#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) {
  int c = a % b;
  while (c != 0) {
    a = b;
    b = c;
    c = a % b;
  }
  return b;
}
long long extGCD(long long a, long long b, long long &x, long long &y) {
  if (b == 0) {
    x = 1;
    y = 0;
    return a;
  }
  long long d = extGCD(b, a % b, y, x);
  y -= a / b * x;
  return d;
}
struct UnionFind {
  vector<int> par;
  UnionFind(int N) : par(N) {
    for (int i = 0; i < N; i++) par[i] = i;
  }
  int root(int x) {
    if (par[x] == x) return x;
    return par[x] = root(par[x]);
  }
  void unite(int x, int y) {
    int rx = root(x);
    int ry = root(y);
    if (rx == ry) return;
    par[rx] = ry;
  }
  bool same(int x, int y) {
    int rx = root(x);
    int ry = root(y);
    return rx == ry;
  }
};
long long M = 1000000007;
vector<long long> fac(1000001);
vector<long long> ifac(1000001);
long long mpow(long long x, long long n) {
  long long ans = 1;
  while (n != 0) {
    if (n & 1) ans = ans * x % M;
    x = x * x % M;
    n = n >> 1;
  }
  return ans;
}
long long mpow2(long long x, long long n, long long mod) {
  long long ans = 1;
  while (n != 0) {
    if (n & 1) ans = ans * x % mod;
    x = x * x % mod;
    n = n >> 1;
  }
  return ans;
}
void setcomb() {
  fac[0] = 1;
  ifac[0] = 1;
  for (long long i = 0; i < 1000000; i++) {
    fac[i + 1] = fac[i] * (i + 1) % M;
  }
  ifac[1000000] = mpow(fac[1000000], M - 2);
  for (long long i = 1000000; i > 0; i--) {
    ifac[i - 1] = ifac[i] * i % M;
  }
}
long long comb(long long a, long long b) {
  if (a == 0 && b == 0) return 1;
  if (a < b || a < 0) return 0;
  long long tmp = ifac[a - b] * ifac[b] % M;
  return tmp * fac[a] % M;
}
long long perm(long long a, long long b) {
  if (a == 0 && b == 0) return 1;
  if (a < b || a < 0) return 0;
  return fac[a] * ifac[a - b] % M;
}
long long modinv(long long a) {
  long long b = M, u = 1, v = 0;
  while (b) {
    long long t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= M;
  if (u < 0) u += M;
  return u;
}
vector<vector<long long>> mul(vector<vector<long long>> a,
                              vector<vector<long long>> b, int n) {
  int i, j, k, t;
  vector<vector<long long>> c(n);
  for (i = 0; i < n; i++) {
    for (j = 0; j < n; j++) {
      t = 0;
      for (k = 0; k < n; k++) t = (t + a[i][k] * b[k][j] % M) % M;
      c[i].push_back(t);
    }
  }
  return c;
}
template <typename Monoid>
struct SegmentTree {
  using F = function<Monoid(Monoid, Monoid)>;
  int sz;
  vector<Monoid> seg;
  const F f;
  const Monoid M1;
  SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1) {
    sz = 1;
    while (sz < n) sz <<= 1;
    seg.assign(2 * sz, M1);
  }
  void set(int k, const Monoid &x) { seg[k + sz] = x; }
  void build() {
    for (int k = sz - 1; k > 0; k--) {
      seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
    }
  }
  void update(int k, const Monoid &x) {
    k += sz;
    seg[k] = x;
    while (k >>= 1) {
      seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
    }
  }
  Monoid query(int a, int b) {
    Monoid L = M1, R = M1;
    for (a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if (a & 1) L = f(L, seg[a++]);
      if (b & 1) R = f(seg[--b], R);
    }
    return f(L, R);
  }
  Monoid operator[](const int &k) const { return seg[k + sz]; }
  template <typename C>
  int find_subtree(int a, const C &check, Monoid &M, bool type) {
    while (a < sz) {
      Monoid nxt = type ? f(seg[2 * a + type], M) : f(M, seg[2 * a + type]);
      if (check(nxt))
        a = 2 * a + type;
      else
        M = nxt, a = 2 * a + 1 - type;
    }
    return a - sz;
  }
  template <typename C>
  int find_first(int a, const C &check) {
    Monoid L = M1;
    if (a <= 0) {
      if (check(f(L, seg[1]))) return find_subtree(1, check, L, false);
      return -1;
    }
    int b = sz;
    for (a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if (a & 1) {
        Monoid nxt = f(L, seg[a]);
        if (check(nxt)) return find_subtree(a, check, L, false);
        L = nxt;
        ++a;
      }
    }
    return -1;
  }
  template <typename C>
  int find_last(int b, const C &check) {
    Monoid R = M1;
    if (b >= sz) {
      if (check(f(seg[1], R))) return find_subtree(1, check, R, true);
      return -1;
    }
    int a = sz;
    for (b += sz; a < b; a >>= 1, b >>= 1) {
      if (b & 1) {
        Monoid nxt = f(seg[--b], R);
        if (check(nxt)) return find_subtree(b, check, R, true);
        R = nxt;
      }
    }
    return -1;
  }
};
template <unsigned mod>
struct RollingHash {
  vector<unsigned> hashed, power;
  inline unsigned mul(unsigned a, unsigned b) const {
    unsigned long long x = (unsigned long long)a * b;
    unsigned xh = (unsigned)(x >> 32), xl = (unsigned)x, d, m;
    asm("divl %4; \n\t" : "=a"(d), "=d"(m) : "d"(xh), "a"(xl), "r"(mod));
    return m;
  }
  RollingHash(const string &s, unsigned base = 10007) {
    int sz = (int)s.size();
    hashed.assign(sz + 1, 0);
    power.assign(sz + 1, 0);
    power[0] = 1;
    for (int i = 0; i < sz; i++) {
      power[i + 1] = mul(power[i], base);
      hashed[i + 1] = mul(hashed[i], base) + s[i];
      if (hashed[i + 1] >= mod) hashed[i + 1] -= mod;
    }
  }
  unsigned get(int l, int r) const {
    unsigned ret = hashed[r] + mod - mul(hashed[l], power[r - l]);
    if (ret >= mod) ret -= mod;
    return ret;
  }
  unsigned connect(unsigned h1, int h2, int h2len) const {
    unsigned ret = mul(h1, power[h2len]) + h2;
    if (ret >= mod) ret -= mod;
    return ret;
  }
  int LCP(const RollingHash<mod> &b, int l1, int r1, int l2, int r2) {
    int len = min(r1 - l1, r2 - l2);
    int low = -1, high = len + 1;
    while (high - low > 1) {
      int mid = (low + high) / 2;
      if (get(l1, l1 + mid) == b.get(l2, l2 + mid))
        low = mid;
      else
        high = mid;
    }
    return (low);
  }
};
using RH = RollingHash<1000000007>;
int main() {
  long long n, m, i, d[111111], u[222222], v[222222], w[222222], used[111111],
      mi[111111], check[111111], c[111111];
  cin >> n >> m;
  for (i = 0; i < n; i++) cin >> d[i];
  vector<pair<long long, long long>> li[111111];
  for (i = 0; i < m; i++) {
    cin >> u[i] >> v[i];
    u[i]--;
    v[i]--;
    li[u[i]].push_back(make_pair(v[i], i));
    li[v[i]].push_back(make_pair(u[i], i));
  }
  for (i = 0; i < n; i++) used[i] = 0;
  priority_queue<
      pair<pair<long long, long long>, pair<long long, long long>>,
      vector<pair<pair<long long, long long>, pair<long long, long long>>>,
      greater<pair<pair<long long, long long>, pair<long long, long long>>>>
      que;
  for (i = 0; i < n; i++) {
    mi[i] = 1e18;
    for (auto &e : li[i]) {
      mi[i] = min(mi[i], d[e.first]);
    }
    if (mi[i] == d[i])
      check[i] = 1;
    else if (mi[i] > d[i]) {
      cout << "-1" << endl;
      return 0;
    } else
      check[i] = 0;
  }
  for (i = 0; i < n; i++) {
    long long flag = 1;
    if (check[i] == 0) flag = 0;
    for (auto &e : li[i]) {
      if (check[e.first] == 0 && d[e.first] <= d[i]) flag = 0;
    }
    if (flag) {
      check[i] = 0;
      c[i] = 0;
      used[i]++;
      for (auto &e : li[i]) {
        if (used[e.first] == 0) {
          que.push(make_pair(make_pair(d[e.first], e.first),
                             make_pair(e.second, 1)));
        }
      }
    }
  }
  while (!que.empty()) {
    pair<pair<long long, long long>, pair<long long, long long>> a = que.top();
    long long nd = a.first.first, now = a.first.second, ne = a.second.first,
              nc = a.second.second;
    que.pop();
    if (used[now] == 0) {
      used[now]++;
      w[ne] = nd;
      c[now] = nc;
      for (auto &e : li[now]) {
        if (used[e.first] == 0)
          que.push(make_pair(make_pair(d[e.first], e.first),
                             make_pair(e.second, nc ^ 1)));
      }
    } else
      w[ne] = 1000000000;
  }
  for (i = 0; i < n; i++) {
    if (c[i] == 0)
      cout << "B";
    else
      cout << "W";
  }
  cout << endl;
  for (i = 0; i < m; i++) cout << w[i] << endl;
}
