#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)
	{
		int x = puteri[i];
		int z = 2;
		while (x < n)
		{
			n -= 5;
			x *= z;
			z ++;
		}
		i++;
	}
	ofstream out("fact.out");
	out << n;
	return 0;
}