#include <iostream>
#include <fstream>

using namespace std;
ifstream fin("fact.in");
ofstream fout("fact.out");




int main(){
	long long int rez, n, check = 1;
	fin >> n;
	long long int num = n;
	rez = num/5;
	num = rez;
	while(num > 0){
		if(check > 0){
		rez = rez + (num / 5);
		//check = -1;
		} else {
			rez = rez - (num / 5);
			check = 1;
		}
		num = num / 5;
	}
	fout << rez << "\n";
	  
	return 0;
}