//
// Created by munaiyi on 2020/3/19.
//

#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;
        int uv = -1, id = 0;
        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]) {
                uv = v;
                id = e[j].id;
                break;
            } else if (d[v] == d[dd[i].second]) {
                uv = v;
                id = e[j].id;
            }
        }
        if (uv == -1) {
            printf("-1\n");
            return 0;
        }
        if (d[uv] < d[dd[i].second]) {
            vis[dd[i].second] = vis[uv];
            ans[id] = d[dd[i].second] - d[uv];
        } else {
            if (vis[uv] == -1) {
                vis[uv] = 0;
            }
            vis[dd[i].second] = vis[uv] ^ 1;
            ans[id] = d[dd[i].second];
        }
    }
    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;
}