#include <bits/stdc++.h>
using namespace std;
const double PI = 2 * acos(0.0);
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng_64(chrono::steady_clock::now().time_since_epoch().count());
const string DIGITS = "0123456789";
const string ALPH = "abcdefghijklmnopqrstuvwxyz";
template <class T0, class T1>
inline ostream &operator<<(ostream &out, pair<T0, T1> &a) {
  return out << "{" << a.first << ", " << a.second << "}";
}
template <class T0, class T1>
inline istream &operator>>(istream &in, pair<T0, T1> &a) {
  return in >> a.first >> a.second;
}
template <class T0, class T1, class T2>
inline ostream &operator<<(ostream &out, tuple<T0, T1, T2> &a) {
  return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a)
             << "}";
}
template <class T0, class T1, class T2, class T3>
inline ostream &operator<<(ostream &out, tuple<T0, T1, T2, T3> &a) {
  return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a)
             << ", " << get<3>(a) << "}";
}
template <class T>
inline ostream &operator<<(ostream &out, vector<T> &a) {
  out << "[";
  for (int i = 0; i < int(a.size()); ++i)
    out << a[i] << vector<string>{", ", "]  "}[i + 1 == a.size()];
  return out;
}
void smain();
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cout << setprecision(12) << fixed;
  smain();
  return 0;
}
const int N = 2e5 + 10;
int n, m;
int d[N];
vector<int> g[N];
bool col[N];
int used[N];
map<pair<int, int>, int> id;
int ans[N];
void dfs(int v) {
  used[v] = 1;
  for (int to : g[v])
    if (d[to] >= d[v]) {
      if (used[to] == 0) {
        col[to] = col[v];
        ans[id[{min(to, v), max(to, v)}]] = d[to];
        dfs(to);
      } else if (used[to] == 1) {
        ans[id[{min(to, v), max(to, v)}]] = 1000 * 1000 * 1000;
      }
    }
  used[v] = 2;
}
void smain() {
  cin >> n >> m;
  for (int i = 1; i <= n; ++i) {
    cin >> d[i];
  }
  int min_d = *min_element(d + 1, d + n + 1);
  bool have = false;
  int f = 0, s = 0;
  for (int i = 0; i < int(m); ++i) {
    int u, v;
    cin >> u >> v;
    id[{min(u, v), max(u, v)}] = i;
    g[u].push_back(v);
    g[v].push_back(u);
    if (!have && d[u] == d[v] && d[u] == min_d) {
      col[u] = 1;
      f = u, s = v;
      have = true;
    }
  }
  if (!have) {
    cout << "-1\n";
    return;
  }
  used[f] = 1, used[s] = 1;
  dfs(f);
  dfs(s);
  ans[id[{min(f, s), max(f, s)}]] = min_d;
  for (int i = 0; i < int(n); ++i) {
    if (col[i + 1])
      cout << "B";
    else
      cout << "W";
  }
  cout << '\n';
  for (int i = 0; i < int(m); ++i) {
    cout << ans[i] << '\n';
  }
}
