#include <stdio.h>
void main()

{
	double i,r,s,p=3.14;
	scanf("%lf",&i);
	r=i/2.0/p;
	s=3.14*r*r; 
	printf("%.2f",s);
 }
/**************************************************************
	Problem: 1337
	User: admin
	Language: C
	Result: Accepted
	Time:9 ms
	Memory:1144 kb
****************************************************************/