#include <bits/stdc++.h>

using namespace std;

inline void Open(const string Name) {
    #ifndef ONLINE_JUDGE
        (void)!freopen((Name + ".in").c_str(), "r", stdin);
        (void)!freopen((Name + ".out").c_str(), "w", stdout);
    #endif
}

int N, l, r, m, q, ans;

int main() {
    //Open("");

    cin >> N;

    l = 1, r = N;
    while(l <= r) {
        m = (l + r) / 2;

        cout << "? " << m << "\n";
        cout.flush();

        cin >> q;

        if(q == 0)
            l = m + 1;
        else {
            ans = m;
            r = m - 1;
        }
    }

    cout << "? " << m << "\n";
    cout.flush();

    return 0;
}