#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n, i, j, k, lef, rig, mid;
  cin >> n;
  if (n % 4 == 2) {
    cout << "! -1" << endl;
    return 0;
  }
  bool cac = true;
  cout << "? 1" << endl;
  cin >> j;
  cout << "? " << 1 + n / 2 << endl;
  cin >> k;
  if (j - k == 0) {
    cout << "! 1" << endl;
    return 0;
  }
  if (j - k < 0) {
    cac = false;
  }
  lef = 2;
  rig = n / 2 + 1;
  while (rig > lef) {
    mid = (lef + rig) / 2;
    cout << "? " << mid << endl;
    cin >> j;
    cout << "? " << mid + n / 2 << endl;
    cin >> k;
    if (cac) {
      if (j - k > 0) {
        lef = mid + 1;
      } else {
        rig = mid;
      }
    } else {
      if (j - k < 0) {
        lef = mid + 1;
      } else {
        rig = mid;
      }
    }
  }
  cout << "! " << lef << endl;
}
