#include <bits/stdc++.h>

using namespace std;

#define SZ(x) (int)(x.size())

using ll = long long;
using ld = long double;
using P = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vector<int>>;
using vll = vector<ll>;
using vvll = vector<vector<ll>>;
const double eps = 1e-10;
const ll MOD = 1000000007;
const int INF = 1000000000;
const ll LINF = 1ll<<50;

template<typename T>
void printv(const vector<T>& s) {
  for(int i=0;i<(int)(s.size());++i) {
    cout << s[i];
    if(i == (int)(s.size())-1) cout << endl;
    else cout << " ";
  }
}

template<typename T1, typename T2>
ostream& operator<<(ostream &os, const pair<T1, T2> p) {
  os << p.first << ":" << p.second;
  return os;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  int n, m; cin >> n >> m;
  vll d(n);
  for(int i=0;i<n;++i) {
    cin >> d[i];
  }

  map<P, int> edges;
  vector<vector<int>> g(n);
  vector<ll> cost(m, INF);
  vector<int> col(n, -1);
  for(int i=0;i<m;++i) {
    int u, v; cin >> u >> v;
    u--; v--;
    if(u > v) swap(u, v);
    
    g[u].push_back(v);
    g[v].push_back(u);
    edges[{u, v}] = i;
  }

  map<int, vi> ma;
  for(int i=0;i<n;++i) {
    ma[d[i]].push_back(i);
  }

  for(auto it: ma) {
    int val = it.first;
    auto vs = it.second;
    for(auto v: vs) {
      if(col[v] != -1) continue;
      bool exist = false;
      for(auto e: g[v]) {
        int nv = e;
        if(d[v] < d[nv]) continue;
        exist = true;
        if(v < nv) cost[edges[{v, nv}]] = val;
        else cost[edges[{nv, v}]] = val;
        if(col[nv] == -1) {
          col[v] = 0;
          col[nv] = 1;
        }
        else col[v] = 1 - col[nv];
        break;
      }
      
      if(!exist) {
        cout << -1 << endl;
        return 0;
      }
    }
  }

  for(int i=0;i<n;++i) {
    cout << (col[i] ? 'W' : 'B');
  }
  cout << endl;
  for(int i=0;i<m;++i) {
    cout << cost[i] << endl;
  }

}
