input: def select_largest_even(x, y):
      """This function takes two positive integers x and y and returns the
      largest even number within the range [x, y] inclusive. If no such 
      number exists, the function should return -1.
  
      For example:
      select_largest_even(12, 15) = 14
      select_largest_even(13, 12) = -1
      """
output:     largest_even = -1
      for num in range(y, x - 1, -1):
          if num % 2 == 0:
              largest_even = num
              break
      return largest_even