#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;
	printf("%.2lf",s);
	return 0;
}

/**************************************************************
	Problem: 1337
	User: zhangziang
	Language: C
	Result: Compile Error
****************************************************************/