#include <bits/stdc++.h>
#define debug(x) cerr << #x << " " << x << "\n"
#define debugs(x) cerr << #x << " " << x << " "
#pragma GCC optimize("Ofast")

using namespace std;
typedef long long ll;
typedef pair <ll, ll> pii;

const ll NMAX = 2000001;
const ll INF = 1e9;
const ll nrbits = 17;
const ll MOD = 998244353;


int OK(int x){
    cout << "? " << x << endl;
    cin >> x;
    return x;
}

int main() {
//#ifdef HOME
//    ifstream cin(".in");
//    ofstream cout(".out");
//#endif // HOME
    int n;
    cin >> n;
    int r = 0, pas = (1 << 30);
    while(pas){
        if(r + pas <= n && !OK(r + pas))
            r += pas;
        pas /= 2;
    }
    r++;
    cout << r;
}