#include <bits/stdc++.h>
using namespace std;
int n, a[1000];
bool ask(int vt) {
  int x, y;
  cout << "? " << vt << endl;
  cin >> x;
  cout << "? " << vt + n / 2 << endl;
  cin >> y;
  if (x == y) {
    cout << "! " << vt << endl;
    exit(0);
  }
  return (x < y);
}
int main() {
  cin >> n;
  int tmp = ask(1);
  int d = 1, c = n / 2;
  while (d <= c) {
    int mid = (d + c) / 2;
    if (ask(mid) == tmp)
      d = mid + 1;
    else
      c = mid - 1;
  }
  cout << "! -1";
  return 0;
}
