#include <fstream>
#include <math.h>
using namespace std;

int main()
{
	long int p = 0;
	ifstream fin("fact.in");
	ofstream fout("fact.out");
	fin >> p;
	
	long int max = p * 5;
	long int nr = 0;
    for (int i = 2; pow(5.0, (double)i) < max; i++)
        nr += (i-1);
	
	p -= nr;
	fout << p * 5;
	
 	fout.close();
	fin.close();

	return 0;
}