import java.util.Scanner;
class N089{
	public static void main(String args[]){
		Scanner s = new Scanner(System.in);
		int c = s.nextInt();
		int i = s.nextInt();
		int o = s.nextInt();
		double r = (o - i) / 2d;
		double R = (o + i) / 2d;
		System.out.println(2 * Math.pow(Math.PI * r, 2) * R * c);
	}
}