#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int l = 1;
  int r = n / 2 + 1;
  int al0, al1, bl, br;
  cout << "? " << l << endl;
  cin >> al0;
  cout << "? " << r << endl;
  cin >> al1;
  bl = al1 - al0;
  br = -bl;
  if (bl % 2 != 0) {
    cout << "! -1" << endl;
    return 0;
  }
  if (bl == 0) {
    cout << "! 1" << endl;
    return 0;
  }
  while (l <= r) {
    int m = (l + r) / 2;
    int m1, m2, bm;
    cout << "? " << m << endl;
    cin >> m1;
    cout << "? " << (m + n / 2 - 1) % n + 1 << endl;
    cin >> m2;
    bm = m2 - m1;
    if (bm == 0) {
      cout << "! " << m << endl;
      return 0;
    }
    if (bm * bl > 0) {
      bl = bm;
      l = m + 1;
    } else {
      br = bm;
      r = m - 1;
    }
  }
  return 0;
}
