// 2019 IUSB Programming Competition
// Round 1 Problem 3
// Count the number of training 0s in n!
// Solution by Liguo Yu

import java.util.Scanner;

public class round1_p3
{
    public static void main(String[] main)
    {
        Scanner scanner = new Scanner(System.in);
        int num = scanner.nextInt();

        int count = 0;

        for (int i = 5; num / i >= 1; i *= 5)
        {
            count += num / i;
        }

        System.out.println(count);
    }
}