import java.util.Scanner;

class no89{
	public static void main(String[] args) {
		Scanner stdIn=new Scanner(System.in);
		double c,i,o,t,p=3.141592653589793;
		c=stdIn.nextDouble();
		i=stdIn.nextDouble();
		o=stdIn.nextDouble();
		t=.5*(o-i);
		System.out.print(t*t*(i+t)*p*p*c*2);
	}
}