#include <bits/stdc++.h>
typedef long long ll;
using namespace std;
const ll mod = 1000000007;
const ll mod998 = 998244353;
const ll intmax = 2147483647;
const ll llmax = 9223372036854775807;
const char sp = ' ';

struct edge {
	int to, id;
};


int N, M, D[100000], dist[200000], uf[100000], col[100000];
vector<edge>e[100000];
vector<pair<int, int>>pp;
vector<int>a[100000];

int par(int x) {
	if (uf[x] == x)return x;
	return uf[x] = par(uf[x]);
}

void uni(int x, int y) {
	x = par(x);
	y = par(y);
	uf[x] = y;
	if (col[y] == -1) {
		col[y] = col[x];
	}
}

void dfs(int x, int from) {
	for (int i : a[x]) {
		if (i == from)continue;
		col[i] = (col[x] ^ 1);
		dfs(i, x);
	}
}

int main() {
	cin >> N >> M;
	for (int i = 0; i < N; i++) {
		cin >> D[i];
		uf[i] = i;
		col[i] = -1;
	}
	for (int i = 0; i < M; i++) {
		int u, v;
		cin >> u >> v;
		u--;
		v--;
		e[u].push_back({ v,i });
		e[v].push_back({ u,i });
	}
	for (int i = 0; i < N; i++) {
		bool f = false;
		for (int j = 0; j < e[i].size(); j++) {
			if (D[e[i][j].to] < D[i]) {
				f = true;
				dist[e[i][j].id] = D[i] - D[e[i][j].to];
				uni(e[i][j].to, i);
				break;
			}
		}
		if (!f) {
			for (int j = 0; j < e[i].size(); j++) {
				if (D[e[i][j].to] == D[i]) {
					f = true;
					dist[e[i][j].id] = D[i];
					pp.push_back({ i,e[i][j].to });
					break;
				}
			}
		}
		if (!f) {
			cout << -1 << endl;
			return 0;
		}
	}
	for (auto at : pp) {
		a[par(at.first)].push_back(par(at.second));
		a[par(at.second)].push_back(par(at.first));
	}
	for (int i = 0; i < N; i++) {
		if (col[par(i)] == -1) {
			col[par(i)] = 1;
			dfs(par(i), -1);
		}
	}
	for (int i = 0; i < N; i++) {
		cout << (col[par(i)] ? 'W' : 'B');
	}
	cout << endl;
	for (int i = 0; i < M; i++) {
		if (!dist[i])dist[i] = 1000000000;
		cout << dist[i] << endl;
	}
}