#include <bits/stdc++.h>
using namespace std;
int n;
int ask(int x) {
  cout << "? " << x << endl;
  cout << "? " << x + n / 2 << endl;
  int a, b;
  cin >> a >> b;
  if (a == b) {
    cout << "! " << x << endl;
    exit(0);
  }
  return a < b;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout << fixed << setprecision(11);
  cerr << fixed << setprecision(6);
  cin >> n;
  if (n % 4 == 2) {
    cout << "! -1\n";
    return 0;
  }
  int po = 1;
  int ko = 1 + n / 2;
  int jak = ask(1);
  while (1) {
    int m = (po + ko) / 2;
    if (ask(m) == jak) {
      po = m;
    } else {
      ko = m;
    }
  }
}
