#include <bits/stdc++.h>
using namespace std;
template<typename T>
void fin(T a){
  cout<<a<<endl;
  exit(0);
}
const int INF=1e9;
template<typename T>
void chmin(T &a,T b){
  if(a>b)a=b;
}
typedef pair<int,int> P;

signed main(){
  int n,m;cin>>n>>m;
  vector<int> d(n),u(m),v(m);
  for(int i=0;i<n;i++)cin>>d[i];
  for(int i=0;i<m;i++){
    cin>>u[i]>>v[i];
    u[i]--;v[i]--;
  }
  vector<int> edge[n];
  for(int i=0;i<m;i++){
    edge[u[i]].push_back(v[i]);
    edge[v[i]].push_back(u[i]);
  }
  for(int i=0;i<n;i++){
    int A=INF;
    for(int p:edge[i])chmin(A,d[p]);
    if(A>d[i])fin(-1);
  }
  set<P> s;
  vector<int> edge2[n];
  map<P,int> ma;
  for(int i=0;i<n;i++){
    int idx,D=INF+1;
    for(int p:edge[i])if(D>d[p])D=d[p],idx=p;
    int A=idx,B=i;
    if(A>B)swap(A,B);
    if(s.count(P(A,B)))continue;
    s.insert(P(A,B));
    edge2[A].push_back(B);
    edge2[B].push_back(A);
    ma[P(A,B)]=d[i];
  }
  vector<int> col(n,0);
  queue<int> que;
  for(int i=0;i<n;i++){
    if(col[i])continue;
    col[i]=1;
    que.push(i);
    while(que.size()){
      int p=que.front();que.pop();
      for(int q:edge2[p]){
        if(col[q])continue;
        col[q]=-col[p];
        que.push(q);
      }
    }
  }
  for(int i=0;i<n;i++)cout<<(~col[i]?'B':'W');cout<<endl;
  for(int i=0;i<m;i++){
    if(u[i]>v[i])swap(u[i],v[i]);
    cout<<(ma[P(u[i],v[i])]?ma[P(u[i],v[i])]:INF)<<endl;
  }
}
