#include <bits/stdc++.h>
using namespace std;
int n;
int a[1005];
int ask(int vt) {
  cout << "? " << vt + 1 << '\n';
  int ans;
  cin >> ans;
  return ans;
}
void Res(int ans) {
  cout << "! " << ans;
  exit(0);
}
int main() {
  cin >> n;
  int x, y;
  x = ask(0);
  y = ask(n / 2);
  if (x == y) {
    Res(1);
  }
  if (abs(x - y) % 2) {
    Res(-1);
  }
  int L = 1, R = n / 2, a, b;
  while (L + 1 < R) {
    int mid = (L + R) / 2;
    a = ask(mid);
    b = ask(mid + n / 2);
    if (a == b) {
      Res(mid + 1);
    }
    if ((a < b) == (x < y))
      L = mid;
    else
      R = mid;
  }
  Res(-1);
  return 0;
}
