2839: 설탕 배달

import java.util.Scanner;

public class Problem_2839 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int sugar = sc.nextInt();
		int cnt = 0;

		while (true) {

			if (sugar % 5 == 0) {
				cnt += sugar / 5;
				break;
			} 
			
			else if(sugar < 3) {
				cnt = -1;
				break;
			} 
			
			else {
				sugar -= 3;
				cnt++;
			}
			
		}

		System.out.println(cnt);

	}

}