#include <iostream>

using namespace std;

int main()
{
    int n,pas,poz = 0,ok;
    cin >> n;
    for (pas = 1;pas < n;pas <<= 1);
    while (pas) {
        if (poz + pas <= n) {
            cout << "? " << poz + pas << "\n";
            cout.flush();
            cin >> ok;
            if (ok == 1)
                poz += pas;
        }
        pas >>= 1;
    }
    cout << "! " << poz << "\n";
    cout.flush();
    return 0;
}