#include <bits/stdc++.h>
using namespace std;
vector<int> g[100005];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, k, x, y;
  cin >> n >> k;
  for (int i = 0; i < n - 1; ++i) {
    cin >> x >> y;
    g[x].push_back(y);
    g[y].push_back(x);
  }
  int kol = 0;
  for (int i = 1; i <= n; ++i) {
    if ((int)g[i].size() == 1) kol++;
  }
  cout << fixed << setprecision(7) << k / (double)(kol)*2 << '\n';
  return 0;
}
