#include<fstream>
using namespace std;
int main()
{long n,k=0,i,x;
	ifstream f("fact.in");ofstream g("fact.out");
	f>>n;i=1;x=1;
	if(n==0)g<<1;else 
	if(n%2==0)
	{
	 while(k<2*n-1)
	 {
		i++;
		x*=i;
		while(x%10==0)
		{
			x/=10;
			k++;
		}
		x=i;
	 }
	 g<<i-1;
	}
	else 
	if(n%2)
	{
	 while(k<2*n-1)
	 {
		i++;
		x*=i;
		while(x%10==0)
		{
			x/=10;
			k++;
		}
		x=i;
	 }
	 g<<i;
	}
	f.close();g.close();
return 0;}