#include <bits/stdc++.h>
using namespace std;
long long v[100006];
void query(int x) {
  printf("? %d\n", x);
  fflush(stdout);
  scanf("%lld", &v[x]);
}
int main() {
  int n;
  scanf("%d", &n);
  int l = 1, r = n / 2, h = n / 2;
  query(l);
  query(l + h);
  if (v[1] == v[l + h]) {
    printf("! %d\n", 1);
    return 0;
  } else if (abs(v[l + h] - v[l]) % 2 == 1) {
    printf("! -1\n");
    return 0;
  }
  query(r);
  query(r + h);
  if (v[r] == v[r + h]) {
    printf("! %d\n", r);
    return 0;
  }
  while (l <= r) {
    int mid = l + r >> 1;
    query(mid);
    query(mid + h);
    if (v[mid] == v[mid + h]) {
      printf("! %d\n", mid);
      return 0;
    } else if ((v[mid] - v[mid + h]) * (v[l] - v[l + h]) > 0LL)
      l = mid;
    else
      r = mid;
  }
}
