[#10] Summation of primes

The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.
Find the sum of all the primes below two million.

def isPrime(n):
        if n == 2 or n == 3:
            return True
        if n < 2 or n % 2 == 0:
            return False
        if n < 9:
            return True
        if n % 3 == 0:
            return False
        r = int(n**0.5)
        f = 5
        while f <= r:
            if n % f == 0:
                return False
            if n % (f + 2) == 0:
                return False
            f += 6
        return True


def problem10(n):
        '''
        The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.
        Find the sum of all the primes below n.
        '''
        if n < 2:
            return 0
        if n < 3:
            return 2
        result = 2
        for i in range(3, n, 2):
            if isPrime(i):
                result += i
        return result