#include <bits/stdc++.h>
#pragma GCC optimize("O3")
#pragma comment(linker, "/STACK:256000000")
#pragma warning(disable : 4996)
using namespace std;
const int inf = 1e9;
const long long llinf = 1e18;
const long double pi =
    3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825;
signed main() {
  srand(time(NULL));
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long n;
  cin >> n;
  long long m = n / 2;
  long long l = 0, r = n / 2, a1, a2, b1, b2;
  if (m & 1) return cout << "! -1" << endl, 0;
  cout << "? 1" << endl;
  cin >> a1;
  cout << "? " << m + 1 << endl;
  cin >> a2;
  b1 = a2;
  b2 = a1;
  if (a1 == a2) return cout << "! 1" << endl, 0;
  long long c1, c2;
  while (1) {
    long long tmp = (r + l) >> 1;
    cout << "? " << tmp + 1 << endl;
    cin >> c1;
    cout << "? " << tmp + m + 1 << endl;
    cin >> c2;
    if (c1 == c2) {
      cout << "! " << tmp + 1 << endl;
      break;
    }
    if ((a2 - a1 > 0 && c2 - c1 < 0) || (a2 - a1 < 0 && c2 - c1 > 0)) {
      r = tmp;
      b1 = c1;
      b2 = c2;
    } else {
      l = tmp;
      a1 = c1;
      a2 = c2;
    }
  }
  return 0;
}
