#include<bits/stdc++.h>
using namespace std;

void dfs(map<int,vector<int>> &hm,vector<int> &door,int K, int src,int par,int count,int *totalLastroom,map<int,int> &room){
    if(door[src]==1){
        count++;
    }

    if(count>K){
        return;
    }

    if(hm[src].size()==1 && src!=1 && count<=K){
        (*totalLastroom)++;
        room[src] = count;
        return;
    }

    for(auto i : hm[src]){
        if(i!=par){
            dfs(hm,door,K,i,src,count,totalLastroom,room);
        }
    }
}

void roomsReached(int N, int K, vector<int> &door, vector<pair<int,int>> &edge){
    map<int,vector<int>> hm;

    for(int i=0;i<N-1;i++){
        hm[edge[i].first].push_back(edge[i].second);
        hm[edge[i].second].push_back(edge[i].first);
    }

    int totalLastRoom = 0;
    map<int,int> lastRoomReached;

    dfs(hm,door,K,1,-1,0,&totalLastRoom,lastRoomReached);

    cout<<totalLastRoom<<endl;
    for(auto i : lastRoomReached){
        cout<<i.first<<" "<<i.second<<endl;
    }
}

int main(){
    int N,K;
    cin>>N>>K;

    vector<int> door(N+1);
    for(int i=1;i<=N;i++){
        cin>>door[i];
    }

    vector<pair<int,int>> edge(N-1);
    for(int i=0;i<N-1;i++){
        int U,V;
        cin>>U>>V;
        edge[i] = make_pair(U,V);
    }

    roomsReached(N,K,door,edge);
}