LL mult(LL a, LL b, LL c)
{
    if (a < b) swap(a, b);
    LL r = 0, m = a;
    for (int i = 0; i < 64; ++i)
    {
        if ((1LL<<i) > b) break;
        if (b&1LL<<i) r = (r+m)%c;
        m = (m*2)%c;
    }
    return r;
}