input: def select_largest_even(x, y):
      """This function accepts two positive integers, x and y, and returns the
      largest even number within the inclusive range [x, y]. If no such number
      exists, the function should return -1.
  
      For instance:
      select_largest_even(12, 15) = 14
      select_largest_even(13, 12) = -1
      """
output:     # Ensure x is less than or equal to y
      if x > y:
          return -1
  
      # Iterate from y down to x to find the largest even number
      for num in range(y, x - 1, -1):
          if num % 2 == 0:
              return num
      
      # If no even number found, return -1
      return -1