#include <bits/stdc++.h>
using namespace std;
int n, m, lo, hi, mi;
int ta, tb, tc, td;
int ask(int a) {
  printf("? %d\n", a + 1);
  fflush(stdout);
  scanf("%d", &ta);
  printf("? %d\n", a + m + 1);
  fflush(stdout);
  scanf("%d", &tb);
  return tb - ta;
}
int main() {
  scanf("%d", &n);
  m = n / 2;
  if (m & 1)
    printf("! -1\n"), fflush(stdout);
  else {
    tc = ask(0);
    if (tc == 0)
      printf("! 1\n"), fflush(stdout);
    else {
      lo = 1;
      hi = m - 1;
      while (lo < hi) {
        mi = (lo + hi) / 2;
        td = ask(mi);
        if (td == 0) {
          printf("! %d\n", mi + 1), fflush(stdout);
          return 0;
        }
        if ((td > 0) == (tc > 0))
          lo = mi + 1;
        else
          hi = mi;
      }
      printf("! %d\n", lo + 1), fflush(stdout);
    }
  }
  return 0;
}
