#include <iostream>

using namespace std;

int n;

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