import java.util.*;
import java.math.*;
import java.io.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int C = sc.nextInt();
		int ri = sc.nextInt();
		int ro = sc.nextInt();
		double rr = (ro - ri)/2.0;
		double a = (ri + ro) /2.0;
		System.out.printf("%.20f\n",C * Math.PI * rr * rr * 2 * Math.PI * a);
		
	}
}