#include <bits/stdc++.h>
using namespace std;
int n;
int ask_someone_who_knows__if_you_read_it_ill_find_you_and_ill_kill_you_hah_its_a_joke(
    int x) {
  x++;
  int a, b;
  printf("? %d\n", x);
  fflush(stdout);
  scanf("%d", &a);
  printf("? %d\n", x + (n / 2));
  fflush(stdout);
  scanf("%d", &b);
  if (a == b) {
    printf("! %d\n", x);
    fflush(stdout);
    exit(0);
  }
  return a < b;
}
int main() {
  scanf("%d", &n);
  int l = 0, r = n / 2,
      tp =
          ask_someone_who_knows__if_you_read_it_ill_find_you_and_ill_kill_you_hah_its_a_joke(
              0);
  while (r - l > 1) {
    int m = (l + r) / 2;
    if (ask_someone_who_knows__if_you_read_it_ill_find_you_and_ill_kill_you_hah_its_a_joke(
            m) == tp) {
      l = m;
    } else {
      r = m;
    }
  }
  printf("! -1");
  fflush(stdout);
  return 0;
}
