input: Write a function to find the largest palindrome made from the product of two 3-digit numbers.
  assert L_plaindrome(3) == 906609
  assert L_plaindrome(2) == 9009
  assert L_plaindrome(1) == -1
output: def L_plaindrome(n):
      if n < 1:
          return -1
      
      upper_limit = 10**n
      lower_limit = 10**(n-1)
      max_palindrome = -1
  
      for i in range(upper_limit - 1, lower_limit - 1, -1):
          for j in range(i, lower_limit - 1, -1):
              product = i * j
              if str(product) == str(product)[::-1] and product > max_palindrome:
                  max_palindrome = product
                  
      return max_palindrome