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