#include <bits/stdc++.h>
using namespace std;
const int N = 3005;
int n, i, l, r, la, ra, lb, rb;
int ask(int x) {
  printf("? %d\n", x);
  fflush(stdout);
  int t;
  scanf("%d", &t);
  return t;
}
void end() {
  fflush(stdout);
  getchar();
  getchar();
}
int main() {
  scanf("%d", &n);
  n /= 2;
  if (n % 2 == 1) {
    cout << "! " << -1 << endl;
    return 0;
  }
  l = 1;
  r = n;
  la = ask(1);
  lb = ask(1 + n);
  ra = ask(n);
  rb = ask(n * 2);
  if (la == lb) {
    cout << "! " << 1 << endl;
    return 0;
  }
  if (ra == rb) {
    cout << "! " << n << endl;
    return 0;
  }
  while (1) {
    int mid = (l + r) / 2;
    int mida = ask(mid);
    int midb = ask(mid + n);
    if (mida == midb) {
      cout << "! " << mid << endl;
      return 0;
    }
    if ((mida < midb && la < lb) || (mida > midb && la > lb)) {
      l = mid;
      la = mida;
      lb = midb;
    } else {
      r = mid;
      ra = mida;
      rb = midb;
    }
  }
  return 0;
}
