#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n;
  cin >> n;
  int st = 1;
  int dr = 1 + n / 2;
  int ST = 0;
  int DR = 0;
  cout << "? " << st << '\n';
  cout.flush();
  cin >> ST;
  cout << "? " << dr << '\n';
  cout.flush();
  cin >> DR;
  int old = ST;
  ST -= DR;
  DR -= old;
  if (ST % 2) {
    cout << "! -1" << '\n';
    cout.flush();
    return 0;
  }
  if (ST == 0) {
    cout << "! 1" << '\n';
    cout.flush();
    return 0;
  }
  while (st <= dr) {
    int mij = st + dr;
    mij /= 2;
    int st_now = 0;
    int dr_now = 0;
    cout << "? " << mij << '\n';
    cout.flush();
    cin >> st_now;
    cout << "? " << mij + n / 2 << '\n';
    cout.flush();
    cin >> dr_now;
    int old = st_now;
    st_now -= dr_now;
    dr_now -= old;
    if (st_now == 0) {
      cout << "! " << mij << '\n';
      cout.flush();
      return 0;
    }
    if (st_now < 0) {
      if (ST < 0) {
        st = mij;
        ST = st_now;
      } else {
        dr = mij;
        DR = st_now;
      }
    } else {
      if (ST > 0) {
        st = mij;
        ST = st_now;
      } else {
        dr = mij;
        DR = st_now;
      }
    }
    cout.flush();
  }
  return 0;
}
