#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 5;
const double esp = 1e-7;
const int mod = 1e9 + 7;
vector<int> adj[N];
int leaf = 0;
void dfs(int node = 1, int par = 0) {
  int cnt = 0;
  for (int child : adj[node]) {
    if (child == par) continue;
    dfs(child, node);
    cnt++;
  }
  if (!cnt) {
    leaf++;
  }
}
int main() {
  int n, s, u, v;
  double ans;
  scanf("%d %d", &n, &s);
  for (int i = 1; i < n; i++) {
    scanf("%d %d", &u, &v);
    adj[u].push_back(v);
    adj[v].push_back(u);
  }
  dfs();
  if (adj[1].size() == 1) leaf++;
  if (n == 2) {
    leaf = 1;
    ans = s;
  } else {
    ans = 2.0 * s / double(leaf);
  }
  printf("%.09f\n", ans);
  return 0;
}
