#include<iostream>
#include<iomanip>
using namespace std;
main(){
	double c,r1,r2,p=3.14159265358979323846;cin>>c>>r1>>r2;
	cout<<fixed<<setprecision(9)<<p*p*(r1+r2)*(r1-r2)*(r1-r2)*c/4<<endl;
}