#include<cstdio>
#include<iostream>
using namespace std;
long long n,ans;
int main()
{
	scanf("%lld",&n);
	
	if(n & 1)
	return 0 & (long long)(puts("0"));
	n>>=1;
	while(n)
	{
		ans+=n/5;
		n/=5;
	}
	
	printf("%lld\n",ans);
	return 0;
}