#include <bits/stdc++.h>
using namespace std;
long long MOD = 1e9 + 7;
void init() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
}
long long n;
long long query(long long x) {
  long long res1, res2;
  cout << "? " << x << endl;
  cin >> res1;
  cout << "? " << x + n / 2 << endl;
  cin >> res2;
  return res1 - res2;
}
void solve() {
  cin >> n;
  long long l = 1;
  long long h = n / 2;
  long long z = query(1);
  if (z == 0) {
    cout << "! " << 1 << endl;
  } else {
    if (abs(z % 2) == 1) {
      cout << "! " << -1 << endl;
    } else {
      while (l <= h) {
        long long mid = l + (h - l) / 2;
        long long y = query(mid);
        if (y == 0) {
          cout << "! " << mid << endl;
          break;
        } else if (z > 0 && y < 0) {
          h = mid - 1;
        } else if (z < 0 && y > 0) {
          h = mid - 1;
        } else {
          l = mid + 1;
        }
      }
    }
  }
}
int main() {
  long long t = 1;
  while (t--) {
    solve();
  }
}
