#include <bits/stdc++.h>
using namespace std;
int n;
int ask(int x) {
  x %= n;
  if (x < 0) x += n;
  cout << "? " << x + 1 << endl;
  int r;
  cin >> r;
  return r;
}
void ans(int x) {
  x %= n;
  if (x < 0) x += n;
  cout << "! " << x + 1 << endl;
}
int main() {
  cin >> n;
  if (n % 4 == 2) {
    cout << "! " << -1 << endl;
    return 0;
  }
  int half = n / 2;
  int s = 0;
  int e = s + half;
  int sdiff = ask(s) - ask(e);
  if (sdiff == 0) {
    ans(s);
    return 0;
  }
  while (s + 1 < e) {
    int m = (s + e) / 2;
    if (s + 2 == e) {
      ans(m);
      return 0;
    }
    int qdiff = ask(m) - ask(m + half);
    if (qdiff == 0) {
      ans(m);
      return 0;
    }
    if ((qdiff > 0) == (sdiff > 0)) {
      sdiff = qdiff;
      s = m;
    } else {
      e = m;
    }
  }
}
