import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int C = sc.nextInt();
        int Rin = sc.nextInt();
        int Rout= sc.nextInt();
        double r = (Rout-Rin)/2.0;
        double d = (Rout+Rin)/2.0;
        double total = C*r*r*Math.PI * 2*d*Math.PI;
        System.out.println(total);
    }
}