#include <bits/stdc++.h>
 
#define f first
#define s second
#define pb push_back
#define mp make_pair
 
using namespace std;
 
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;
 
const int N = (int)2e5 + 123, inf = 1e9, mod = 1e9 + 7;
const ll INF = 1e18;
 
vector< int > g[N];
bool ok[N];
int col[N], d[N], ans[N];
int n, m, s[N], t[N];
void f(int v){
	for(auto i : g[v]){
		int to = s[i] ^ t[i] ^ v;
		if(col[to] == -1){
			col[to] = col[v] ^ 1;
			ans[i] = d[v];
			f(to);
		}
	}
}
int main()
{
	scanf("%d%d", &n, &m);
	for(int i = 1; i <= n; i++)
		scanf("%d", &d[i]);
	for(int i = 1; i <= m; i++){
		scanf("%d%d", &s[i], &t[i]);
		if(d[s[i]] == d[t[i]]){
			g[s[i]].pb(i);
			g[t[i]].pb(i);
			ans[i] = d[s[i]];
			ok[s[i]] = ok[t[i]] = 1;
		}else{
			if(d[s[i]] < d[t[i]])	
				ok[t[i]] = 1;
			else
				ok[s[i]] = 1;
		}
	}   	
	for(int i = 1; i <= n; i++)
		if(!ok[i]){
			printf("-1");
			return 0;
		}

	for(int i = 1; i <= n; i++)
		col[i] = -1;

	for(int i = 1; i <= n; i++){
		if(col[i] == -1){
			col[i] = 0;
			f(i);
		}
	}
	for(int i = 1; i <= n; i++)
		if(col[i] == -1)
			col[i] = 0;
	for(int i = 1; i <= m; i++){
		if(d[s[i]] != d[t[i]]){
			if(col[s[i]] == col[t[i]])
				ans[i] = abs(d[s[i]] - d[t[i]]);
			else
				ans[i] = max(d[s[i]], d[t[i]]);
		}
	}
	for(int i = 1; i <= n; i++)
		printf("%c", (col[i] ? 'B' : 'W'));
	printf("\n");
	for(int i = 1; i <= m; i++)
		printf("%d\n", ans[i]);
}
