#include <bits/stdc++.h>
using namespace std;
int n;
int Ask(int x) {
  cout << "? " << x << '\n';
  cout << "? " << x + n / 2 << endl;
  int y1, y2;
  cin >> y1 >> y2;
  return y1 - y2;
}
void Answer(int x) {
  cout << "! " << x << endl;
  exit(0);
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cin >> n;
  if (n % 4) {
    Answer(-1);
  }
  int low = 1, high = n / 2;
  int a_low = Ask(low);
  if (a_low == 0) {
    Answer(low);
  }
  while (low < high) {
    int mid = (low + high) >> 1;
    int a_mid = Ask(mid);
    if (a_mid == 0) {
      Answer(mid);
    }
    if (signbit(a_low) == signbit(a_mid)) {
      low = mid + 1;
      a_low = a_mid;
    } else {
      high = mid - 1;
    }
  }
  Answer(low);
  return 0;
}
