#include <bits/stdc++.h>
using namespace std;
const long long INFL = 1LL << 60;
const long long INF = 1 << 30;
long long MOD = 1000000007;
vector<long long> dy = {0, 1, 0, -1, 1, 1, -1, -1, 0};
vector<long long> dx = {1, 0, -1, 0, 1, -1, 1, -1, 0};
void pres(double A, long long x = 10) {
  cout << fixed << setprecision(x) << A << endl;
}
void BinarySay(long long x, long long y = 60) {
  for (long long i = 0; i < (y); i++) cout << (x >> (y - 1 - i) & 1);
  cout << endl;
}
long long cnt_bit(long long x) { return __builtin_popcountll(x); }
long long pow_long(long long x, long long k) {
  long long res = 1;
  while (k > 0) {
    if (k % 2) res *= x;
    x *= x;
    k /= 2;
  }
  return res;
}
long long pow_mod(long long x, long long k) {
  long long res = 1;
  while (k > 0) {
    if (k % 2) {
      res *= x;
      res %= MOD;
    }
    x *= x;
    x %= MOD;
    k /= 2;
  }
  return res;
}
long long inverse(long long x) { return pow_mod(x, MOD - 2); };
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long lcm(long long x, long long y) {
  long long res = x / gcd(x, y);
  res *= y;
  return res;
};
const int MAXcomb = 200010;
long long fac[MAXcomb], finv[MAXcomb], inv[MAXcomb];
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAXcomb; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
long long comb(int n, int k) {
  if (n < k) return 0;
  if (n < 0 || k < 0) return 0;
  return fac[n] * finv[k] % MOD * finv[n - k] % MOD;
}
const int MAXkai = 200010;
long long kai_memo[MAXkai];
long long kai(long long N) {
  if (kai_memo[N] != 0) return kai_memo[N];
  if (N <= 1) return 1;
  return kai_memo[N] = N * kai(N - 1) % MOD;
}
long long N, M;
vector<pair<long long, long long>> D;
vector<long long> E[100010];
vector<pair<long long, long long>> edge;
long long cost[200010];
long long col[100010];
map<pair<long long, long long>, long long> e_id;
vector<pair<long long, pair<long long, long long>>> sames;
void solve() {
  cin >> N >> M;
  D.resize(N);
  for (long long i = 0; i < (N); i++) {
    cin >> D[i].first;
    D[i].second = i;
  }
  for (long long i = 0; i < (M); i++) {
    long long u, v;
    cin >> u >> v;
    u--, v--;
    E[u].push_back(v);
    E[v].push_back(u);
    edge.push_back({u, v});
    e_id[{u, v}] = i;
    e_id[{v, u}] = i;
    if (D[u].first == D[v].first) {
      pair<long long, pair<long long, long long>> p;
      p.first = D[u].first;
      p.second.first = u;
      p.second.second = v;
      sames.push_back(p);
    }
  }
  sort(sames.begin(), sames.end());
  sames.push_back({INFL, {-1LL, -1LL}});
  priority_queue<pair<long long, long long>, vector<pair<long long, long long>>,
                 greater<pair<long long, long long>>>
      pque;
  for (long long i = 0; i < (sames.size()); i++) {
    if (sames[i].first == INFL) break;
    long long val = sames[i].first;
    long long u = sames[i].second.first, v = sames[i].second.second;
    if (col[u] == 0 && col[v] == 0) {
      col[u] = 1;
      col[v] = 2;
      long long eid = e_id[{u, v}];
      cost[eid] = val;
    } else if (col[u] == 0) {
      col[u] = 3 - col[v];
      long long eid = e_id[{u, v}];
      cost[eid] = val;
    } else if (col[v] == 0) {
      col[v] = 3 - col[u];
      long long eid = e_id[{u, v}];
      cost[eid] = val;
    }
    for (long long to : E[u]) {
      long long eid = e_id[{u, to}];
      pque.push({D[to].first, eid});
    }
    for (long long to : E[v]) {
      long long eid = e_id[{v, to}];
      pque.push({D[to].first, eid});
    }
    cout << pque.size() << endl;
    while (!pque.empty()) {
      pair<long long, long long> p = pque.top();
      if (p.first >= sames[i + 1].first) break;
      pque.pop();
      long long u = edge[p.second].first;
      long long v = edge[p.second].second;
      if (D[u].first > D[v].first) swap(u, v);
      if (col[v] != 0) continue;
      cost[p.second] = D[v].first;
      col[v] = 3 - col[u];
      for (long long to : E[v]) {
        long long eid = e_id[{v, to}];
        pque.push({D[to].first, eid});
      }
    }
  }
  for (long long i = 0; i < (N); i++) {
    if (col[i] == 0) {
      cout << -1 << endl;
      return;
    }
  }
  for (long long i = 0; i < (N); i++) {
    if (col[i] == 1)
      cout << 'B';
    else
      cout << 'W';
  }
  cout << endl;
  for (long long i = 0; i < (M); i++) {
    if (cost[i] == 0) cost[i] = 1000000000;
    cout << cost[i] << endl;
  }
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
}
