#include <bits/stdc++.h>
using namespace std;

const double PI = acos( -1.0 );

signed main() {
	double C;
	double Rin, Rout;
	double r1, r2;
	cin >> C >> Rin >> Rout;
	r1 = ( Rout - Rin ) / 2;
	r2 = Rin + r1;
	cout << fixed << setprecision( 13 ) << C *( ( PI * r1 *r1 ) * ( 2.0 * PI * r2 ) ) << endl;
	return 0;
}