#include<fstream.h>
long double st,dr,p,x,m,n;
long double check(long double x)
	{
	long double s=0,q=5;
	while(q<=x)
		{
		s+=x/q;
		q*=5;
		}
	return s;
	}
int main()
{
ifstream f("fact.in");
ofstream g("fact.out");
f>>p;
st=5;
dr=10000000000;
if(p==0)g<<1;
else 
{while(1)
	{
	m=(st+dr)>>1;
	x=check(m);
	if(x<p)st=m;
	else if(x>p)dr=m;
	else {n=m;break;}
	}
n=n-n%5;
g<<n;}
return 0;
}