#include <bits/stdc++.h>
using namespace std;
int n;
int cal(int mid);
int main() {
  scanf("%d", &n);
  if (n % 4) {
    printf("! -1\n");
    return 0;
  }
  int l = 1;
  int r = n / 2;
  int mid = (l + r) / 2;
  int fl = cal(l);
  int fr = cal(r);
  if (fl == 0) {
    printf("! %d\n", l);
    return 0;
  }
  if (fr == 0) {
    printf("! %d\n", r);
    return 0;
  }
  while (r > l) {
    int fm = cal(mid);
    if (fm == 0) {
      printf("! %d\n", mid);
      return 0;
    }
    if (fm == fl) {
      l = mid;
    } else {
      r = mid;
    }
    mid = (l + r) / 2;
  }
  return 0;
}
int cal(int mid) {
  int a, b;
  printf("? %d\n", mid);
  fflush(stdout);
  scanf("%d", &a);
  printf("? %d\n", mid + n / 2);
  fflush(stdout);
  scanf("%d", &b);
  if (b - a == 0) return 0;
  if (b - a > 0) return 1;
  return -1;
}
