package main

import (
	"fmt"
	"math"
)

func main() {
	var c float64
	var r1, r2 float64
	fmt.Scanf("%f", &c)
	fmt.Scanf("%f %f", &r1, &r2)
	R := (r2 - r1) / 2
	R2 := (r1 + r2) / 2
	res := c * math.Pi * R * R * (2.0 * math.Pi * R2)
	fmt.Printf("%.12f\n", res)
}