D.cpp

#include "bits/stdc++.h"
using namespace std;

#define int long long
#define FOR(i, a, b) for (int i = (a), _##i = (b); i <= _##i; ++i)
#define FORD(i, a, b) for (int i = (a), _##i = (b); i >= _##i; --i)
#define REP(i, a) for (int i = 0, _##i = (a); i < _##i; ++i)
#define REPD(i,n) for(int i = (n)-1; i >= 0; --i)

#define DEBUG(X) { auto _X = (X); cerr << "L" << __LINE__ << ": " << #X << " = " << (_X) << endl; }
#define PR(A, n) { cerr << "L" << __LINE__ << ": " << #A << " = "; FOR(_, 1, n) cerr << A[_] << ' '; cerr << endl; }
#define PR0(A, n) { cerr << "L" << __LINE__ << ": " << #A << " = "; REP(_, n) cerr << A[_] << ' '; cerr << endl; }

#define __builtin_popcount __builtin_popcountll
#define SZ(x) ((int)(x).size())
#define ALL(a) (a).begin(), (a).end()

int32_t main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout << (fixed) << setprecision(9) << boolalpha;
    int n, s; cin >> n >> s;
    vector<int> deg(n);
    FOR(i,1,n-1) {
        int u, v; cin >> u >> v;
        --u; --v;
        deg[u]++;
        deg[v]++;
    }
    int cnt = 0;
    REP(i,n) cnt += deg[i] == 1;

    cout << (2.0 * s / cnt) << endl;
    return 0;
}