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

pair<int,int> dfs(map<int,vector<int>> &hm,vector<int> &S, int src,int par,map<int,int> &room){
    int fireAlpino = 0;
    int iceTender = 0;
    if(S[src]==1){
        fireAlpino++;
    }
    else{
        iceTender++;
    }

    if(hm[src].size()==1 && src!=1){
        return {fireAlpino, iceTender};
    }

    for(auto i : hm[src]){
        if(i!=par){
            pair<int,int> temp =dfs(hm,S,i,src,room);
            fireAlpino+=temp.first;
            iceTender+=temp.second;
        }
    }
    if(fireAlpino == iceTender){
        room[src] = fireAlpino;
    }

    return {fireAlpino, iceTender};
}

void heavenlyRooms(int N, vector<int> &S, 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);
    }

    map<int,int> heavenlyRoom;

    dfs(hm,S,1,-1,heavenlyRoom);

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

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

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

    vector<pair<int,int>> edge(N-1);
    for(int i=2;i<=N;i++){
        int U,V;
        cin>>U>>V;
        edge[i-2] = make_pair(U,V);
    }

    heavenlyRooms(N,S,edge);
}