input: Write a function to determine if a number provided by the user is prime or not. If the number is prime, return True; otherwise, return False. If the number is less than 2, return False immediately.
  assert is_prime(17)==True
  assert is_prime(18)==False
  assert is_prime(1)==False
  assert is_prime(2)==True
  assert is_prime(29)==True
output: def is_prime(n):
      if n < 2:
          return False
      for i in range(2, int(n**0.5) + 1):
          if n % i == 0:
              return False
      return True