#include <stdio.h>
FILE *f=fopen("fact.in","r"), *g=fopen("fact.out","w");
long long nr5(long long x)
{
	int o=0;
	while (x%5==0)
		{
		o++;
		x/=5;
		}
	return o;
}
int main()
{
long long j,p,n,i=0,k;
fscanf(f,"%lld",&n);
i=n*5;
for (j=5;j<=i;j+=5)
{
	k=nr5(j);
	for (p=1;p<k;p++)
		i-=5;
}
fprintf(g,"%lld\n",i);
fcloseall();
return 0;
}