVal

def main():
   first = 10
   last = 20
   msg = "Enter an int between " + str(first)
   msg += " and " + str(last) + " "

   ans = input(msg)
   while not canBeInt(ans) or\
        (int(ans) < first or int(ans) > last):
      if not canBeInt(ans):
         print("The value must be an int")         
      else:
         print("The number must be between",first,"and",last)
      ans = input(msg)
   print("Finally, we have a valid answer:",ans)

main()