#include <bits/stdc++.h>
using namespace std;
bool ask(int n, int x) {
  x++;
  int a, b;
  cout << "? " << x << endl;
  cin >> a;
  cout << "? " << (x + n / 2) << endl;
  cin >> b;
  if (a == b) {
    cout << "! " << x << endl;
    exit(0);
  }
  return a < b;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  ;
  bool type = ask(n, 0);
  int l = 0;
  int r = n / 2;
  while (l < r) {
    int m = (l + r) / 2;
    if (ask(n, m) == type) {
      l = m + 1;
    } else {
      r = m;
    }
  }
  cout << "! -1" << endl;
}
