https://codeforces.com/contest/1087/problem/D

题解：只要求有几个端点，把s都平均到每个端点，再乘以2就是答案

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=100000+100;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,k,q,s,u,v;
int d[N];
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    scanf("%d%d",&n,&s);
    for(int i = 1; i < n; i++) {
		cin>>u>>v;
		d[u]++;
		d[v]++;
	}
	int cnt=0;
	for(int i = 1; i <= n; i++) {
		if(d[i]==1)
            cnt++;
	}
	printf("%.12lf",(double)s/cnt*2);
    //cout << "Hello world!" << endl;
    return 0;
}