import java.util.Scanner;
import java.lang.Math;

class Yuki89 {
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    double c = sc.nextDouble();
    double ri = sc.nextDouble();
    double ro = sc.nextDouble();
    double r = (double)((ro-ri)/2);
    double g = r+ri;
    double x = 0;
    double pi = Math.PI;
    double gd = 2*pi*g;
    double s = r*r*pi;
    x=c*gd*s;
    System.out.println(x);
  }
}