#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
int n;
int ask(int x) {
  cout << "? " << x << endl;
  cout << "? " << x + n / 2 << endl;
  int a, b;
  cin >> a >> b;
  if (a == b) {
    cout << "! " << x << endl;
    exit(0);
  }
  if ((a - b + 2) % 2 == 1) {
    cout << "! -1" << endl;
    exit(0);
  }
  return a - b;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie();
  cout.tie();
  cin >> n;
  int l = 1, r = 1 + n / 2, lv = ask(l), rv = -lv;
  while (1) {
    int mid = (l + r) >> 1, mv = ask(mid);
    if ((mv < 0) ^ (lv < rv))
      r = mid;
    else
      l = mid + 1;
  }
}
