#include <bits/stdc++.h>
void debug_out() { std::cerr << std::endl; }
template <typename Head, typename... Tail>
void debug_out(Head h, Tail... t) {
  std::cerr << " " << h;
  debug_out(t...);
}
template <typename T1, typename T2>
std::ostream& operator<<(std::ostream& os, std::pair<T1, T2> pa) {
  return os << pa.first << " " << pa.second;
}
template <typename T>
std::ostream& operator<<(std::ostream& os, std::vector<T> vec) {
  for (int i = 0; i < vec.size(); i++)
    os << vec[i] << (i + 1 == vec.size() ? "" : " ");
  return os;
}
template <typename T1, typename T2>
inline bool chmax(T1& a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2>
inline bool chmin(T1& a, T2 b) {
  return a > b && (a = b, true);
}
long long pow_mod(long long a, long long b, long long mod = -1) {
  if (b == 0) return 1;
  if ((a == 0) || (mod != -1 && (a + mod) % mod == 0)) {
    return 0;
  }
  long long x = 1;
  while (b > 0) {
    if (b & 1) {
      x = (mod != -1) ? (x * a) % mod : x * a;
    }
    a = (mod != -1) ? (a * a) % mod : a * a;
    b >>= 1;
  }
  return x;
}
const long long MOD = 1e9 + 7;
using ll = long long;
using P = std::pair<long long, long long>;
int main() {
  std::cin.tie(0);
  std::ios::sync_with_stdio(false);
  ll n, m;
  std::cin >> n >> m;
  ll min = MOD;
  std::vector<P> d(n);
  std::vector<ll> d_inv(n);
  for (long long(i) = (0); i < (n); i++) {
    ll temp;
    std::cin >> temp;
    d_inv[i] = temp;
    d[i] = {temp, i};
  }
  std::sort(d.begin(), d.end());
  if (d[0].first != d[1].first) {
    std::cout << -1 << "\n";
    return 0;
  }
  ll root = d[0].second;
  d_inv[root] = 0;
  d[0].first = 0;
  std::vector<std::vector<P>> graph(n);
  std::vector<ll> cost(m, 1e9);
  for (long long(i) = (0); i < (m); i++) {
    ll u, v;
    std::cin >> u >> v;
    u--;
    v--;
    graph[u].push_back({v, i});
    graph[v].push_back({u, i});
  }
  std::set<ll> used;
  used.insert(root);
  for (long long(i) = (1); i < (n); i++) {
    bool isok = false;
    for (P next : graph[d[i].second]) {
      if (used.find(next.first) != used.end() &&
          (next.first == root || d_inv[next.first] < d[i].first)) {
        cost[next.second] = d[i].first - d_inv[next.first];
        used.insert(d[i].second);
        isok = true;
        break;
      }
    }
    if (!isok) {
      std::cout << -1 << "\n";
      return 0;
    }
  }
  for (long long(i) = (0); i < (n); i++) {
    if (i != root)
      std::cout << "B";
    else
      std::cout << "W";
  }
  std::cout << "\n";
  for (long long(i) = (0); i < (m); i++) {
    std::cout << cost[i] << "\n";
  }
  return 0;
}
