#include <bits/stdc++.h>
using namespace std;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
const int eps = 1e-6;
const int N = 2e5 + 5;
int deg[N];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n, s;
  cin >> n >> s;
  for (int i = 0, x, y; i < n - 1; i++) {
    cin >> x >> y;
    --x, --y;
    deg[x]++, deg[y]++;
  }
  int leaves = 0;
  for (int i = 0; i < n; i++) {
    if (deg[i] == 1) leaves++;
  }
  double sum = (2.0 * s) / (1.0 * leaves);
  cout << fixed << setprecision(15) << sum;
  return 0;
}
