#include <bits/stdc++.h>
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define rep(i,N) for(long long i = 0; i < (long long)(N); i++)
#define repr(i,N) for(long long i = (long long)(N) - 1; i >= 0; i--)
#define rep1(i,N) for(long long i = 1; i <= (long long)(N) ; i++)
#define repr1(i,N) for(long long i = (N) ; (long long)(i) > 0 ; i--)
#define each(x,v) for(auto& x : v)
#define all(v) (v).begin(),(v).end()
#define sz(v) ((int)(v).size())
#define ini(...) int __VA_ARGS__; in(__VA_ARGS__)
#define inl(...) long long __VA_ARGS__; in(__VA_ARGS__)
#define ins(...) string __VA_ARGS__; in(__VA_ARGS__)
using namespace std; void solve();
using ll = long long; template<class T = ll> using V = vector<T>;
using vi = V<int>; using vl = V<>; using vvi = V< V<int> >;
constexpr int inf = 1001001001; constexpr ll infLL = (1LL << 61) - 1;
struct IoSetupNya {IoSetupNya() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); cerr << fixed << setprecision(7);} } iosetupnya;
template<typename T, typename U> inline bool amin(T &x, U y) { return (y < x) ? (x = y, true) : false; }
template<typename T, typename U> inline bool amax(T &x, U y) { return (x < y) ? (x = y, true) : false; }
template<typename T, typename U> ostream& operator <<(ostream& os, const pair<T, U> &p) { os << p.first << " " << p.second; return os; }
template<typename T, typename U> istream& operator >>(istream& is, pair<T, U> &p) { is >> p.first >> p.second; return is; }
template<typename T> ostream& operator <<(ostream& os, const vector<T> &v) { int s = (int)v.size(); rep(i,s) os << (i ? " " : "") << v[i]; return os; }
template<typename T> istream& operator >>(istream& is, vector<T> &v) { for(auto &x : v) is >> x; return is; }
void in(){} template <typename T,class... U> void in(T &t,U &...u){ cin >> t; in(u...);}
void out(){cout << "\n";} template <typename T,class... U> void out(const T &t,const U &...u){ cout << t; if(sizeof...(u)) cout << " "; out(u...);}
template<typename T>void die(T x){out(x); exit(0);}
#ifdef NyaanDebug
  #include "NyaanDebug.h"
  #define trc(...) do { cerr << #__VA_ARGS__ << " = "; dbg_out(__VA_ARGS__);} while(0)
  #define trca(v,N) do { cerr << #v << " = "; array_out(v , N);cout << endl;} while(0)
#else
  #define trc(...)
  #define trca(...)
  int main(){solve();}
#endif

using P = pair<int,int>; using vp = V<P>;
constexpr int MOD = /**/ 1000000007; //*/ 998244353;
//////////

void solve(){
  ini(N , M);
  vi D(N); in(D);
  vi U(M) , V(M);
  vvi g(N);
  rep(i , M) {
    in(U[i] , V[i]); 
    U[i]-- , V[i]--;
    g[U[i]].pb(V[i]);
    g[V[i]].pb(U[i]);
  }
  map<P , int> m; 
  rep(i , M) m[minmax(U[i] , V[i])] = i;

  vi ord(N); iota(all(ord),0);
  sort(all(ord),[&](int x,int y){return D[x] < D[y];});

  // visited 0 黒 1 白
  vi visited(N , -1);
  vi ans(M , 1000000000);

  each(x , ord){
    trc(x);
    if(visited[x] != -1) continue;
    each(dst , g[x]){
      trc(dst);
      if(D[dst] > D[x]) continue;
      if(visited[dst] == -1) visited[dst] = 0 , visited[x] = 1;
      else visited[x] = visited[dst] ^ 1;
      ans[m[minmax(x , dst)]] = D[x];
      trc(D[dst] , D[x] , visited[x]);
      break;
    }
    if(visited[x] == -1) die(-1);
  }

  rep(i , N) cout << (visited[i] == 0 ? 'B' : 'W');
  cout << "\n";
  each(x , ans) out(x);

}

Measured memory_bytes: 24576