#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
constexpr ll inf = 1000000000;
constexpr ll mod = 1000000007;
constexpr long double eps = 1e-15;
template <typename T1, typename T2>
ostream& operator<<(ostream& os, pair<T1, T2> p) {
  os << to_string(p.first) << " " << to_string(p.second);
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, vector<T>& v) {
  for (ll i = 0; i < (int)v.size(); i++) {
    if (i) os << " ";
    os << to_string(v[i]);
  }
  return os;
}
struct modint {
  ll n;

 public:
  modint(const ll n = 0) : n((n % mod + mod) % mod) {}
  static modint pow(modint a, int m) {
    modint r = 1;
    while (m > 0) {
      if (m & 1) {
        r *= a;
      }
      a = (a * a);
      m /= 2;
    }
    return r;
  }
  modint& operator++() {
    *this += 1;
    return *this;
  }
  modint& operator--() {
    *this -= 1;
    return *this;
  }
  modint operator++(int) {
    modint ret = *this;
    *this += 1;
    return ret;
  }
  modint operator--(int) {
    modint ret = *this;
    *this -= 1;
    return ret;
  }
  modint operator~() const { return (this->pow(n, mod - 2)); }
  friend bool operator==(const modint& lhs, const modint& rhs) {
    return lhs.n == rhs.n;
  }
  friend bool operator<(const modint& lhs, const modint& rhs) {
    return lhs.n < rhs.n;
  }
  friend bool operator>(const modint& lhs, const modint& rhs) {
    return lhs.n > rhs.n;
  }
  friend modint& operator+=(modint& lhs, const modint& rhs) {
    lhs.n += rhs.n;
    if (lhs.n >= mod) lhs.n -= mod;
    return lhs;
  }
  friend modint& operator-=(modint& lhs, const modint& rhs) {
    lhs.n -= rhs.n;
    if (lhs.n < 0) lhs.n += mod;
    return lhs;
  }
  friend modint& operator*=(modint& lhs, const modint& rhs) {
    lhs.n = (lhs.n * rhs.n) % mod;
    return lhs;
  }
  friend modint& operator/=(modint& lhs, const modint& rhs) {
    lhs.n = (lhs.n * (~rhs).n) % mod;
    return lhs;
  }
  friend modint operator+(const modint& lhs, const modint& rhs) {
    return modint(lhs.n + rhs.n);
  }
  friend modint operator-(const modint& lhs, const modint& rhs) {
    return modint(lhs.n - rhs.n);
  }
  friend modint operator*(const modint& lhs, const modint& rhs) {
    return modint(lhs.n * rhs.n);
  }
  friend modint operator/(const modint& lhs, const modint& rhs) {
    return modint(lhs.n * (~rhs).n);
  }
};
istream& operator>>(istream& is, modint m) {
  is >> m.n;
  return is;
}
ostream& operator<<(ostream& os, modint m) {
  os << m.n;
  return os;
}
long long extgcd(long long a, long long b, long long& x, long long& y) {
  long long d = a;
  if (b != 0) {
    d = extgcd(b, a % b, y, x);
    y -= (a / b) * x;
  } else {
    x = 1;
    y = 0;
  }
  return d;
}
long long mod_inverse(long long a, long long m) {
  long long x, y;
  if (extgcd(a, m, x, y) == 1)
    return (m + x % m) % m;
  else
    return -1;
}
vector<long long> fact(3030303 + 1, inf);
long long mod_fact(long long n, long long& e) {
  if (fact[0] == inf) {
    fact[0] = 1;
    if (3030303 != 0) fact[1] = 1;
    for (ll i = 2; i <= 3030303; ++i) {
      fact[i] = (fact[i - 1] * i) % mod;
    }
  }
  e = 0;
  if (n == 0) return 1;
  long long res = mod_fact(n / mod, e);
  e += n / mod;
  if ((n / mod) % 2 != 0) return (res * (mod - fact[n % mod])) % mod;
  return (res * fact[n % mod]) % mod;
}
long long mod_comb(long long n, long long k) {
  if (n < 0 || k < 0 || n < k) return 0;
  long long e1, e2, e3;
  long long a1 = mod_fact(n, e1), a2 = mod_fact(k, e2),
            a3 = mod_fact(n - k, e3);
  if (e1 > e2 + e3) return 0;
  return (a1 * mod_inverse((a2 * a3) % mod, mod)) % mod;
}
using mi = modint;
mi mod_pow(mi a, ll n) {
  mi ret = 1;
  mi tmp = a;
  while (n > 0) {
    if (n % 2) ret *= tmp;
    tmp = tmp * tmp;
    n /= 2;
  }
  return ret;
}
ll gcd(ll a, ll b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  vector<int> dd(n);
  vector<pair<int, int>> d(n);
  for (ll i = 0; i < (int)n; i++) {
    cin >> d[i].first;
    d[i].second = i;
    dd[i] = d[i].first;
  }
  sort(d.begin(), d.end());
  vector<int> ans1(n, 0);
  vector<int> ans2(m, -1);
  vector<vector<pair<int, int>>> g(n);
  for (ll i = 0; i < (int)m; i++) {
    int u, v;
    cin >> u >> v;
    u--;
    v--;
    g[u].push_back({v, i});
    g[v].push_back({u, i});
  }
  for (ll i = 0; i < (int)n; i++) {
    int p = d[i].second;
    bool ok = false;
    for (ll j = 0; j < (int)g[p].size(); j++) {
      if (ok) {
        ans2[g[p][j].second] = inf;
        continue;
      }
      if (dd[p] == dd[g[p][j].first]) {
        ans2[g[p][j].second] = dd[p];
        ans1[p] = !ans1[g[p][j].first];
        ok = true;
      } else if (dd[p] > dd[g[p][j].first]) {
        ans2[g[p][j].second] = dd[p] - dd[g[p][j].first];
        ans1[p] = ans1[g[p][j].first];
        ok = true;
      } else {
        ans2[g[p][j].second] = inf;
      }
    }
    if (!ok) {
      cout << -1 << endl;
      return 0;
    }
  }
  string ans3 = "";
  for (ll i = 0; i < (int)n; i++) {
    if (ans1[i])
      ans3 += 'B';
    else
      ans3 += 'W';
  }
  cout << ans3 << endl;
  for (ll i = 0; i < (int)m; i++) {
    cout << ans2[i] << endl;
  }
  return 0;
}
