#include <bits/stdc++.h>
using namespace std;
int n;
int ask(int i) {
  cout << "? " << i << endl;
  cout << "? " << i + n / 2 << endl;
  int a, b;
  cin >> a >> b;
  if (a == b) {
    cout << "! " << i << endl;
    return 2;
  }
  return a < b;
}
int main() {
  cin >> n;
  if (n % 4 == 2)
    cout << "! " << -1 << endl;
  else {
    int f = ask(1);
    int l = 1, r = n / 2 + 1;
    while (1) {
      int m = (l + r) / 2;
      int f2 = ask(m);
      if (f2 == 2) break;
      if (f2 == f)
        l = m;
      else
        r = m;
    }
  }
  return 0;
}
