#include <bits/stdc++.h>
using namespace std;
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &v : vec) is >> v;
  return is;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i];
    if (i + 1 != vec.size()) os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end()) os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end()) os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end()) os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end()) os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << itr->first << ":" << itr->second;
    auto titr = itr;
    if (++titr != mp.end()) os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << itr->first << ":" << itr->second;
    auto titr = itr;
    if (++titr != mp.end()) os << " ";
  }
  return os;
}
using ll = long long int;
using P = pair<int, int>;
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1000000007;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};
struct fast_io {
  fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(20);
  }
} fast_io_;
set<pair<int, ll>> edge[100010];
int u[200100], v[200100];
ll w[200100];
int main(int argc, char const *argv[]) {
  int n, m;
  cin >> n >> m;
  vector<ll> d(n);
  cin >> d;
  for (int i = (0); i < (m); ++i) {
    cin >> u[i] >> v[i], u[i]--, v[i]--;
    edge[u[i]].insert(make_pair(d[v[i]], v[i]));
    edge[v[i]].insert(make_pair(d[u[i]], u[i]));
  }
  map<P, int> idx;
  for (int i = (0); i < (m); ++i) {
    idx[make_pair(u[i], v[i])] = i;
    idx[make_pair(v[i], u[i])] = i;
    w[i] = inf;
  }
  map<ll, vector<int>> mp;
  for (int i = (0); i < (n); ++i) mp[d[i]].emplace_back(i);
  set<int> bl, re;
  for (auto itr = (mp).begin(); itr != (mp).end(); ++itr) {
    if (itr == mp.begin()) {
      for (auto e : itr->second) {
        auto tr = edge[e].lower_bound(make_pair(d[e], -inf));
        if (tr->first != d[e]) {
          cout << -1 << endl;
          return 0;
        }
        bool ins = false;
        while (tr != edge[e].end()) {
          if (tr->first != d[e]) break;
          if (bl.find(tr->second) != bl.end()) {
            re.insert(e);
            w[idx[make_pair(e, tr->second)]] = d[e];
            ins = true;
            break;
          }
          if (re.find(tr->second) != re.end()) {
            bl.insert(e);
            w[idx[make_pair(e, tr->second)]] = d[e];
            ins = true;
            break;
          }
          tr++;
        }
        if (!ins) {
          bl.insert(e);
        }
      }
    } else {
      for (auto e : itr->second) {
        bool ok = false;
        bool ex = false;
        for (auto v : edge[e]) {
          int V = v.second;
          if (d[V] == d[e]) {
            ex = true;
            if (bl.find(V) != bl.end()) {
              re.insert(e);
              ok = true;
              w[idx[make_pair(e, V)]] = d[e];
              break;
            }
            if (re.find(V) != re.end()) {
              bl.insert(e);
              w[idx[make_pair(e, V)]] = d[e];
              ok = true;
              break;
            }
          }
        }
        if (ex) {
          bl.insert(e);
          ok = true;
        }
        if (!ok) {
          for (auto v : edge[e]) {
            int V = v.second;
            if (d[V] < d[e]) {
              if (bl.find(V) != bl.end()) {
                bl.insert(e);
                ok = true;
                w[idx[make_pair(e, V)]] = d[e] - d[V];
                break;
              } else {
                re.insert(e);
                ok = true;
                w[idx[make_pair(e, V)]] = d[e] - d[V];
                break;
              }
            }
          }
        }
        if (!ok) {
          cout << -1 << endl;
          return 0;
        }
      }
    }
  }
  string s = "";
  for (int i = (0); i < (n); ++i) s += "1";
  for (auto itr = (bl).begin(); itr != (bl).end(); ++itr) {
    s[*itr] = 'B';
  }
  for (auto itr = (re).begin(); itr != (re).end(); ++itr) {
    s[*itr] = 'W';
  }
  cout << s << endl;
  for (int i = (0); i < (m); ++i) cout << w[i] << endl;
  return 0;
}
