#include <cstdio>
#include <math.h>

const int vec[11]={25 ,125 ,625 ,3125 ,15625 ,78125 ,390625 ,1953125 ,9765625 ,48828125};

using namespace std;

int main()
{
	freopen("fact.out","w",stdout);
	freopen("fact.in","r",stdin);
	int p,s,i,j;
	scanf("%d",&p);
	s=0;
	s=5*p;
	for(i=0;i<11; i++)
		if(s<vec[i])
		{
			for(j=0;j<i;j++)
				s=s-(5*(j+1));
			break;
		}
	printf("%d",s);
	return 0;
}