#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
  for (T e : v) {
    os << e << ' ';
  }
  return os;
}
static constexpr ll inf = 1e18;
int ask(int x) {
  cout << "? " << x << endl;
  int tmp;
  cin >> tmp;
  return tmp;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  if (n % 4 == 2) {
    cout << "! -1\n";
    return 0;
  }
  int l = 1, r = 1 + n / 2;
  int a = ask(1), b = ask(1 + n / 2);
  int c = a - b;
  b = a;
  a = c;
  if (a == 0) {
    cout << "! 1\n";
    return 0;
  }
  while (true) {
    int mid = (l + r) / 2;
    int diff = ask(mid) - ask(mid + n / 2);
    if (diff == 0) {
      cout << "! " << mid << endl;
      return 0;
    }
    if (1LL * a * diff < 0) {
      r = mid - 1;
      b = diff;
    } else {
      l = mid + 1;
      a = diff;
    }
  }
  return 0;
}
