input: Write a function to return the sum of the first n prime numbers.
  assert sum_of_primes(10) == 129
  assert sum_of_primes(5) == 28
  assert sum_of_primes(0) == 0
output: def sum_of_primes(n):
      def is_prime(num):
          if num < 2:
              return False
          for i in range(2, int(num ** 0.5) + 1):
              if num % i == 0:
                  return False
          return True
      
      prime_sum = 0
      count = 0
      current_num = 2
      
      while count < n:
          if is_prime(current_num):
              prime_sum += current_num
              count += 1
          current_num += 1
      
      return prime_sum