private static void Problem10()
{
    long sum = 0;
    for (int i = 2; i < 2000000; i++)
    {
        if (Helpers.IsPrime(i))
        {
            sum += i;
        }
    }

    Console.WriteLine(sum);
}

public static class Helpers
{
    public static bool IsPrime(int candidate)
    {
        if (candidate == 1) return false;
        // Check even.
        if ((candidate & 1) == 0)
        {
            return candidate == 2;
        }

        for (int i = 3; i*i <= candidate; i+=2)
        {
            if (candidate % i == 0)
            {
                return false;
            }
        }

        return true;
    }
}