#include <bits/stdc++.h>
using namespace std;
int n;
int main() {
  scanf("%d", &n);
  int l = 1, r = 1 + n / 2;
  int ll, rr, hh;
  printf("? %d\n", l);
  fflush(stdout);
  scanf("%d", &ll);
  printf("? %d\n", r);
  fflush(stdout);
  scanf("%d", &rr);
  if (ll == rr) {
    puts("! 1");
    fflush(stdout);
    return 0;
  }
  ll -= rr;
  rr = -ll;
  while (l + 1 < r) {
    int md = l + r >> 1;
    printf("? %d\n", md);
    fflush(stdout);
    int hh;
    scanf("%d", &hh);
    int oo = md + n / 2;
    if (oo > n) oo -= n;
    printf("? %d\n", oo);
    fflush(stdout);
    scanf("%d", &oo);
    hh -= oo;
    if (hh == 0) {
      printf("! %d\n", md);
      fflush(stdout);
      return 0;
    } else if (hh * ll > 0) {
      l = md;
      ll = hh;
    } else {
      r = md;
      rr = hh;
    }
  }
  puts("! -1");
  fflush(stdout);
}
