/*
* @Author: Twynzz
* @Last Modified by: Twynzz
* @Note For This Problem: https://wbd.ms/share/v2/aHR0cHM6Ly93aGl0ZWJvYXJkLm1pY3Jvc29mdC5jb20vYXBpL3YxLjAvd2hpdGVib2FyZHMvcmVkZWVtLzM3MDM5NDkwNGRkNDQ1NTc5Y2Y3NWFhNjI3ZmMxOGIyX0JCQTcxNzYyLTEyRTAtNDJFMS1CMzI0LTVCMTMxRjQyNEUzRF9iZmM4ZDc5MC05ZTEwLTQ4MGEtYjliYy04OGRhZDVhNTJlZWQ=
*/
#include <bits/stdc++.h>
using namespace std;
// ------------ Define ------------
#define el "\n"
#define int long long
#define ld long double
#define str string
#define file(name) {freopen(name".inp","r",stdin);freopen(name".out","w",stdout);}
#define IOS {std::ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);}
#define _TBN_ signed main()
#define setp(n) setprecision(n)<< fixed
#define fi first
#define se second
#define ii pair<int,int>
#define gcd(a,b) __gcd(a,b)
#define iii tuple<int, int, int>
#define pb push_back
#define pf push_front
#define eb emplace_back
#define sz(v) (int)v.size()
const int maxn = 1e6+9;
const long long oo = 1e18;
const int MOD = 1e9+7, Mod = 1e9+9;
const int base = 311, base2 = 293;
template<class X>
bool maximize(X &a, const X &b)
{
if (a<b)
{
a = b;
return 1;
}
return 0;
}
template<class X>
bool minimize(X &a, const X &b)
{
if (a>b)
{
a = b;
return 1;
}
return 0;
}
namespace sub1
{
bool checkSub1();
void sub1();
}
namespace sub2
{
bool checkSub2();
void sub2();
}
namespace sub3
{
bool checkSub3();
void sub3();
}
int lcm(int a, int b)
{
return a/gcd(a,b)*b;
}
int sqr(int x) {return x*x;}
inline bool MASK(int mask, int bit) {return ((mask>>bit)&1);}
inline long long F(int n) {return n*(n+1)/2;}
inline void add(int &a, int b, int m = MOD)
{
(a += b) %= m;
if (a >= m) a -= m;
}
inline void sub(int &a, int b, int m = MOD)
{
(a -= b) %= m;
if (a < 0) a += m;
}
inline void mul(int &a, int b, int m = MOD)
{
a %= m, b %= m;
(a *= b) %= m;
}
int dx[] = {-1, 1, 0, 0};
int dy[] = {0, 0, -1, 1};
// ------------ Code ------------
int n, k, h[maxn], len[maxn];
vector<ii> adj[maxn];
int heavy[maxn], in[maxn], out[maxn], timer, id[maxn];
int dfs(int u, int par)
{
in[u] = ++timer;
id[timer] = u;
int mx = 0, cursz = 1;
for(ii cur : adj[u])
{
int v = cur.fi, l = cur.se;
if (v == par) continue;
h[v] = h[u] + 1;
len[v] = len[u] + l;
int sz = dfs(v, u);
cursz += sz;
if (maximize(mx, sz)) heavy[u] = v;
}
out[u] = timer;
return cursz;
}
int d[maxn * 3];
void them(int u)
{
minimize(d[len[u]], h[u]);
}
int ans, mx;
void Sack(int u, int par, bool keep)
{
for(ii cur : adj[u])
{
int v = cur.fi;
if (v == par || v == heavy[u]) continue;
Sack(v, u, 0);
}
if (heavy[u]) Sack(heavy[u], u, 1);
for(ii cur : adj[u])
{
int v = cur.fi;
if (v == par || v == heavy[u]) continue;
for(int i = in[v]; i <= out[v]; i++)
{
int rem = k - (len[id[i]] - len[u]);
if (rem < 0) continue;
int cur_need = len[u] + rem;
if (cur_need > mx) continue;
minimize(ans, d[cur_need] + h[id[i]] - 2 * h[u]);
}
for(int i = in[v]; i <= out[v]; i++) them(id[i]);
}
them(u);
if (len[u] + k <= mx) minimize(ans, d[len[u] + k] - h[u]);
// cout<< u<< ' '<< len[u] + k<< ' '<< h[len[u] + k]<< el;
if (!keep)
{
for(int i = in[u]; i <= out[u]; i++) d[len[id[i]]] = oo;
}
}
_TBN_
{
IOS;
if (fopen("WSHIPPER.inp","r")) file("WSHIPPER");
cin >>n >>k;
for(int i = 1; i < n; i++)
{
int u, v, l;
cin >>u >>v >>l;
u++, v++;
adj[u].eb(v, l);
adj[v].eb(u, l);
}
for(int i = 1; i <= 1; i++)
{
if (sub1::checkSub1()) {sub1::sub1(); continue;}
// if (sub2::checkSub2()) {sub2::sub2(); continue;}
// if (sub3::checkSub3()) {sub3::sub3(); continue;}
}
return 0;
}
namespace sub1
{
bool checkSub1()
{
return 1;
}
void sub1()
{
dfs(1, -1);
ans = oo;
mx = *max_element(len + 1, len + 1 + n);
for(int i = 0; i <= mx; i++)
d[i] = oo;
Sack(1, -1, 0);
if (ans > (int)1e9) ans = -1;
cout<< ans<< el;
}
}
