bool is_prime(int n) {
    if (n < 2)
        return false;
    if (n == 2 || n == 3 || n == 5)
        return true;
    if (n % 2 == 0 || n % 3 == 0 || n % 5 == 0)
        return false;

    int i = 7;
    int step = 4;
    int finish_number = sqrt(n) + 1;

    while (i <= finish_number) {
        if (n % i == 0)
            return false;
        i += step;
        step = 6 - step;
    }
    return true;
}