#include <bits/stdc++.h>
using namespace std;
long long inf = 1e9 + 7;
long long mod = 998244353;
using namespace std;
vector<long long> g[200005];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long T, N, i, j, k, L, l, m, n;
  long long s;
  cin >> N >> s;
  for (i = 0; i < N - 1; i++) {
    long long u, v;
    cin >> u >> v;
    u--;
    v--;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  long double ans = 0.0;
  if (N == 2)
    ans = (long double)s / 2.0;
  else {
    long long cnt = 0;
    for (i = 0; i < N; i++) {
      if (g[i].size() == 1) cnt++;
    }
    ans = (long double)s / (long double)cnt;
  }
  printf("%.12Lf\n", 2 * ans);
}
