有一种奇怪的临时(较大)整数乘法：

using ull = unsigned long long;
ull bmul(ull a, ull b, ull m) {
    ull c = a * b - (ull)((long double)a / m * b + 0.5L) * m;
    if (c < m) return c;
    return c + m;
}