#include <bits/stdc++.h>
 
using namespace std;

int n, m;
int D[100005];
int U[200005];
int V[200005];
vector<int> edge[100005];
int col[100005];

int dfs(int x) {
    int ret = 1;
    for (int i : edge[x]) {
        if (col[i]) continue;
        if (D[i] != D[x]) continue;
        col[i] = 3 - col[x];
        ret += dfs(i);
    }
    return ret;
}

int main() {
    ios_base::sync_with_stdio(0); cin.tie(0);
    cin >> n >> m;
    for (int i = 1; i <= n; i++) {
        cin >> D[i];
    }
    for (int i = 1; i <= m; i++) {
        cin >> U[i] >> V[i];
        edge[U[i]].push_back(V[i]);
        edge[V[i]].push_back(U[i]);
    }
    int prv = 2;
    for (int x = 1; x <= n; x++) {
        if (col[x]) continue;
        bool en = false;
        for (int i : edge[x]) {
            if (D[i] < D[x]) {
                en = true;
                break;
            }
        }
        prv = 3 - prv;
        col[x] = prv;
        if (dfs(x) == 1 && !en) {
            cout << -1;
            return 0;
        }
    }
    for (int i = 1; i <= n; i++) {
        cout << "BW"[col[i] - 1];
    }
    cout << '\n';
    for (int i = 1; i <= m; i++) {
        int c1 = col[U[i]], c2 = col[V[i]];
        if (c1 == c2) {
            cout << max(1, abs(D[U[i]] - D[V[i]])) << '\n';
        }
        else {
            cout << max(D[U[i]], D[V[i]]) << '\n';
        }
    }
}