long modinv(long a, long m) {
    long b = m, u = 1, v = 0;
    while (b) {
        long t = a / b;
        a -= t * b; swap(a, b);
        u -= t * v; swap(u, v);
    }
    u %= m;
    if (u < 0) u += m;
    return u;
}