class Main{
	public static void main (String[] args){
		java.util.Scanner in = new java.util.Scanner(System.in); 
		long n = in.nextInt(), m = 5, s = 0;
		while(n >= m){
			s = s + (n / m); 
			m = m * 5;
		}
		System.out.println(s);
	}
}