三分

auto cal = [&](int x) {
    int res = 0;

    return res;
};
int l = 0, r = 2e9;
while (l < r) {
    int a = l + (r - l) / 3;
    int b = l + (r - l) * 2 / 3;
    int ra = cal(a);
    int rb = cal(b);
    if (ra > rb) {
        l = a + 1;
    } else if (ra < rb) {
        r = b - 1;
    } else {
        l = a, r = b;
    }
}
cout << cal(l) << "\n";