#include <bits/stdc++.h>
using namespace std;
int query(int i, int n) {
  int v1, v2;
  cout << "? " << i << endl;
  cin >> v1;
  cout << "? " << i + n / 2 << endl;
  cin >> v2;
  if (v1 == v2)
    return 0;
  else if (v1 < v2)
    return -1;
  return 1;
}
void solve() {
  int n;
  cin >> n;
  if (n % 4 > 0) {
    cout << "! -1" << endl;
    return;
  }
  int v = query(1, n);
  if (v == 0) {
    cout << "! " << 1 << endl;
    return;
  }
  int low = 1, high = n / 2;
  while (low <= high) {
    int mid = low + (high - low) / 2;
    int q = query(mid, n);
    if (q == 0) {
      cout << "! " << mid << endl;
      return;
    }
    if (v == q)
      low = mid + 1;
    else
      high = mid - 1;
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  solve();
  return 0;
}
