JaeHyeonKim19

import java.util.*;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int kg = sc.nextInt();
		int totalNum = 0;
		int fiveKgNum = kg/5;
		while((kg-5*fiveKgNum)%3!=0) {
			fiveKgNum--;
			if(fiveKgNum<0) {
				totalNum = -1;
				break;
			}
		}
		if(totalNum!=-1)totalNum = fiveKgNum + (kg-5*fiveKgNum)/3;
		System.out.print(totalNum);
	}
}