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