#include <bits/stdc++.h>
using namespace std;
int n;
int f(int x) {
  int a, b;
  cout << "? " << x << endl;
  cin >> a;
  cout << "? " << x + (n / 2) << endl;
  cin >> b;
  return a - b;
}
int main() {
  cin >> n;
  if (n % 2 != 0) {
    cout << "! -1" << endl;
    return 0;
  }
  int fl = f(1), fr;
  if (fl == 0) {
    cout << "! 1" << endl;
  } else {
    fr = -fl;
    int l = 1, r = 1 + (n >> 1), m = (l + r) >> 1, fm;
    while (r - l > 1 && (fm = f(m)) != 0) {
      if ((fm < 0) == (fl < 0)) {
        l = (l + r) >> 1;
      } else {
        r = (l + r) >> 1;
      }
      m = l + r >> 1;
    }
    if (fm == 0)
      cout << "! " << m << endl;
    else
      cout << "! -1" << endl;
  }
  return 0;
}
