/*
 * app.cpp
 *
 *  Created on: Jul 12, 2014
 *      Author: Aurelian
 */
#include <iostream>
#include <stdio.h>
#include <fstream>

using namespace std;
int main(){

	int nr;
	ifstream fin("fact.in");
	fin>>nr;

	fin.close();
	ofstream fout("fact.out");
	if(nr==0){
			fout<<1;
			return 0;
	}
	if(nr<0){
		fout<<-1;
		return 0;
	}
	int final=0;
	for(int i=1;i<=nr;i++){
		final++;
		int j=i;
		while(j/5 &&j%5==0){
			i++;
			j=j/5;
		}
	}
	fout<<final*5;
	return 0;
}