#include<stdio.h>
long long k,u1,n,i,aux,ax,nn;
int main ()
{
	freopen("fact.in","r",stdin);
	freopen("fact.out","w",stdout);
	scanf("%lld",&n);
	if(n==0)
		printf("0");
	i=5;
	nn=n;
	u1=n;
	while(nn%i==0)
	{
		ax=nn/i;
		u1-=ax;
		if(u1/i!=n/i)
		{
			u1+=n/i-u1/i;
		}
		i*=5;
	}
	printf("%lld",5*u1);
}