int gcd(int a, int b) {
    while (b != 0) {
        int r = a % b;
        a = b;
        b = r;
    }
    return a;       // khi b = 0 thi a la UCLN
}

// boi chung nho nhat:
long long lcm(int a, int b) {
    return (long long)a / gcd(a, b) * b;   // chia truoc de tranh tran
}