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

typedef long long lint;
typedef long double louble;

template<typename T1,typename T2> inline T1 max(T1 a,T2 b){return a<b?b:a;}
template<typename T1,typename T2> inline T1 min(T1 a,T2 b){return a<b?a:b;}

const char lf = '\n';

namespace ae86
{
	const int bufl = 1 << 15;

	char buf[bufl],*s=buf,*t=buf;

	inline int fetch()
	{
		if(s==t){t=(s=buf)+fread(buf,1,bufl,stdin);if(s==t)return EOF;}
		return *s++;
	}

	inline int ty()
	{
		int a=0,b=1,c=fetch();
		while(!isdigit(c))b^=c=='-',c=fetch();
		while(isdigit(c))a=a*10+c-48,c=fetch();
		return a;
	}
}
using ae86::ty;

const int _ = 100007 , __ = 200007 , inf = 1e9;
const int _e = __ << 1;

int to[_e],ne[_e],ev[_e],he[_]={0},ecnt=1;
inline void adde(int a,int b,int c){to[++ecnt]=b,ev[ecnt]=c,ne[ecnt]=he[a],he[a]=ecnt;}
inline void addde(int a,int b,int c){adde(a,b,c),adde(b,a,c);}

int n,m,ned[_]={0},ea[__],eb[__],eva[__]={0};
int col[_]={0};

struct cxk
{
	int x,d;
	cxk(int x_=0,int d_=0){x=x_,d=d_;}
	friend bool operator < (cxk a,cxk b){return a.d<b.d;}
};
priority_queue<cxk> dlq;

void failure()
{
	cout<<-1<<lf;
	exit(0);
}

int main()
{
	ios::sync_with_stdio(0),cout.tie(nullptr);

	n=ty(),m=ty();
	for(int i=1;i<=n;i++)ned[i]=ty();
	for(int i=1;i<=m;i++)ea[i]=ty(),eb[i]=ty(),addde(ea[i],eb[i],i);

	int got=0,spc=0;
	for(int i=1;i<=m;i++)
	{
		int a=ea[i],b=eb[i];
		if(ned[a]==ned[b] && !col[a] && !col[b])
		{
			dlq.emplace(a,ned[a]),dlq.emplace(b,ned[b]);
			col[a]=1,col[b]=2,spc+=2;
			eva[i]=ned[a],got=1;
		}
	}

	if(!got)failure();

	while(!dlq.empty())
	{
		int a=dlq.top().x;dlq.pop();
		for(int i=he[a];i;i=ne[i])
		{
			int b=to[i];
			if(col[b] || ned[b]<ned[a])continue;
			col[b]=3-col[a],eva[ev[i]]=ned[b],spc++;
			dlq.emplace(b,ned[b]);
		}
	}

	if(spc<n)failure();
	for(int i=1;i<=n;i++)cout<<"BW"[col[i]-1];
	cout<<lf;
	for(int i=1;i<=m;i++)
	{
		if(!eva[i])eva[i]=inf;
		cout<<eva[i]<<lf;
	}

	return 0;
}