#include <bits/stdc++.h>
using namespace std;
long long n,ans;
int main()
{
	cin>>n;
	if(n%2==1)
	printf("0");
	else
	{
		ans+=n/10;
		n/=10;
		while(n)
		{
			ans+=n/5;
			n/=5;
		}
		printf("%lld",ans);
	}
	return 0;
}