#include <bits/stdc++.h>
const int N = 100010;
int ask(int sit) {
  printf("? %d\n", sit);
  fflush(stdout);
  int ret;
  scanf("%d", &ret);
  return ret;
}
inline int sig(int n) { return (n > 0) - (n < 0); }
int main() {
  int n;
  scanf("%d", &n);
  if (n >> 1 & 1) {
    puts("! -1");
    return 0;
  }
  int left = 1, right = (n >> 1) + 1;
  int vl = ask(left) - ask(right);
  if (!vl) {
    puts("! 1");
    return 0;
  }
  while (true) {
    int mid = left + right >> 1;
    int x = ask(mid) - ask((n >> 1) + mid);
    if (!x) {
      printf("! %d\n", mid);
      return 0;
    }
    if (sig(x) == sig(vl)) {
      left = mid + 1;
    } else {
      right = mid - 1;
    }
  }
}
