#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 100;
int n;
int ask(int id) {
  cout << "? " << id << endl;
  int ret;
  cin >> ret;
  return ret;
}
int sign(int id) {
  int a = ask(id), b = ask(id + n / 2);
  if (a == b) {
    cout << "! " << id << endl;
    exit(0);
  }
  return a - b > 0;
}
int main() {
  ios_base::sync_with_stdio(false), cin.tie(0);
  cin >> n;
  if ((n / 2) % 2 == 1) return cout << "! -1" << endl, 0;
  assert(sign(1) != sign(n / 2));
  int f = sign(1);
  int l = 1, r = n / 2;
  while (true) {
    int mid = (l + r + 1) / 2;
    if (sign(mid) != f)
      r = mid - 1;
    else
      l = mid;
  }
  return 0;
}
