#include <bits/stdc++.h>

using namespace std;

int main() {
  int n;
  cin >> n;
  int p = 0, q = n;
  for (; q - p > 1;) {
    int m = (p + q) >> 1;
    (cout << "? " << m << '\n').flush();
    int o;
    cin >> o;
    if (o == -1) {
      return 1;
    } else {
      if (o) {
        q = m;
      } else {
        p = m;
      }
    }
  }
  (cout << "! " << q << '\n').flush();
}