// https://www.infoarena.ro/problema/cbinteractiv
#include <bits/stdc++.h>
#define endl '\n'
using namespace std;

int main() {
  int n;
  cin>>n;

  int l=1, r=n, res=0;
  while (l <= r) {
    int mid = (l+r)/2, verdict;
    cout<<"? "<<mid;
    cout.flush();
    cin>>verdict;

    if (verdict==0) l=mid+1;
    else if (verdict==1) {
      r = mid-1;
      res = mid;
    }
  }

  cout<<res;
}