#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const long long LINF = 0x3f3f3f3f3f3f3f3f;
const long long mod = 1e9 + 7;
const double PI = acos(-1.0);
const double eps = 1e-10;
const int N = 1e5 + 5;
int n, m;
bool ask(int id) {
  printf("? %d\n? %d\n", id, id + m);
  fflush(stdout);
  int x, y;
  scanf("%d %d", &x, &y);
  if (x == y) {
    printf("! %d\n", id);
    exit(0);
  }
  return x < y;
}
int main() {
  scanf("%d", &n);
  m = n >> 1;
  if (m & 1)
    puts("! -1");
  else {
    int l = 1, r = m + 1;
    bool sign = ask(1);
    while (r - l > 1) {
      int mid = l + r >> 1;
      if (ask(mid) == sign)
        l = mid;
      else
        r = mid;
    }
    printf("! %d\n", l);
  }
  return 0;
}
