///
/// There's a reason for your defeat, DIO. One simple reason...
/// You pissed me off.
///
#include <bits/stdc++.h>
#define Loop(x,l,r) for(ll x = ll(l); x < ll(r); ++x)
#define LoopR(x,l,r) for(ll x = ll(r)-1; x >= ll(l); --x)
#define Kill(x) exit((cout << (x) << '\n', 0))
typedef long long ll;
typedef std::pair<int,int> pii;
typedef std::pair<ll,ll> pll;
using namespace std;
#ifndef DARD
#include "race.h"
#endif
const int N = 200'010;
vector<pll> A[N];
bool block[N];
int cnt[N];
int n, k;
int dfs_cnt_rem(int v, int p)
{
cnt[v] = 1;
Loop (i,0,A[v].size()) {
if (block[A[v][i].first]) {
swap(A[v][i], A[v].back());
A[v].pop_back();
--i;
} else if (A[v][i].first != p) {
cnt[v] += dfs_cnt_rem(A[v][i].first, v);
}
}
return cnt[v];
}
int dfs_cent(int v, int p, int cnt)
{
for (auto [u, _] : A[v]) {
if (u == p)
continue;
if (::cnt[u] > cnt/2)
return dfs_cent(u, v, cnt);
}
return v;
}
void dfs_len(int v, int p, pll cur, vector<pll>& vec)
{
vec.push_back(cur);
for (auto [u, w] : A[v]) {
if (u == p)
continue;
dfs_len(u, v, {cur.first+w, cur.second+1}, vec);
}
}
int solve(int v)
{
dfs_cnt_rem(v, -1);
v = dfs_cent(v, -1, cnt[v]);
map<ll, int> mp; mp[0] = 0;
vector<pll> vec;
int ans = N;
for (auto [u, w] : A[v]) {
vec.clear();
dfs_len(u, v, {w, 1}, vec);
for (auto [x, y] : vec) {
auto it = mp.find(k-x);
if (it != mp.end())
ans = min(ans, (int)(y + it->second));
}
for (auto [x, y] : vec) {
int &z = mp[x];
if (!z || z > y)
z = y;
}
}
block[v] = 1;
for (auto [u, _] : A[v])
ans = min(ans, solve(u));
return ans;
}
int best_path(int n, int k, int h[][2], int l[])
{
::n = n; ::k = k;
Loop (i,0,n-1) {
A[h[i][0]].push_back({h[i][1], l[i]});
A[h[i][1]].push_back({h[i][0], l[i]});
}
int ans = solve(0);
return ans == N? -1: ans;
}
#ifdef DARD
int main()
{
// int constexpr n = 4;
// int constexpr k = 3;
// int h[n-1][2] = {{0, 1}, {1, 2}, {1, 3}};
// int l[n-1] = {1, 2, 4};
int constexpr n = 3;
int constexpr k = 3;
int h[n-1][2] = {{0, 1}, {1, 2}};
int l[n-1] = {1, 1};
// int constexpr n = 11;
// int constexpr k = 12;
// int h[n-1][2] = {{0, 1}, {0, 2}, {2, 3}, {3, 4}, {4, 5}, {0, 6}, {6, 7}, {6, 8}, {8, 9}, {8, 10}};
// int l[n-1] = {3, 4, 5, 4, 6, 3, 2, 5, 6, 7};
cout << best_path(n, k, h, l) << '\n';
}
#endif
