#include<bits/stdc++.h>
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define pb push_back
using namespace std;
using ll = long long;
using vi = vector<ll>;
using pi = pair<ll, ll>;
const int maxn = 2e5 + 33, mod = 1e9 + 7;
int n, m, d[maxn], col[maxn];
vector<vi> g;
vector<pi> e;
queue<int> q;
map<pi, int> cost;
pi minmax(pi a) {
	if(a.second<a.first) swap(a.second, a.first);
	return a;
}
int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	cin >> n >> m;
	for(int i = 1; i <= n; i++) {
		cin >> d[i];
	}
	g.resize(n+1);
	for(int f, t, i = 0; i < m; i++) {
		cin >> f >> t;
		g[f].pb(t);
		g[t].pb(f);
		e.pb({f, t});
		if(d[f]==d[t]&&(!col[f]||!col[t])) {
			if(!col[f])
				q.push(f);
			if(!col[t])
				q.push(t);
			if(col[t])
				col[f]=col[t]^3;
			else if(col[f])
				col[t]=col[f]^3;
			else
				col[t] = 1, col[f] = 2;
			cost[minmax({f, t})] = d[f];
		}
	}
	while(!q.empty()) {
		int u = q.front();
		q.pop();
		for(auto i : g[u]) {
			if(col[i]||d[i]<d[u]) continue;
			col[i] = col[u];
			cost[minmax({u, i})] = d[i]-d[u];
			q.push(i);
		}
	}
	for(int i = 1; i <= n; i++) if(!col[i]) return cout << -1, 0;
	for(int i = 1; i <= n; i++) cout << ("BW"[col[i]-1]);
	cout << '\n';
	for(auto i : e) {
		if(!cost.count(minmax(i))) cout << "1000000000\n";
		else cout << cost[minmax(i)] << '\n';
	}
}

