#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 10;
int n;
int ask(int x) {
  if (x > n) x = x - n;
  cout << "? " << x << endl;
  fflush(stdout);
  cin >> x;
  return x;
}
int b(int x) { return ask(x) - ask(x + n / 2); }
int main() {
  cin >> n;
  if ((n / 2) & 1)
    cout << "! -1";
  else {
    int l = 2, r = n / 2;
    int fl = b(1), fr = b(1 + n / 2);
    if (fl == 0) {
      cout << "! " << 1;
      return 0;
    }
    while (l <= r) {
      int mid = (l + r) >> 1;
      int res = b(mid);
      if (res == 0) {
        cout << "! " << mid;
        return 0;
      } else if (res > 0) {
        if (fl > 0)
          l = mid + 1;
        else
          r = mid - 1;
      } else {
        if (fl > 0)
          r = mid - 1;
        else
          l = mid + 1;
      }
    }
  }
  return 0;
}
