#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e5;
int edges[MAXN] = {0};
int main() {
  int n, s;
  cin >> n >> s;
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    edges[a]++, edges[b]++;
  }
  int amt = 0;
  for (int i = 0; i < n; i++) {
    if (edges[i] == 1) amt++;
  }
  double ans = ((double)s) / ((double)amt);
  cout << fixed << setprecision(10) << ans * 2 << endl;
}
