#include <bits/stdc++.h>
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
long long ask(long long x) {
  cout << "? " << x << endl;
  long long ret;
  cin >> ret;
  return ret;
}
long long go(long long x, long long y) {
  x++;
  y++;
  long long a = ask(x);
  long long b = ask(y);
  if (a == b) {
    cout << "! " << x << endl;
    exit(0);
  }
  return a - b;
}
signed main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  long long n;
  cin >> n;
  if ((n / 2) % 2 == 1) {
    cout << "! -1" << endl;
    ;
    return 0;
  }
  long long cur = go(0, n / 2);
  long long l = 0, r = n / 2;
  while (1) {
    long long mid = (l + r) / 2;
    long long x = go(mid, mid + n / 2);
    if (x * cur > 0)
      l = mid;
    else
      r = mid;
  }
}
