#include <bits/stdc++.h>

using namespace std;

int main()
{
    int n;
    cin >> n;
    int st = 1, dr = n, sol = -1;
    while (st < dr)
    {
        int mijl = (st + dr) / 2;
        cout << "? " << mijl << "\n";
        cout.flush();
        int ras;
        cin >> ras;
        if (ras == 1)
        {
            sol = mijl;
            st = mijl + 1;
        }
        else if (ras == 0)
        {
            dr = mijl;
        }
        else
        {
            return 0;
        }
    }
    cout << "! " << sol;
    return 0;
}