package no089;

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double c = sc.nextInt();
		double rin = sc.nextInt();
		double rout = sc.nextInt();
		double r = (rout - rin) / 2;
		double s = r * r * Math.PI;
		double l = (rout + rin) / 2 * Math.PI * 2;
		System.out.println(s * l * c);
	}
}