#include<bits/stdc++.h>
using namespace std;
int main(){

	float r,c,s;
	const float pi=3.14;
	cin>>c;
	r=c/2.0/pi;
	s=pi*r*r;
	printf("%.2f",s);
	return 0;
}


/**************************************************************
	Problem: 1337
	User: tangkaixuan
	Language: C++
	Result: Accepted
	Time:6 ms
	Memory:2072 kb
****************************************************************/