#include <bits/stdc++.h>
using namespace std;
const long long INF = 2e9 + 7, N = 3e6 + 5;
int n;
int ask(int x) {
  int a, b, y = x + n / 2;
  if (x >= n / 2 + 1) y = x - n / 2;
  cout << "? " << x << endl;
  cin >> a;
  cout << "? " << y << endl;
  cin >> b;
  return b - a;
}
signed main() {
  int val;
  cin >> n;
  val = ask(1);
  if (val % 2 == 0) {
    int l = 1, r = n / 2 + 1;
    while (l + 1 < r) {
      int mid = (l + r) / 2;
      if (ask(mid) * -val > 0) {
        r = mid;
      } else {
        l = mid;
      }
    }
    cout << "! " << l << endl;
  } else {
    cout << "! " << -1 << endl;
  }
  return 0;
}
