#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
struct bian
{
	int nxt,to;
}bi[400040];
int n,m,d[100010],head[100010],num,w[200020],p[100010];
char col[100010];
inline void add(int from,int to)
{
	bi[++num]=bian{head[from],to};
	head[from]=num;
}
int cnt;
bool vis[100010];
void dfs(int x,int c)
{
	++cnt;vis[x]=1;col[x]=c?'W':'B';
	for(int i=head[x],y;i;i=bi[i].nxt)
	{
		y=bi[i].to;
		if(d[y]==d[x]&&!vis[y])w[(i+1)/2]=d[x],dfs(y,c^1);
	}
}
void work(int x)
{
	cnt=0;
	dfs(x,0);
	if(cnt==1)col[x]=0;
}
int main()
{
	scanf("%d %d",&n,&m);
	for(int i=1;i<=n;++i)scanf("%d",&d[i]),p[i]=i;
	for(int i=1,x,y;i<=m;++i)
	{
		scanf("%d %d",&x,&y);
		add(x,y),add(y,x);
	}
	sort(p+1,p+1+n,[&](int x,int y){return d[x]<d[y];});
	for(int i=1,r;i<=n;i=r+1)
	{
		r=i;
		while(r<n&&d[p[r+1]]==d[p[i]])++r;
		for(int j=i;j<=r;++j)if(!col[p[j]])work(p[j]);
		for(int j=i;j<=r;++j)
			if(!col[p[j]])
			{
				bool flag=0;
				for(int k=head[p[j]];k;k=bi[k].nxt)
					if(d[bi[k].to]<d[p[j]])
					{
						w[(k+1)/2]=d[p[j]]-d[bi[k].to];
						col[p[j]]=col[bi[k].to];
						flag=1;
						break;
					}
				if(!flag)
				{
					printf("-1");
					exit(0);
				}
			}
	}
	for(int i=1;i<=n;++i)putchar(col[i]);putchar('\n');
	for(int i=1;i<=m;++i)
		if(w[i])printf("%d\n",w[i]);
		else printf("%d\n",1000000000);
	return 0;
}
