#include <bits/stdc++.h>
using namespace std;
long long n, a, b, mid, t;
int ask(int p) {
  cout << "? " << p << endl;
  cin >> a;
  cout << "? " << p + n << endl;
  cin >> b;
  if (a == b) {
    cout << "! " << p << endl;
    exit(0);
  }
  return a < b;
}
int main() {
  cin >> n;
  if (n % 4 == 2) {
    cout << "! -1";
    return 0;
  }
  long long l = 1, r = n / 2 + 1;
  n /= 2;
  t = ask(1);
  while (l + 1 < r) {
    mid = (l + r) / 2;
    if (ask(mid) == t) {
      l = mid;
    } else {
      r = mid;
    }
  }
  cout << "! " << l << endl;
}
