#include <bits/stdc++.h>
using namespace std;
int n;
int get(int i) {
  int a, b;
  cout << "? " << i << endl;
  cin >> a;
  cout << "? " << i + n / 2 << endl;
  cin >> b;
  return a - b;
}
int main() {
  cin >> n;
  int sv = get(1), l = 1, r = n / 2;
  if (sv == 0) {
    cout << "! 1" << endl;
    return 0;
  }
  while (l <= r) {
    int pv = (l + r) / 2, val;
    val = get(pv);
    if (val == 0) {
      cout << "! " << pv << endl;
      return 0;
    } else if ((sv > 0) ^ (val > 0)) {
      r = pv - 1;
    } else {
      l = pv + 1;
    }
  }
  cout << "! -1" << endl;
  return 0;
}
