#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
using namespace std;
template <class T, class S>
ostream &operator<<(ostream &o, const pair<T, S> &p) {
  return o << '(' << p.first << ", " << p.second << ')';
}
template <template <class, class...> class T, class... A>
typename enable_if<!is_same<T<A...>, string>(), ostream &>::type operator<<(
    ostream &o, T<A...> V) {
  o << '[';
  for (auto a : V) o << a << ", ";
  return o << ']';
}
long long int n, delta;
long long int ask(long long int i) {
  cout << "? " << i << endl;
  long long int r1;
  cin >> r1;
  cout << "? " << i + n / 2 << endl;
  long long int r2;
  cin >> r2;
  if (r1 == r2) {
    cout << "! " << i << endl;
    exit(0);
  }
  return r2 - r1;
}
long long int bSearch(long long int l, long long int r) {
  long long int m = (l + r) / 2, d = ask(m);
  return ((d < 0) == (delta < 0)) ? bSearch(m + 1, r) : bSearch(l, m - 1);
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  if (n % 4) {
    cout << "! -1" << endl;
    exit(0);
  }
  delta = ask(1);
  bSearch(2, n / 2);
}
