#include <bits/stdc++.h>
using namespace std;
int query(int pos) {
  cout << "? " << pos << '\n';
  fflush(stdout);
  int res;
  cin >> res;
  return res;
}
void win(int ans) {
  cout << "! " << ans << '\n';
  exit(0);
}
int main() {
  int n, n2;
  cin >> n;
  n2 = n / 2;
  int initd = query(1) - query(1 + n2);
  if (abs(initd) % 2 == 1) win(-1);
  if (initd == 0) win(1);
  int l = 2, h = n2;
  while (l != h) {
    int m = (l + h) / 2;
    int qres = query(m) - query(m + n2);
    if (((qres > 0) && (initd > 0)) || ((qres < 0) && (initd < 0)))
      l = m + 1;
    else
      h = m;
  }
  win(l);
}
