#include<bits/stdc++.h>
using namespace std;
int main(){
	float x;
	float a;
	cin>>x;
	a=x/3.14/2.0;
	cout<<a*a*3.14;
}
 
/**************************************************************
	Problem: 1337
	User: lvrenxiang
	Language: C++
	Result: Wrong Answer
****************************************************************/