input: 
  def choose_num(x, y):
      """This function takes two positive numbers x and y and returns the
      biggest even integer number that is in the range [x, y] inclusive. If 
      there's no such number, then the function should return -1.
  
      For example:
      choose_num(12, 15) = 14
      choose_num(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