#include <iostream>

using namespace std;

int main() {
    int N;
    cin >> N;

    int st = 1, dr = N, sol = -1;

    while(st <= dr) {
        int mid = (st + dr) >> 1;
        cout << "? " << mid << endl;
        int r; cin >> r;
        if(r == 0) {
            sol = mid + 1;
            st = mid + 1;
        } else {
            dr = mid - 1;
        }
    }
    cout << "! " << sol << endl;
    return 0;
}