#include <bits/stdc++.h>
const int N = 1e5 + 10;
const int INF = 0x3f3f3f3f;
const int mod = 1e9 + 7;
using namespace std;
int ask(int x) {
  cout << "? " << x << endl;
  int p;
  cin >> p;
  return p;
}
int a[N];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  if ((n / 2) & 1) {
    cout << "! -1" << endl;
    return 0;
  }
  int m = n / 2;
  a[1] = ask(1), a[1 + m] = ask(1 + m);
  if (a[1] == a[1 + m]) return cout << "! 1" << endl, 0;
  int l = 2, r = m;
  while (l <= r) {
    int mid = (l + r) >> 1;
    a[mid] = ask(mid), a[mid + m] = ask(mid + m);
    if (a[mid] == a[mid + m]) return cout << "! " << mid << endl, 0;
    if ((a[1] < a[1 + m] && a[mid] < a[mid + m]) ||
        (a[1] >= a[1 + m] && a[mid] >= a[mid + m])) {
      l = mid + 1;
    } else
      r = mid - 1;
  }
  cout << "! -1" << endl;
  return 0;
}
