#include <iostream>
using namespace std;

int main() {
	
	long long n;
	cin>>n;
	
	long long count5=0;
	for(long long i=5;i<=n;i+=5)
	{
		long long num=i;
		while(num%5 == 0)
		{
			num=num/5;
			count5++;
		}
		//cout<<"count2="<<count2<<" count5="<<count5<<endl;
	}
	
	cout<<count5<<endl;
	
	return 0;
}