```go
package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Scan()
	var n, s int
	fmt.Sscanf(scanner.Text(), "%d %d", &n, &s)
	deg := make([]int, n+1)
	for i := 0; i < n-1; i++ {
		scanner.Scan()
		var a, b int
		fmt.Sscanf(scanner.Text(), "%d %d", &a, &b)
		deg[a]++
		deg[b]++
	}
	l := 0
	for i := 1; i <= n; i++ {
		if deg[i] == 1 {
			l++
		}
	}
	ans := 2.0 * float64(s) / float64(l)
	fmt.Printf("%.10f\n", ans)
}
```