JAVA题解：

import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] line = br.readLine().trim().split(" ");
        int N = Integer.parseInt(line[0]);
        int M = Integer.parseInt(line[1]);
        
        // 构建邻接表
        List<Pair>[] adj = new ArrayList[N + 1];
        for (int i = 1; i <= N; i++) {
            adj[i] = new ArrayList<>();
        }
        
        for (int i = 0; i < N - 1; i++) {
            line = br.readLine().trim().split(" ");
            int u = Integer.parseInt(line[0]);
            int v = Integer.parseInt(line[1]);
            int c = Integer.parseInt(line[2]);
            adj[u].add(new Pair(v, c));
            adj[v].add(new Pair(u, c));
        }
        
        // BFS 建立父节点与拓扑序
        int[] fa = new int[N + 1];
        List<Integer> order = new ArrayList<>();
        Queue<Integer> q = new LinkedList<>();
        boolean[] visited = new boolean[N + 1];
        
        q.offer(1);
        visited[1] = true;
        
        while (!q.isEmpty()) {
            int u = q.poll();
            order.add(u);
            for (Pair p : adj[u]) {
                int v = p.node;
                if (!visited[v]) {
                    visited[v] = true;
                    fa[v] = u;
                    q.offer(v);
                }
            }
        }
        
        // 子树计数
        int[] cntS = new int[N + 1];
        int[] cntT = new int[N + 1];
        
        for (int i = 0; i < M; i++) {
            line = br.readLine().trim().split(" ");
            int s = Integer.parseInt(line[0]);
            int t = Integer.parseInt(line[1]);
            cntS[s]++;
            cntT[t]++;
        }
        
        // 后序汇总
        for (int i = order.size() - 1; i > 0; i--) {
            int u = order.get(i);
            if (u != 1) { // 跳过根节点
                cntS[fa[u]] += cntS[u];
                cntT[fa[u]] += cntT[u];
            }
        }
        
        long sumS = 0, sumT = 0;
        // 累加各边权
        for (int u = 2; u <= N; u++) {
            int p = fa[u];
            // 找到边权
            for (Pair pair : adj[u]) {
                if (pair.node == p) {
                    if (cntS[u] > 0) {
                        sumS += pair.weight;
                    }
                    if (cntT[u] > 0) {
                        sumT += pair.weight;
                    }
                    break;
                }
            }
        }
        
        long ans = 2 * (sumS + sumT);
        System.out.println(ans);
    }
    
    static class Pair {
        int node;
        int weight;
        
        public Pair(int node, int weight) {
            this.node = node;
            this.weight = weight;
        }
    }
}