#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
template <class T, class U>
istream &operator>>(istream &in, pair<T, U> &p) {
  in >> p.first >> p.second;
  return in;
}
template <class T, class U>
ostream &operator<<(ostream &out, const pair<T, U> &p) {
  out << p.first << ' ' << p.second;
  return out;
}
template <class T>
istream &operator>>(istream &in, vector<T> &v) {
  for (auto &i : v) {
    in >> i;
  }
  return in;
}
template <class T>
ostream &operator<<(ostream &out, const vector<T> &v) {
  for (auto &i : v) {
    out << i << ' ';
  }
  return out;
}
inline void io(int x = 15) {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  srand(time(0));
  cout << fixed << setprecision(x);
}
vector<int> prefix_function(string s) {
  int n = s.length();
  vector<int> pi(n);
  for (int i = 1; i < n; ++i) {
    int j = pi[i - 1];
    while (j > 0 && s[i] != s[j]) {
      j = pi[j - 1];
    }
    if (s[i] == s[j]) ++j;
    pi[i] = j;
  }
  return pi;
}
bool prime_test(long long n) {
  for (long long i = 2; i * i <= n; ++i)
    if ((n >= 0 && n < 2) || n % i == 0) return false;
  return true;
}
void prime_nums(int n, vector<char> &prime) {
  prime[0] = prime[1] = false;
  for (long long i = 2; i * i <= n; ++i) {
    if (prime[i]) {
      for (int j = i * i; j <= n; j += i) {
        prime[j] = false;
      }
    }
  }
}
long long bin_pow(long long a, long long b, long long m = 0) {
  if (m == 0)
    return b == 0 ? 1 : (b % 2 ? bin_pow(a, b - 1) * a : bin_pow(a * a, b / 2));
  if (b == 0) return 1;
  long long temp = bin_pow(a, b / 2, m);
  if (b % 2 == 0)
    return (temp * temp) % m;
  else
    return (a * temp * temp) % m;
}
bool nextperm(vector<char> &a) {
  int n = a.size();
  for (int i = n - 2; i >= 0; --i) {
    if (a[i] < a[i + 1]) {
      int mnv = a[i + 1], mni = i + 1;
      for (int j = i + 2; j < n; ++j) {
        if (a[j] < mnv && a[j] > a[i]) {
          mni = j;
          mnv = a[j];
        }
      }
      swap(a[i], a[mni]);
      reverse(a.begin() + i + 1, a.end());
      return true;
    }
  }
  return false;
}
int main() {
  io();
  int n, s;
  cin >> n >> s;
  vector<int> used(n);
  for (int i = 0; i < n - 1; ++i) {
    int l, r;
    cin >> l >> r;
    ++used[--l];
    ++used[--r];
  }
  int x = 0;
  for (int i = 0; i < n; ++i) {
    if (used[i] == 1) {
      ++x;
    }
  }
  cout << (long double)s / x * 2;
  return 0;
}
