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

typedef long long ll;
const int maxn = 200005, maxm = 200005;

inline int gi()
{
    char c = getchar();
    while (c < '0' || c > '9') c = getchar();
    int sum = 0;
    while ('0' <= c && c <= '9') sum = sum * 10 + c - 48, c = getchar();
    return sum;
}

#define pb push_back

int n, m, d[maxn];

struct edge
{
    int to, next,vis;
} e[maxm * 2];
int h[maxn], tot = 1;
int w[maxm], col[maxn], fa[maxn], fe[maxn];
vector<int> vec[maxn];

inline void add(int u, int v)
{
    e[++tot] = (edge) {v, h[u]}; h[u] = tot;
    e[++tot] = (edge) {u, h[v]}; h[v] = tot;
}

void dfs(int u)
{
    for (int i=h[u];i;i=e[i].next) {
	if(!e[i].vis)continue;
	int v=e[i].to;
	if (col[v] == -1) col[v] = col[u] ^ 1, dfs(v);
    }
}

int main()
{
    n = gi(); m = gi();
    for (int i = 1; i <= n; ++i) d[i] = gi();
    for (int i = 1; i <= m; ++i) add(gi(), gi());
    fill(w + 1, w + m + 1, 1e9);

    d[0] = 2e9;
    for (int u = 1; u <= n; ++u) {
	int p = h[u] >> 1, mn = e[h[u]].to;
	for (int i = h[u], v; v = e[i].to, i; i = e[i].next)
	    if (d[v] < d[mn] || (d[v] == d[mn] && v < mn)) mn = v, p = i >> 1;
	if (d[mn] > d[u]) return puts("-1"), 0;
	w[p]=d[u];fa[u] = mn;
	if(e[p<<1].to==u)e[p<<1].vis=1;
	else e[p<<1|1].vis=1;
    }
	
    fill(col + 1, col + n + 1, -1);
    for (int i = 1; i <= n; ++i) if (fa[fa[i]] == i && col[i] == -1) col[i] = 0, dfs(i); 
    for (int i = 1; i <= n; ++i) putchar(col[i] == 0 ? 'B' : 'W');
    puts("");
    for (int i = 1; i <= m; ++i) printf("%d\n", w[i]);
	
    return 0;
}
