public static long checkPrime(int n){ long m3 = (n - 1) / 3; long sum3 = 3 * m3 * (m3 + 1) / 2;

    long m5 = (n - 1) / 5;
    long sum5 = 5 * m5 * (m5 + 1) / 2;

    long m15 = (n - 1) / 15;
    long sum15 = 15 * m15 * (m15 + 1) / 2;

    return sum3 + sum5 - sum15;
}