#include <bits/stdc++.h>
using namespace std;
int n;
int query(int x) {
  printf("? %d\n", x);
  fflush(stdout);
  int ret;
  scanf("%d", &ret);
  return ret;
}
void ans(int x) {
  printf("! %d", x);
  exit(0);
}
int main() {
  scanf("%d", &n);
  int m = n / 2;
  if (m & 1) {
    ans(-1);
    return 0;
  }
  int l = 1, r = 1 + m, a, b, c, d;
  d = a = query(l);
  c = b = query(r);
  if (a == b) ans(l);
  while (1) {
    int mid = l + r >> 1, tx, ty;
    tx = query(mid);
    ty = query(mid + m);
    if (tx == ty) ans(mid);
    if (1LL * (a - b) * (tx - ty) < 0) {
      c = tx;
      d = ty;
      r = mid - 1;
    } else {
      a = tx;
      b = ty;
      l = mid + 1;
    }
  }
  return 0;
}
