input: Write a function to find the largest prime factor of a given number. If the given number is less than 2, return -1. Otherwise, return the largest prime factor of the number.
  assert largest_prime_factor(331) == 331
  assert largest_prime_factor(13195) == 29
  assert largest_prime_factor(600851475143) == 6857
  assert largest_prime_factor(0) == -1
  assert largest_prime_factor(1) == -1
output: def largest_prime_factor(n):
      if n < 2:
          return -1
      largest_factor = -1
      for i in range(2, int(n**0.5) + 1):
          while n % i == 0:
              largest_factor = i
              n //= i
      if n > 1:
          largest_factor = n
      return largest_factor