using System;
using System.Linq;
class Program {
    static void Main() {
        int c = int.Parse(Console.ReadLine());
        int[] r = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
        int dif = r[1] - r[0];
        Console.WriteLine(c * dif * dif * r.Sum() * 0.25 * Math.PI * Math.PI);
    }
}