#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3fffffff;
const int SINF = 0x7fffffff;
const long long LINF = 0x3fffffffffffffff;
const long long SLINF = 0x7fffffffffffffff;
const int MAXN = 0;
int n;
void init();
void input();
void work();
int query(int n) {
  cout << "? " << n << endl;
  int ret;
  cin >> ret;
  return ret;
}
void out(int x) {
  cout << "! " << x << endl;
  exit(0);
}
int main() {
  init();
  input();
  work();
}
void init() { ios::sync_with_stdio(false); }
void input() { cin >> n; }
void work() {
  if (n % 4) out(-1);
  int m = n >> 1;
  int l = 1, r = m + 1, mid, vl = query(1 + m) - query(1), vr = -vl, vm;
  if (!vl) out(1);
  while (l < r) {
    mid = (l + r) >> 1;
    vm = query(mid + m) - query(mid);
    if (!vm) out(mid);
    if (vm<0 == vl> 0)
      r = mid, vr = vm;
    else
      l = mid, vl = vm;
  }
}
