#include<bits/stdc++.h>
using namespace std;
const int N = 5e5 + 5;
int n, m, tot, c[N], a[N], head[N], nex[N], to[N], id[N];
void add(int u, int v, int i) {to[++tot] = v; nex[tot] = head[u]; head[u] = tot; id[tot] = i;}
bool vis[N];
int dis[N];
int ans;
struct node
{
	int u, x;
	node(int u = 0, int x = 0): u(u), x(x) {}
	bool operator<(const node&o)const
	{
		return x < o.x;
	}
};
priority_queue<node>q;
int main()
{
	scanf("%d%d", &n, &m);
	for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
	bool flag = false;
	for (int i = 1; i <= m; i++)
	{
		int u, v; scanf("%d%d", &u, &v);
		add(u, v, i); add(v, u, i);
		if (a[u] == a[v] && !vis[u] && !vis[v])
		{
			q.push(node(u, a[u]));
			q.push(node(v, a[v]));
			c[u] = 1; c[v] = 0;
			vis[u] = vis[v] = true;
			ans += 2;
			dis[i] = a[u];
			flag = true;
		}
	}
	if (!flag) {printf("-1\n"); return 0;}
	while (!q.empty())
	{
		node now = q.top(); q.pop();
		for (int i = head[now.u]; i; i = nex[i])
		{
			int v = to[i]; if (vis[v] || a[v] < now.x) continue;
			vis[v] = true; ans++;
			dis[id[i]] = a[v];
			c[v] = c[now.u] ^ 1;
			q.push(node(v, a[v]));
		}
	}
	if (ans != n) {printf("-1\n"); return 0;}
	for (int i = 1; i <= n; i++)
		putchar(c[i] ? 'B' : 'W');
	putchar('\n');
	for (int i = 1; i <= m; i++)
		printf("%d\n", !dis[i] ? 1000000000 : dis[i]);
}

