import java.util.*; public class Donut{ public static void main(String[] arg){ Scanner sc = new Scanner(System.in); double ri,ro,c,res; System.out.println(Math.PI); c = sc.nextDouble(); ri = sc.nextDouble(); ro = sc.nextDouble(); res = 2*Math.PI*(ri+(ro-ri)/2)*Math.PI*(ro-ri)*(ro-ri)*c/4; System.out.printf("%.10f\n",res); } }