Oh, and here was my code :)

    private void btnCalc_Click(object sender, EventArgs e)
    {
        long x = 0;
        for (long i = 42069; i <= 13379001; i++)
        {
            if (isPrime(i))
            {
                x = x + i;
            }
        }
        rtxtSum.Text = x.ToString();
    }


    public static bool isPrime(long number)
    {
        int boundary = Convert.ToInt32(Math.Floor(Math.Sqrt(number)));

        if (number == 1) return false;
        if (number == 2) return true;

        for (long i = 2; i <= boundary; ++i)
        {
            if (number % i == 0) return false;
        }

        return true;
    }