import java.util.*;
public class Donut{
	public static void main(String[] arg){
		Scanner sc = new Scanner(System.in);
		double ri,ro,c,res;
		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);
	}
}