#include <bits/stdc++.h>
using namespace std;
int n;
const int maxn = 1e5 + 10;
int nu[maxn];
int ask(int x) {
  printf("? %d\n", x);
  fflush(stdout);
  scanf("%d", &x);
  return x;
}
int main() {
  scanf("%d", &n);
  int l = 2, r = n / 2;
  int a1 = ask(1);
  int b1 = ask(1 + n / 2);
  if (a1 == b1) printf("! 1\n");
  while (l <= r) {
    int mid = l + (r - l) / 2;
    int aa = ask(mid), bb = ask(mid + n / 2);
    if (aa == bb) {
      printf("! %d\n", mid);
      return 0;
    }
    if ((a1 > b1 && aa < bb) || (a1 < b1 && aa > bb))
      r = mid - 1;
    else
      l = mid + 1;
  }
  printf("! -1\n");
  return 0;
}
