#include <bits/stdc++.h>
using namespace std;
const int N = 100010;
int n;
int ask(int i) {
  printf("? %d\n", i);
  fflush(stdout);
  int res;
  scanf("%d", &res);
  return res;
}
void finish(int a) {
  printf("! %d\n", a);
  fflush(stdout);
  exit(0);
}
int main() {
  cin >> n;
  if (n % 4 != 0) {
    puts("! -1");
    fflush(stdout);
    return 0;
  }
  int a = ask(1 + n / 2) - ask(1);
  if (a == 0) finish(1);
  int low = 2, high = n / 2, mid;
  while (high >= low) {
    mid = (low + high) / 2;
    int k = ask(mid + n / 2) - ask(mid);
    if (k == 0) finish(mid);
    if ((a > 0) ^ (k > 0))
      high = mid - 1;
    else
      low = mid + 1;
  }
  assert(false);
  return 0;
}
