#include <bits/stdc++.h>
using namespace std;
int get(int l, int n) {
  int v1, v2;
  cout << "? " << l << endl;
  fflush(stdout);
  cin >> v1;
  cout << "? " << (l + n / 2 - 1) % n + 1 << endl;
  fflush(stdout);
  cin >> v2;
  return v2 - v1;
}
int main() {
  int n;
  cin >> n;
  int l, r;
  r = (n / 2 + 2) / 2;
  l = 1;
  int v_r, v_l;
  v_r = get(r, n);
  v_l = get(l, n);
  if (v_r == 0) {
    cout << "! " << r << endl;
    return 0;
  }
  if (v_l == 0) {
    cout << "! " << l << endl;
    return 0;
  }
  if (((v_r > 0) - (v_r < 0)) * ((v_l > 0) - (v_l < 0)) == 1) {
    int tmp = r;
    r = l + n / 2;
    l = tmp;
  }
  int m, v_m;
  while (r - l > 1) {
    m = (r + l) / 2;
    v_m = get(m, n);
    if (v_m == 0) {
      cout << "! " << m << endl;
      return 0;
    }
    if (((v_m > 0) - (v_m < 0)) * ((v_l > 0) - (v_l < 0)) == 1) {
      l = m;
      v_l = v_m;
    } else {
      r = m;
      v_r = v_m;
    }
  }
  cout << "! " << -1 << endl;
  return 0;
}
