백준 14916

import java.io.BufferedReader;
import java.io.InputStreamReader;

class Main14916 {
   //https://www.acmicpc.net/problem/14916

   public static void main(String[] args) throws Exception {
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

      int n = Integer.parseInt(br.readLine());

      if (n == 1 || n == 3) {
         System.out.println(-1);
         return;
      }

      int answer = n / 5;
      n %= 5;

      if (n % 2 != 0) {
         answer--;
         n += 5;
      }

      answer += n / 2;

      System.out.println(answer);
   }
}