#include <bits/stdc++.h>
using namespace std;
using lint = long long;
const lint mod = 1e9 + 7;
const int inf9 = 1e9;
const lint inf18 = 1e18;
template <class T>
inline void YES(T condition) {
  if (condition)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
template <class T>
inline void Yes(T condition) {
  if (condition)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
template <class T = string, class U = char>
int character_count(T text, U character) {
  int ans = 0;
  for (U i : text) {
    ans += (i == character);
  }
  return ans;
}
lint power(lint base, lint exponent, lint module) {
  if (exponent % 2) {
    return power(base, exponent - 1, module) * base % module;
  } else if (exponent) {
    lint root_ans = power(base, exponent / 2, module);
    return root_ans * root_ans % module;
  } else {
    return 1;
  }
}
struct position {
  int y, x;
};
position mv[4] = {{0, -1}, {1, 0}, {0, 1}, {-1, 0}};
template <class T, class U>
string to_string(pair<T, U> x) {
  return to_string(x.first) + "," + to_string(x.second);
}
string to_string(string x) { return x; }
template <class itr>
void array_output(itr start, itr goal) {
  string ans;
  for (auto i = start; i != goal; i++) ans += to_string(*i) + " ";
  if (!ans.empty()) ans.pop_back();
  cout << ans << endl;
}
template <class itr>
void cins(itr first, itr last) {
  for (auto i = first; i != last; i++) {
    cin >> (*i);
  }
}
template <class T>
T gcd(T a, T b) {
  if (a && b) {
    return gcd(min(a, b), max(a, b) % min(a, b));
  } else {
    return a;
  }
}
template <class T>
T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
struct combination {
  vector<lint> fact, inv;
  combination(int sz) : fact(sz + 1), inv(sz + 1) {
    fact[0] = 1;
    for (int i = 1; i <= sz; i++) {
      fact[i] = fact[i - 1] * i % mod;
    }
    inv[sz] = power(fact[sz], mod - 2, mod);
    for (int i = sz - 1; i >= 0; i--) {
      inv[i] = inv[i + 1] * (i + 1) % mod;
    }
  }
  lint P(int n, int r) {
    if (r < 0 || n < r) return 0;
    return (fact[n] * inv[n - r] % mod);
  }
  lint C(int p, int q) {
    if (q < 0 || p < q) return 0;
    return (fact[p] * inv[q] % mod * inv[p - q] % mod);
  }
};
template <class itr>
bool next_sequence(itr first, itr last, int max_bound) {
  itr now = last;
  while (now != first) {
    now--;
    (*now)++;
    if ((*now) == max_bound) {
      (*now) = 0;
    } else {
      return true;
    }
  }
  return false;
}
template <class itr, class itr2>
bool next_sequence2(itr first, itr last, itr2 first2, itr2 last2) {
  itr now = last;
  itr2 now2 = last2;
  while (now != first) {
    now--, now2--;
    (*now)++;
    if ((*now) == (*now2)) {
      (*now) = 0;
    } else {
      return true;
    }
  }
  return false;
}
inline int at(lint i, int j) { return (i >> j) & 1; }
struct UnionFind {
  vector<int> data;
  UnionFind(int sz) { data.assign(sz, -1); }
  bool unite(int x, int y) {
    x = find(x), y = find(y);
    if (x == y) return (false);
    if (data[x] > data[y]) swap(x, y);
    data[x] += data[y];
    data[y] = x;
    return (true);
  }
  int find(int k) {
    if (data[k] < 0) return (k);
    return (data[k] = find(data[k]));
  }
  int size(int k) { return (-data[find(k)]); }
};
struct BipartiteGraph : UnionFind {
  vector<int> color;
  BipartiteGraph(int v) : color(v + v, -1), UnionFind(v + v) {}
  bool bipartite_graph_coloring() {
    for (int i = 0; i < color.size() / 2; i++) {
      int a = find(i);
      int b = find(i + (int)color.size() / 2);
      if (a == b) return (false);
      if (color[a] < 0) color[a] = 0, color[b] = 1;
    }
    return (true);
  }
  bool operator[](int k) { return (bool(color[find(k)])); }
};
const lint inf = 1e9;
int N, M;
vector<lint> D;
vector<vector<int>> roads;
vector<lint> is_covered;
vector<pair<int, int>> ans;
vector<lint> ans_cost;
vector<bool> is_equal;
void dfs(int now, lint type) {
  if (is_covered[now] != -1) {
    return;
  }
  is_covered[now] = type;
  for (int i : roads[now]) {
    if (D[now] < D[i] && is_covered[i] != type) {
      dfs(i, type);
      ans.push_back({now, i});
      is_equal.push_back(true);
      ans_cost.push_back(D[i] - D[now]);
    }
  }
}
int main() {
  cin >> N >> M;
  D.resize(N);
  cins((D).begin(), (D).end());
  roads.resize(N);
  vector<pair<int, int>> roads_set(M);
  for (int i = 0; i < M; i++) {
    int U, V;
    cin >> U >> V;
    U--, V--;
    if (U > V) {
      swap(U, V);
    }
    roads_set[i] = {U, V};
    roads[U].push_back(V);
    roads[V].push_back(U);
  }
  map<lint, vector<int>> vertex_by_D;
  for (int i = 0; i < N; i++) {
    vertex_by_D[D[i]].push_back(i);
  }
  is_covered.resize(N, -1);
  for (auto i : vertex_by_D) {
    lint cost = i.first;
    vector<int> vertex = i.second;
    sort((vertex).begin(), (vertex).end());
    UnionFind uf(int(vertex.size()));
    for (int j = 0; j < vertex.size(); j++) {
      for (int k : roads[vertex[j]]) {
        int pos = int(lower_bound((vertex).begin(), (vertex).end(), k) -
                      vertex.begin());
        if (pos < vertex.size() && vertex[pos] == k) {
          uf.unite(j, pos);
        }
      }
    }
    for (int x = 0; x < vertex.size(); x++) {
      int j = vertex[x];
      if (is_covered[j] == -1 && uf.size(x) == 1) {
        cout << -1 << endl;
        return 0;
      }
    }
    map<int, vector<int>> group_of_root;
    for (int x = 0; x < vertex.size(); x++) {
      int j = vertex[x];
      if (uf.size(x) != 1) {
        group_of_root[uf.find(x)].push_back(j);
      }
    }
    for (auto j : group_of_root) {
      vector<int> group = i.second;
      sort((group).begin(), (group).end());
      assert(group.size() >= 2);
      vector<bool> is_grouped(group.size(), false);
      is_grouped[0] = true;
      vector<int> grouped{0};
      for (int k = 0; k < group.size(); k++) {
        for (int l : roads[grouped[k]]) {
          int pos = int(lower_bound((group).begin(), (group).end(), l) -
                        group.begin());
          if (pos == group.size() || group[pos] != l) {
            continue;
          }
          if (is_grouped[l]) {
            continue;
          }
          ans.push_back({grouped[k], l});
          ans_cost.push_back(cost);
          grouped.push_back(l);
          is_grouped[l] = true;
          is_equal.push_back(false);
        }
      }
      for (int k = 0; k < group.size(); k++) {
        dfs(group[k], cost);
      }
    }
  }
  BipartiteGraph graph(N);
  assert(ans.size() == N - 1);
  for (int i = 0; i < N - 1; i++) {
    if (is_equal[i]) {
      graph.unite(ans[i].first, ans[i].second);
    } else {
      graph.unite(ans[i].first, ans[i].second + N);
    }
  }
  graph.bipartite_graph_coloring();
  for (int i = 0; i < N; i++) {
    if (graph[i]) {
      cout << "B";
    } else {
      cout << "W";
    }
  }
  cout << endl;
  vector<lint> ans_cost_output(M, inf);
  map<pair<int, int>, int> roads_set_map;
  for (int i = 0; i < M; i++) {
    roads_set_map[roads_set[i]] = i;
  }
  for (int i = 0; i < N - 1; i++) {
    if (ans[i].first > ans[i].second) {
      swap(ans[i].first, ans[i].second);
    }
    ans_cost_output[roads_set_map[ans[i]]] = ans_cost[i];
  }
  for (int i = 0; i < M; i++) {
    cout << ans_cost_output[i] << endl;
  }
}
