#include <bits/stdc++.h>
using namespace std;
inline bool EQ(double a, double b) { return fabs(a - b) < 1e-9; }
const int INF = (((1 << 30) - 1) << 1) + 1;
const int nINF = 1 << 31;
int n, m;
int ask(int idx) {
  int res;
  cout << "? " << idx << endl;
  cin >> res;
  return res;
}
void answer(int idx) {
  cout << "! " << idx << endl;
  exit(0);
}
int getd(int idx) {
  int a1 = ask(idx);
  int a2 = ask(idx + m);
  return a2 - a1;
}
inline int sign(int x) { return x > 0 ? 1 : -1; }
signed main() {
  ios::sync_with_stdio(false);
  cin >> n;
  m = n / 2;
  int s = getd(1);
  if (s & 1) answer(-1);
  int l = 1, r = 1 + m;
  while (l < r) {
    int mid = (l + r) / 2;
    int d = getd(mid);
    if (d == 0)
      answer(mid);
    else if (sign(d) == sign(s))
      l = mid + 1;
    else
      r = mid - 1;
  }
  answer(l);
  return 0;
}
