#include <bits/stdc++.h>
using namespace std;
const int dx[] = {0, 1, 0, -1};
const int dy[] = {1, 0, -1, 0};
const long long inf = 2e9;
const long long mod = 666013;
const int N = 1e6 + 11;
const int NMAX = 1e4 + 11;
const long long INF64 = 3e18 + 1;
const double eps = 1e-14;
const double PI = acos(-1);
int n, mx;
int ask(int x) {
  cout << "? " << x + 1 << endl;
  int y;
  cin >> y;
  return y;
}
void solve() {
  cin >> n;
  int t = n / 2;
  if (t & 1) {
    cout << "! -1" << '\n';
    return;
  }
  int l = 0, r = n - 1;
  while (r - l > 1) {
    int mid = (l + r) >> 1;
    int diff = ask(mid) - ask((mid + t) % n);
    if (diff == 0) {
      cout << "! " << mid + 1 << endl;
      exit(0);
    }
    if (diff > 0)
      r = mid - 1;
    else
      l = mid + 1;
  }
  cout << "! " << l + 1 << endl;
}
int32_t main() {
  ios_base ::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    solve();
  }
}
