#include <bits/stdc++.h>
using namespace std;
int a[200000];
int require(int x) {
  printf("? %d\n", x);
  fflush(stdout);
  int re;
  scanf("%d", &re);
  return a[x] = re;
}
int main() {
  int n;
  scanf("%d", &n);
  require(1);
  require(1 + n / 2);
  if (a[1] == a[1 + n / 2]) {
    printf("! 1\n");
    return 0;
  }
  if (abs(a[1] - a[1 + n / 2]) % 2 != 0) {
    printf("! -1\n");
    return 0;
  }
  int l = 1, r = n / 2;
  int ans = 0;
  while (l <= r) {
    int mid = l + r >> 1;
    require(mid);
    require(mid + n / 2);
    if ((a[mid] - a[mid + n / 2]) * (a[1] - a[1 + n / 2]) >= 0)
      ans = mid, l = mid + 1;
    else
      r = mid - 1;
  }
  printf("! %d\n", ans);
  return 0;
}
