#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;
int h[maxn], mem;
struct {
  int v, next, id;
} e[maxn << 2];
void add(int u, int v, int id) {
  e[mem].v = v;
  e[mem].id = id;
  e[mem].next = h[u];
  h[u] = mem++;
}
int vis[maxn], d[maxn];
pair<int, int> dd[maxn];
int ans[maxn << 1];
int main() {
  memset(h, -1, sizeof(h));
  memset(vis, -1, sizeof(vis));
  memset(ans, -1, sizeof(ans));
  mem = 0;
  int n, m;
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &d[i]);
    dd[i].first = d[i];
    dd[i].second = i;
  }
  sort(dd + 1, dd + 1 + n);
  for (int i = 0; i < m; i++) {
    int u, v;
    scanf("%d%d", &u, &v);
    add(u, v, i);
    add(v, u, i);
  }
  for (int i = 1; i <= n; i++) {
    if (vis[dd[i].second] + 1) continue;
    if (dd[i].first == dd[1].first) {
      for (int j = h[dd[i].second]; j + 1; j = e[j].next) {
        int v = e[j].v;
        if (d[v] == dd[1].first) {
          if (vis[v] == -1) {
            vis[v] = 0;
          }
          vis[dd[i].second] = vis[v] ^ 1;
          ans[e[j].id] = dd[i].first;
          break;
        }
      }
    } else {
      for (int j = h[dd[i].second]; j + 1; j = e[j].next) {
        int v = e[j].v;
        if (vis[v] + 1 && d[v] < d[dd[i].second]) {
          vis[dd[i].second] = vis[v];
          ans[e[j].id] = d[dd[i].second] - d[v];
          break;
        }
      }
      if (vis[dd[i].second] == -1) {
        for (int j = h[dd[i].second]; j + 1; j = e[j].next) {
          int v = e[j].v;
          if (vis[v] + 1 && d[v] == d[dd[i].second]) {
            vis[dd[i].second] = vis[v] ^ 1;
            ans[e[j].id] = d[dd[i].second];
            break;
          }
        }
      }
    }
    if (vis[dd[i].second] == -1) {
      printf("-1\n");
      return 0;
    }
  }
  for (int i = 1; i <= n; i++) {
    putchar(vis[i] ? 'B' : 'W');
  }
  putchar('\n');
  for (int i = 0; i < m; i++) {
    printf("%d\n", ans[i] == -1 ? 1000000000 : ans[i]);
  }
  return 0;
}
