import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int C = scan.nextInt();
		int Rin = scan.nextInt();
		int Rout = scan.nextInt();
		scan.close();
		double r = Rout - Rin;
		double ans = C * r * r * (Rin + r / 2 ) * Math.PI / 4 * 2 * Math.PI;
		System.out.println(ans);

	}
}