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