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

int main() {
	long long N, M;
	vector<long long> D;
	vector<pair<long long, long long> > V;
	vector<long long> ANS;
	vector<bool> check;
	vector<long long> C;
	cin >> N >> M;
	D.resize(N), V.resize(M), C.assign(N, 2), ANS.resize(M), check.assign(N, false);
	for (int i = 0; i < N; i++) cin >> D[i];
	for (int i = 0; i < M; i++) {
		cin >> V[i].first >> V[i].second;
		V[i].first--, V[i].second--;
		if (D[V[i].first] == D[V[i].second]) {
			check[V[i].first] = check[V[i].second] = true;
			ANS[i] = D[V[i].first];
			if (C[V[i].first] == 2) {
				if (C[V[i].second] == 2) C[V[i].first] = 0, C[V[i].second] = 1;
				else C[V[i].first] = 1 - C[V[i].second];
			}
			else {
				if (C[V[i].second] == 2) C[V[i].second] = 1 - C[V[i].first];
			}
		}
		else {
			if (D[V[i].first] > D[V[i].second]) check[V[i].first] = true;
			else check[V[i].second] = true;
		}
	}
	for (int i = 0; i < N; i++) {
		if (!check[i]) {
			cout << -1 << endl;
			return 0;
		}
	}
	for (int i = 0; i < N; i++) {
		if (C[i] == 2) cout << 'W', C[i] = 0;
		else cout << (C[i] == 0 ? 'W' : 'B');
	}
	cout << endl;
	for (int i = 0; i < M; i++) {
		long long S = V[i].first, T = V[i].second;
		if (C[S] == C[T]) {
			if (D[S] == D[T]) ANS[i] = D[S];
			else ANS[i] = abs(D[S] - D[T]);
		}
		else ANS[i] = max(D[S], D[T]);
	}
	for (int i = 0; i < M; i++) cout << ANS[i] << endl;
}