#include <fstream>
using namespace std;

long long puteri[26], n;
int p;

void citire ()
{
	ifstream in("fact.in");
	in >> p;
}

void constructie ()
{
	puteri[1] = 25;
	for (int i = 2; i <= 25; i++)
		puteri[i] = puteri[i-1] * 5;
}

int main ()
{
	citire ();
	constructie ();
	n = 5 * p;
	int i = 1;
	while (puteri[i] <= n)
	{
		n = n - 5*i;
		i++;
	}
	ofstream out("fact.out");
	out << n;
	return 0;
}