input: Write a python function to calculate the sum of prime numbers below a given number.
  assert sum_of_primes_below(10) == 17
  assert sum_of_primes_below(100) == 1060
  assert sum_of_primes_below(1000) == 76127
output: def sum_of_primes_below(n):
      def is_prime(num):
          if num <= 1:
              return False
          if num == 2:
              return True
          if num % 2 == 0:
              return False
          for i in range(3, int(num**0.5) + 1, 2):
              if num % i == 0:
                  return False
          return True
      
      return sum(x for x in range(2, n) if is_prime(x))