#include <bits/stdc++.h>
using namespace std;
template <class T>
ostream& prnt(ostream& out, T v) {
  out << v.size() << '\n';
  for (auto e : v) out << e << ' ';
  return out;
}
template <class T>
ostream& operator<<(ostream& out, vector<T> v) {
  return prnt(out, v);
}
template <class T>
ostream& operator<<(ostream& out, set<T> v) {
  return prnt(out, v);
}
template <class T1, class T2>
ostream& operator<<(ostream& out, map<T1, T2> v) {
  return prnt(out, v);
}
template <class T1, class T2>
ostream& operator<<(ostream& out, pair<T1, T2> p) {
  return out << '(' << p.first << ' ' << p.second << ')';
}
const int N = 100100;
const int MOD = 666013;
int n, k, m, g[N], ans, s, a, b;
int main() {
  ios_base::sync_with_stdio(false);
  cin >> n >> s;
  for (int i = 1; i < n; i++) {
    cin >> a >> b;
    g[a]++;
    g[b]++;
  }
  for (int i = 1; i <= n; i++)
    if (g[i] == 1) ans++;
  cout << fixed << setprecision(7);
  cout << 2. * s / ans << '\n';
}
