#include<bits/stdc++.h>
using namespace std;
double c,r,s;
int main(){
	cin>>c;
	r=c/3.14/2;
	s=3.14*r*r;
	cout<<s;
	return 0;
}

/**************************************************************
	Problem: 1337
	User: zhangziang
	Language: C++
	Result: Wrong Answer
****************************************************************/