#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using P = pair<int, int>;

const int MAXC = 1e9;

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n, m;
    cin >> n >> m;
    vector<int> ids(n);
    iota(ids.begin(), ids.end(), 0);

    vector<ll> d(n);
    for (int i = 0; i < n; ++i) {
        cin >> d[i];
    }
    sort(ids.begin(), ids.end(), [&](int i1, int i2){
        return d[i1] < d[i2];
    });

    if (d[ids[0]] != d[ids[1]]) {
        cout << -1 << "\n";
        return 0;
    }

    vector< vector<int> > g(n);
    vector<int> u(m), v(m);
    map<P, int> memo;
    for (int i = 0; i < m; ++i) {
        cin >> u[i] >> v[i];
        u[i]--; v[i]--;
        if (u[i] > v[i]) swap(u[i], v[i]);
        g[u[i]].push_back(v[i]);
        g[v[i]].push_back(u[i]);
    }

    vector<int> c(n, -1);
    for (int ii = 0; ii < n; ++ii) {
        int i = ids[ii];
        if (c[i] != -1) continue;

        for (int j : g[i]) {
            if (c[j] == -1) continue;
            c[i] = 1 - c[j];
            memo[minmax(i, j)] = d[i];
            break;
        }

        if (c[i] == -1) {
            for (int j : g[i]) {
                if (d[i] != d[j]) continue;
                c[i] = 0;
                c[j] = 1;
                memo[minmax(i, j)] = d[i];
                break;
            }
        }

        if (c[i] == -1) {
            cout << -1 << "\n";
            return 0;
        }
    }

    for (int i = 0; i < n; ++i) {
        cout << "BW"[c[i]];
    }
    cout << "\n";

    for (int i = 0; i < m; ++i) {
        P tar(u[i], v[i]);
        cout << (memo.find(tar) != memo.end() ? memo[tar] : MAXC) << "\n";
    }
    return 0;
}