#include <bits/stdc++.h>
using ll = long long;
enum : int { M = (int)1e9 + 7 };
enum : ll { MLL = (ll)1e18L + 9 };
using namespace std;
template <ostream& out = cout, class... T>
void prints(T&&...) {}
template <ostream& out = cout, class... T>
void printe(T&&...) {}
template <ostream& out = cout, class... T>
void printw(T&&...) {}
template <ostream& out = cout, class... T>
void printew(T&&...) {}
template <ostream& out = cout, class... T>
void printb(T&&...) {}
template <ostream& out = cout, class... T>
void printd(T&&...) {}
template <ostream& out = cout, class... T>
void printde(T&&...) {}
template <ostream& out = cout, class... T>
void printdu(T&&...){};
template <class S, class T>
istream& operator>>(istream& in, pair<S, T>& p) {
  in >> p.first >> p.second;
  return in;
}
template <class T>
istream& operator>>(istream& in, vector<T>& v) {
  for (auto& e : v) {
    in >> e;
  }
  return in;
}
int main() {
  int n;
  cin >> n;
  if (n % 4 == 2) {
    cout << '!' << ' ' << -1 << endl;
    return 0;
  }
  cout << '?' << ' ' << n << endl;
  int a;
  cin >> a;
  cout << '?' << ' ' << 1 << endl;
  int b;
  cin >> b;
  cout << '?' << ' ' << n / 2 << endl;
  int c;
  cin >> c;
  cout << '?' << ' ' << 1 + n / 2 << endl;
  int d;
  cin >> d;
  int l1, l2, u1, u2;
  if (a == c) {
    cout << '!' << ' ' << n << endl;
    return 0;
  } else if (b == d) {
    cout << '!' << ' ' << 1 << endl;
    return 0;
  } else if (a < c) {
    l1 = n;
    l2 = 1;
    u1 = n / 2 + 1;
    u2 = n / 2;
  } else {
    u1 = n;
    u2 = 1;
    l1 = n / 2 + 1;
    l2 = n / 2;
  }
  int cnt = 4;
  while (1) {
    int p1 = (l1 + u1) / 2, p2 = (l2 + u2) / 2;
    cout << '?' << ' ' << p1 << endl;
    int a;
    cin >> a;
    cout << '?' << ' ' << p2 << endl;
    int b;
    cin >> b;
    cnt += 2;
    if (a == b) {
      cout << '!' << ' ' << p1 << endl;
      return 0;
    } else if (a < b) {
      l1 = p1;
      u2 = p2;
    } else {
      u1 = p1;
      l2 = p2;
    }
    if (cnt == 60) {
      cout << '!' << ' ' << -1 << endl;
      return 0;
    }
  }
}
