#include <bits/stdc++.h>
using namespace std;

template <typename T> ostream& operator<<(ostream &os, const vector<T> &v) { os << '{'; string sep; for (const auto &x : v) os << sep << x, sep = ", "; return os << '}'; }
template <typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) { return os << '(' << p.first << ", " << p.second << ')'; }
using i64 = long long int;

const int INF = INT_MAX, MOD = 1e9 + 7;
const double EPS = 1e-9, PI = acos(-1);
const int dx[] = {0, 0, 0, -1, 1, -1, 1, 1, -1};
const int dy[] = {0, -1, 1, 0, 0, -1, 1, -1, 1};

int main() {
  ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
  /// mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

  int N; cin >> N;
  int lo = 0, hi = N, K;
  while (true) {
    int mid = lo + (hi - lo) / 2;
    cout << "? " << mid << "\n";
    cout.flush();

    cin >> K;
    if (K == -1) break;
    else if (K == 0) {
      lo = mid + 1;
    } else {
      hi = mid;
      if (lo == hi) {
        cout << "! " << lo << "\n";
        cout.flush();
        break;
      }
    }
  }

  return 0;
}