#include<bits/stdc++.h>
using namespace std;
int main(){
	float c,s,r;
	cin>>c;
	r=c/3.14/2;
	s=3.14*r*r;
	printf("%.2f",s);
}
/**************************************************************
	Problem: 1337
	User: sucongyi
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/