// https://oj.uz/problem/view/IOI11_race
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
#define ll long long
#define plli pair<ll, int>
#define pll pair<ll, ll>
#define pii pair<int, int>
// Usage: FOR(i, 0, N) {...}
#define FOR(i, start, end) for(int i = start; i < end; i++)
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int uid(int a, int b) { return uniform_int_distribution<int>(a, b)(rng); }
ll uld(ll a, ll b) { return uniform_int_distribution<ll>(a, b)(rng); }
void setIO(string s) {
freopen((s + ".in").c_str(), "r", stdin);
freopen((s + ".out").c_str(), "w", stdout);
}
struct comp {
bool operator() (const plli& a, const plli& b) const {
return a < b;
}
};
typedef tree<plli, null_type, comp, rb_tree_tag, tree_order_statistics_node_update> ordered_multiset;
// Segtree start
const int Nmax = 1e5; // limit for array size
int N; // array size
int t[2 * Nmax];
int oper(int a, int b) {
return a + b;
}
void build() { // build the tree
for (int i = N - 1; i > 0; --i) t[i] = oper(t[i<<1], t[i<<1|1]);
}
void modify(int p, int value) { // set value at position p
for (t[p += N] = value; p > 1; p >>= 1) t[p>>1] = oper(t[p], t[p^1]);
}
int query(int l, int r) { // on interval [l, r)
int res = 0;
for (l += N, r += N; l < r; l >>= 1, r >>= 1) {
if (l&1) res = oper(res, t[l++]);
if (r&1) res = oper(res, t[--r]);
}
return res;
}
// Segtree end
const int MAXN = 4e5;
struct node {
vector<plli> next;
ll dist = 0;
int dep = 0;
};
node graph[MAXN];
map<ll, ll> dists[MAXN];
ll mindist = 1e18;
void dfs(int v, int p, ll K) {
for (auto [w, x] : graph[v].next) {
if (x == p) continue;
graph[x].dist = graph[v].dist + w;
graph[x].dep = graph[v].dep + 1;
dfs(x, v, K);
}
dists[v].insert({graph[v].dist, graph[v].dep});
for (auto [w, x] : graph[v].next) {
if (x == p) continue;
if (dists[x].size() > dists[v].size()) swap(dists[x], dists[v]);
// for (auto [dist, dep] : dists[x]) {
for (auto itr = dists[x].begin(); itr != dists[x].end(); itr++) {
ll dist = itr->first;
ll dep = itr->second;
ll target = K + 2*graph[v].dist - dist;
// dist + target = K + 2*graph[v].dist
// auto itr2 = dists[v].lower_bound({target, 0});
if (dists[v].find(target) != dists[v].end()) {
// if (itr2 != dists[v].end() && itr2->first == target) {
mindist = min(mindist, dep + dists[v][target] - 2*graph[v].dep);
}
if (dists[v].find(dist) == dists[v].end()) dists[v][dist] = dep;
else dists[v][dist] = min(dists[v][dist], dep);
}
}
}
int best_path(int N, int K, int H[][2], int L[]) {
mindist = 1e18;
for (int i = 0; i < N-1; i++) {
auto [u, v] = H[i];
ll w = L[i];
graph[u].next.push_back({w, v});
graph[v].next.push_back({w, u});
}
dfs(0, -1, K);
// cout << mindist << endl;
return (mindist == 1e18 ? -1 : mindist);
}
int main() {
// Comment out for interactive problems
ios::sync_with_stdio(false);
cin.tie(nullptr);
int N;
int K;
cin >> N >> K;
int H[N-1][2];
int L[N-1];
for (int i = 0; i < N-1; i++) {
cin >> H[i][0] >> H[i][1];
} for (int i = 0; i < N-1; i++) cin >> L[i];
cout << best_path(N, K, H, L);
}
