ll lcm(ll m, ll n)
{
    ll g = __gcd(m, n);
    return (m / g) * n;
}

r -= b / (lcm(x, y));

這裡有個呼叫 lcm 時，如果 x, y 太大又互質時會 overflow
可以改個地方避開這個問題

ll lcm(ll m, ll n, ll y)
{
    ll g = __gcd(m, n);
    ll mg = m / g;
    if (y / n < mg) return y + 1;
    return (m / g) * n;
}

r -= b / (lcm(x, y, b));