#include <iostream>
#include<math.h>
#include<stdlib.h>
#include<fstream>

using namespace std;
ifstream in("fact.in");
ofstream out("fact.out");
int main()
{
	int P,n=1,s,i=5,count=0,x=5,cc=0;
	cin>>P;
	while (i<P*5)
	{
		i=i*i;
		cc++;
	}
	i=i/i;
	cc--;
    for(n=1;n<=cc;n++)
	{
		s=s+n;
	}
	s=P*5-5*s;
	/*while (1)
	{
		if (x==i)
		{
			cc++;
			x=x*x;
			count=count+cc-2;
		}
		i=i+5;
		count++;
		if (count==P) break;
	}*/
	cout<<s;
}