#include <bits/stdc++.h>
using namespace std;
const int maxn = 100005, maxm = 200005;
inline int gi() {
  char c = getchar();
  while (c < '0' || c > '9') c = getchar();
  int sum = 0;
  while ('0' <= c && c <= '9') sum = sum * 10 + c - 48, c = getchar();
  return sum;
}
int n, m, d[maxn];
struct edge {
  int to, next;
} e[maxm * 2];
int h[maxn], tot = 1;
int w[maxn], col[maxn], fa[maxn], fe[maxn];
vector<int> vec[maxn];
inline void add(int u, int v) {
  e[++tot] = (edge){v, h[u]};
  h[u] = tot;
  e[++tot] = (edge){u, h[v]};
  h[v] = tot;
}
void dfs(int u) {
  for (int i = h[u], v; v = e[i].to, i; i = e[i].next)
    if (col[v] == -1) {
      col[v] = col[u] ^ 1;
      w[i >> 1] = d[v];
      dfs(v);
    }
}
int main() {
  n = gi();
  m = gi();
  for (int i = 1; i <= n; ++i) d[i] = gi();
  for (int i = 1; i <= m; ++i) add(gi(), gi());
  fill(w + 1, w + m + 1, 1e9);
  for (int u = 1; u <= n; ++u) {
    int p = h[u] >> 1, mn = e[h[u]].to;
    for (int i = h[u], v; v = e[i].to, i; i = e[i].next)
      if (d[v] < d[mn] || (d[v] == d[mn] && v < mn)) mn = v, p = i >> 1;
    if (d[mn] > d[u]) return puts("-1"), 0;
    fe[u] = p;
    fa[u] = mn;
    vec[mn].push_back(u);
  }
  fill(col + 1, col + n + 1, -1);
  for (int i = 1; i <= n; ++i)
    if (fa[fa[i]] == i && col[i] == -1) col[i] = 0, dfs(i);
  for (int i = 1; i <= n; ++i) putchar(col[i] == 0 ? 'B' : 'W');
  puts("");
  for (int i = 1; i <= m; ++i) printf("%d\n", w[i]);
  return 0;
}
