// ※※※ 解答不能 ※※※
// I9992147氏.
// https://atcoder.jp/contests/keyence2020/submissions/9588989
#include <bits/stdc++.h>
using namespace std;
using vi = vector<int>;
using P = pair<int, int>;
#define repex(i, a, b, c) for(int i = a; i < b; i += c)
#define repx(i, a, b) repex(i, a, b, 1)
#define rep(i, n) repx(i, 0, n)
#define repr(i, a, b) for(int i = a; i >= b; i--)
#define pb push_back
#define a first
#define b second
#define all(x) x.begin(), x.end()
int d[202020], a[202020], b[202020], c[202020];
vi G[202020];

int main(){
    int N, M;
    map<P, int> mp;
    scanf("%d %d", &N, &M);
    repx(i, 1, N + 1) scanf("%d", &d[i]);
    repx(i, 1, M + 1){
        scanf("%d %d", &a[i], &b[i]);
        G[a[i]].pb(b[i]);
        G[b[i]].pb(a[i]);
    }
    vector<P> v;
    repx(i, 1, N + 1) v.pb({d[i], i});
    sort(all(v));
    rep(i, v.size()){
        int u = v[i].b, p = 0;
        if(c[u]) continue;
        for(auto v : G[u]){
            if(d[v] <= d[u]){
                p = v;
                break;
            }
        }
        if(!p){
            puts("-1");
            return 0;
        }
        if(!c[p]) c[p] = 1;
        c[u] = -c[p];
        mp[minmax(u, p)] = d[u];
    }
    repx(i, 1, N + 1) putchar(c[i] == 1 ? 'B' : 'W');
    puts("");
    repx(i, 1, M + 1) printf("%d\n", mp.count(minmax(a[i], b[i])) ? mp[minmax(a[i], b[i])] : 1000000000);
    return 0;
}