#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef double dou;
#define pii pair<int, int>
#define pb push_back
#define fi first
#define se second
//const ll mod = 1e9+7;
bool maximize(ll &x, ll y ){
if (x < y) {x = y; return true;};
return false;
}
bool minimize(int &x, int y){
if (x > y) {x = y; return true;}
return false;
}
//void add(ll &x, ll y ){
// x += y;
// if (x >= mod) x -= mod;
//}
//void sub(ll &x, ll y) {
// x -= y;
// if (x < 0) x += mod;
//}
const int maxn = 1e6+5;
const int maxx = 1e9+7;
int n, k;
vector<pii> a[maxn];
bool del[maxn] = {0};
int child[maxn], target;
int ans = maxx;
void couchild(int u, int p) {
child[u] = 1;
for (int i = 0; i < (int)a[u].size(); i++) {
int v = a[u][i].fi;
if (del[v] || v == p) continue;
couchild(v, u);
child[u] += child[v];
}
}
int find_centroid(int u, int p) {
for (int i = 0; i < (int)a[u].size(); i++) {
int v= a[u][i].fi;
if (del[v] || v == p) continue;
if (child[v] * 2 > target) return find_centroid(v, u);
}
return u;
}
int h[maxn], dis[maxn], min_length[maxn];
vector<int> undo;
vector<pii> updates;
void dfs(int u, int p) {
if (dis[u] > k) return;
minimize(ans, h[u] + min_length[k-dis[u]]);
updates.push_back(make_pair(dis[u], h[u]));
for (int i = 0; i < (int)a[u].size(); i++) {
int v = a[u][i].first;
if (del[v] || v == p) continue;
int w = a[u][i].se;
dis[v] = dis[u] + w;
h[v] = h[u] + 1;
dfs(v, u);
}
}
void solve(int u) {
couchild(u, -1);
target = child[u];
u = find_centroid(u, -1);
del[u] = true;
h[u] = dis[u] = 0;
min_length[0] = 0;
//solve
for (int i = 0; i < (int)a[u].size(); i++) {
int v = a[u][i].fi;
if (del[v]) continue;
int w = a[u][i].se;
dis[v] = w;
h[v] = 1;
dfs(v, u);
//update
for (int i = 0; i < (int)updates.size(); i++) {
minimize(min_length[updates[i].fi], updates[i].se);
undo.push_back(updates[i].fi);
}
updates.clear();
}
//clear
for (int i = 0; i < (int)undo.size(); i++) {
min_length[undo[i]] = maxx;
}
undo.clear();
for (int i =0; i < (int)a[u].size(); i++) {
int v = a[u][i].fi;
if (del[v]) continue;
solve(v);
}
}
int best_path(int N, int K, int H[][2], int L[]) {
n = N;
k = K;
for (int i = 0; i < n-1; i++) {
int u = H[i][0], v = H[i][1];
int w = L[i];
++u;
++v;
a[u].pb({v, w});
a[v].pb({u, w});
}
for (int i = 1; i <= k; i++) {
min_length[i] = maxx;
}
solve(1);
return (ans >= maxx ? -1 : ans);
}
//#define Mike_Vui
#ifdef Mike_Vui
int N, K, H[maxn][2], L[maxn];
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> N >> K;
for (int i = 1; i < N; i++) {
cin >> H[i][0] >> H[i][1];
}
for (int i = 1; i < N; i++) {
cin >> L[i];
}
cout << best_path(N, K, H, L);
}
#endif // Mike_Vui
/*
4 3
0 1
1 2
1 3
1
2
4
3 3
0 1
1 2
1
1
11 12
0 1
0 2
2 3
3 4
4 5
0 6
6 7
6 8
8 9
8 10
3
4
5
4
6
3
2
5
6
7
*/
