#include <bits/stdc++.h>
using namespace std;
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<bool> 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;
  }
}
