#include <bits/stdc++.h>
using namespace std;
int n;
int x, y, a, b;
long long L, R, MID;
int main() {
  cin >> n;
  if (n % 4) {
    cout << "! -1" << endl;
    return 0;
  }
  cout << "? 1" << endl;
  cout << "? " << 1 + n / 2 << endl;
  cin >> x >> y;
  if (x == y) {
    cout << "! 1" << endl;
    return 0;
  }
  L = x - y;
  R = y - x;
  int l = 1, r = 1 + n / 2;
  while (l < r) {
    int m = (l + r) / 2;
    cout << "? " << m << endl;
    cin >> a;
    cout << "? " << m + n / 2 << endl;
    cin >> b;
    MID = a - b;
    if (a == b) {
      cout << "! " << m << endl;
      return 0;
    } else if (MID * L < 0)
      r = m;
    else
      l = m + 1;
  }
  cout << "! -1" << endl;
}
