#include <bits/stdc++.h>
using namespace std;
long long int mpow(long long int a, long long int b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    long long int memo = mpow(a, b / 2);
    return memo * memo % (long long int)(1e9 + 7);
  } else
    return mpow(a, b - 1) * a % (long long int)(1e9 + 7);
}
long long int lpow(long long int a, long long int b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    long long int memo = lpow(a, b / 2);
    return memo * memo;
  } else
    return lpow(a, b - 1) * a;
}
long long int gcd(long long int a, long long int b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
vector<long long int> kaijo_memo;
long long int kaijo(long long int n) {
  if (kaijo_memo.size() > n) return kaijo_memo[n];
  if (kaijo_memo.size() == 0) kaijo_memo.push_back(1);
  while (kaijo_memo.size() <= n)
    kaijo_memo.push_back(kaijo_memo[kaijo_memo.size() - 1] * kaijo_memo.size() %
                         (long long int)(1e9 + 7));
  return kaijo_memo[n];
}
vector<long long int> gyaku_kaijo_memo;
long long int gyaku_kaijo(long long int n) {
  if (gyaku_kaijo_memo.size() > n) return gyaku_kaijo_memo[n];
  if (gyaku_kaijo_memo.size() == 0) gyaku_kaijo_memo.push_back(1);
  while (gyaku_kaijo_memo.size() <= n)
    gyaku_kaijo_memo.push_back(
        gyaku_kaijo_memo[gyaku_kaijo_memo.size() - 1] *
        mpow(gyaku_kaijo_memo.size(), (long long int)(1e9 + 7) - 2) %
        (long long int)(1e9 + 7));
  return gyaku_kaijo_memo[n];
}
long long int nCr(long long int n, long long int r) {
  if (n == r) return 1;
  if (n < r || r < 0) return 0;
  long long int ret = 1;
  ret *= kaijo(n);
  ret %= (long long int)(1e9 + 7);
  ret *= gyaku_kaijo(r);
  ret %= (long long int)(1e9 + 7);
  ret *= gyaku_kaijo(n - r);
  ret %= (long long int)(1e9 + 7);
  return ret;
}
vector<int> G[(int)1e5 + 3];
int main(void) {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n, m;
  cin >> n >> m;
  vector<long long int> D;
  vector<pair<long long int, long long int> > nodes;
  for (int i = 0; i < (int)(n); i++) {
    long long int d;
    cin >> d;
    D.push_back(d);
    nodes.push_back({d, i});
  }
  map<pair<long long int, long long int>, long long int> edge_map;
  vector<pair<long long int, long long int> > edges;
  vector<int> color;
  for (int i = 0; i < (int)(n); i++) {
    color.push_back(-1);
  }
  for (int i = 0; i < (int)(m); i++) {
    long long int u, v;
    cin >> u >> v;
    u--;
    v--;
    G[u].push_back(v);
    G[v].push_back(u);
    edges.push_back({min(u, v), max(u, v)});
    edge_map[{min(u, v), max(u, v)}] = (long long int)1e9;
  }
  sort(nodes.begin(), nodes.end());
  for (int i = 0; i < (int)(nodes.size()); i++) {
    int now = nodes[i].second;
    long long int d = nodes[i].first;
    for (int j = 0; j < (int)(G[now].size()); j++) {
      int next = G[now][j];
      if (D[next] <= D[now]) {
        if (D[next] == D[now]) {
          if (color[next] == -1) {
            color[next] = 0;
          }
          color[now] = 1 - color[next];
          edge_map[{min(now, next), max(now, next)}] = D[now];
          break;
        } else {
          color[now] = 1 - color[next];
          edge_map[{min(now, next), max(now, next)}] = D[now];
          break;
        }
      }
      if (j == G[now].size() - 1) {
        cout << -1 << endl;
        return 0;
      }
    }
  }
  for (int i = 0; i < (int)(n); i++) {
    cout << (color[i] == 1 ? "B" : "W");
  }
  cout << endl;
  for (int i = 0; i < (int)(edges.size()); i++) {
    int u = edges[i].first;
    int v = edges[i].second;
    cout << edge_map[{u, v}] << endl;
  }
  return 0;
}
