1085D - Minimum Diameter Tree

using namespace std;
using ll = long long;
using ii = pair<int, int>;
using vi = vector<int>;
int const NMAX = 1e5 + 11;
int n, s, deg[NMAX];
vi g[NMAX];
void dfs(int u, int p)
{
    deg[u] = 0;
    for (auto v : g[u])
    {
        deg[u]++;
        if (v != p)
            dfs(v, u);
    }
}
int main(void)
{
    cin >> n >> s;
    for (int i = 0; i < n - 1; ++i)
    {
        int u, v;
        cin >> u >> v;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    dfs(1, 0);
    int l = count_if(deg + 1, deg + n + 1, [](int d) { return d == 1; });
    cout << fixed << setprecision(12) << (2.0 * s) / (1.0 * l) << endl;
    return 0;
}