#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, al, ar, L, R;
  cin >> n;
  if (n % 4 != 0) {
    printf("! -1\n");
    return 0;
  }
  printf("? 1\n");
  cin >> al;
  printf("? %d\n", n / 2 + 1);
  cin >> ar;
  if (al == ar) {
    printf("! 1\n");
    return 0;
  }
  L = 1, R = n / 2 + 1;
  al = al - ar;
  ar = -al;
  while (L != R) {
    int mid = (L + R) >> 1, tmp, tmp2;
    printf("? %d\n", mid);
    cin >> tmp;
    printf("? %d\n", mid + n / 2);
    cin >> tmp2;
    tmp -= tmp2;
    if (tmp == 0) {
      printf("! %d\n", mid);
      return 0;
    }
    if ((tmp ^ al) >= 0) {
      L = mid;
      al = tmp;
    } else {
      R = mid;
      ar = tmp;
    }
  }
}
