#include <bits/stdc++.h>
using namespace std;
int ask(int x) {
    cout << "? " << x << '\n'; cout.flush();
    int ans; cin >> ans;
    return ans;
}
int main() {
    int N, k = 0; cin >> N;
    for(int step = (1 << 30); step >= 1; step >>= 1) {
        if(step + k <= N && ask(k + step) == 0) {
            k += step;
        }
    }
    ++k;
    cout << "! " << k << '\n';
    return 0;
}