#include <bits/stdc++.h>
using namespace std;
int N;
int getV(int i) {
  cout << "? " << (i + 1) << endl;
  int r;
  cin >> r;
  return r;
}
int diff(int i) {
  int v1 = getV(i);
  int v2 = getV((i + N / 2) % N);
  if (v2 > v1) return 1;
  if (v1 > v2) return -1;
  return 0;
}
int main() {
  cin >> N;
  if ((N / 2) % 2 == 1) {
    cout << "! -1" << endl;
    return 0;
  }
  int d = diff(0);
  if (d == 0) {
    cout << "! 1" << endl;
    return 0;
  }
  bool leftUp = d == 1;
  int l = 0, r = N / 2, m, mv;
  while (l < r) {
    m = (l + r) / 2;
    mv = diff(m);
    if (mv == 0) {
      cout << "! " << (m + 1) << endl;
      return 0;
    }
    if ((mv == 1 && leftUp) || (mv == -1 && !leftUp)) {
      l = m + 1;
    } else {
      r = m;
    }
  }
  cout << "! " << (r + 1) << endl;
  return 0;
}
