#include <bits/stdc++.h>
using namespace std;
bool ask(int n, int p) {
  ++p;
  int a, b;
  cout << "? " << p << endl;
  cin >> a;
  cout << "? " << p + n / 2 << endl;
  cin >> b;
  if (a == b) {
    cout << "! " << p << endl;
    exit(0);
  }
  return a < b;
}
int main() {
  int n;
  cin >> n;
  bool type = ask(n, 0);
  int l = 0, r = n / 2;
  while (r - l > 1)
    ask(n, (l + r) / 2) == type ? l = (l + r) / 2 : r = (l + r) / 2;
  cout << "! -1" << endl;
}
