#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cassert>
#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <tuple>
#include <iterator>
using namespace std;
const int MAXN = 200000;
struct edg
{
int x, w;
};
vector<edg> arr[MAXN + 10];
bool chk[MAXN + 10];
int cnt[MAXN + 10];
int k;
int sz;
void f(int x, int p)
{
cnt[x] = 1;
for(edg &e : arr[x])
{
if(chk[e.x] || e.x == p)
continue;
f(e.x, x);
cnt[x] += cnt[e.x];
}
}
int g(int x, int p)
{
bool u = 1;
int sum = 1;
for(edg &e : arr[x])
{
if(chk[e.x] || e.x == p)
continue;
int t = g(e.x, x);
if(t != -1)
return t;
if(cnt[e.x] > sz / 2)
u = 0;
sum += cnt[e.x];
}
if(!u || sz - sum > sz / 2)
return -1;
return x;
}
map<int, int> mem;
vector<pair<int, int>> add;
void h(int x, int p, int w, int l)
{
add.push_back({ w, l });
for(edg &e : arr[x])
{
if(chk[e.x] || e.x == p)
continue;
if(w + e.w <= k)
h(e.x, x, w + e.w, l + 1);
}
}
int solve(int a)
{
f(a, a);
sz = cnt[a];
int c = g(a, a);
assert(c != -1);
mem.clear();
mem[0] = 0;
int res = -1;
for(edg &e : arr[c])
{
if(chk[e.x])
continue;
add.clear();
h(e.x, c, e.w, 1);
for(auto &e : add)
{
auto it = mem.find(k - e.first);
if(it != mem.end())
{
int t = it->second + e.second;
if(res == -1 || t < res)
res = t;
}
}
for(auto &e : add)
{
auto it = mem.find(e.first);
if(it != mem.end())
it->second = min(it->second, e.second);
else
mem[e.first] = e.second;
}
}
chk[c] = 1;
for(edg &e : arr[c])
{
if(chk[e.x])
continue;
int t = solve(e.x);
if(res == -1 || t != -1 && t < res)
res = t;
}
return res;
}
int best_path(int N, int K, int H[][2], int L[])
{
for(int i = 0; i < N - 1; i++)
{
arr[H[i][0]].push_back({ H[i][1], L[i] });
arr[H[i][1]].push_back({ H[i][0], L[i] });
}
k = K;
return solve(0);
}
