#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using II = pair<long long, long long>;
using VII = vector<II>;
using VVII = vector<VII>;
using VI = vector<long long>;
using VVI = vector<VI>;
using VVVI = vector<VVI>;
template <class T = long long>
inline bool chmax(T& a, const T& b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T = long long>
inline bool chmin(T& a, const T& b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
ostream& operator<<(ostream& s, const vector<T>& d) {
  for (long long i = 0, max_i = (d.size()); i < max_i; i++)
    s << d[i] << (i == d.size() - 1 ? "" : " ");
  return s;
}
template <class T>
ostream& operator<<(ostream& s, const vector<vector<T>>& d) {
  for (long long i = 0, max_i = (d.size()); i < max_i; i++)
    s << d[i] << (i == d.size() - 1 ? "" : "\n");
  return s;
}
template <class T, class S>
ostream& operator<<(ostream& s, const pair<T, S>& p) {
  s << "{" << p.first << ", " << p.second << "}";
  return s;
}
template <class T, class S>
ostream& operator<<(ostream& s, const map<T, S> m) {
  for (pair<T, S> p : m) s << p << endl;
  return s;
}
void dump_func() { cerr << endl; }
template <class Head, class... Tail>
void dump_func(Head&& h, Tail&&... t) {
  cerr << h << (sizeof...(Tail) == 0 ? "" : ", "),
      dump_func(forward<Tail>(t)...);
}
struct Fast {
  Fast() {
    cin.tie(0);
    ios::sync_with_stdio(false);
  }
} fast;
constexpr long long MOD = 1e9 + 7;
signed main() {
  long long n;
  cin >> n;
  double s;
  cin >> s;
  VVI G(n);
  for (long long i = 0, max_i = (n - 1); i < max_i; i++) {
    long long u, v;
    cin >> u >> v;
    u--;
    v--;
    G[u].push_back(v);
    G[v].push_back(u);
  }
  long long r = 0;
  for (long long i = 0, max_i = (n); i < max_i; i++) {
    if (G[i].size() == 1) {
      r++;
    }
  }
  printf("%.10f\n", s * 2 / r);
}
