#include <iostream>
using namespace std;
int main()
{
    int n,st,dr,m,sol;
    bool ok;
    cin>>n;
    st=1,dr=n;
    while(st<=dr)
    {
        m=(st+dr)/2;
        cout<<"? "<<m; cout.flush();
        cin>>ok;
        if(ok==0)
            st=m+1;
        else
            dr=m-1,sol=m;
    }
    cout<<sol;
    return 0;
}