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

#define FAST ios_base::sync_with_stdio(false); cin.tie(0);
#define pb push_back
#define eb emplace_back
#define ins insert
#define f first
#define s second
#define cbr cerr<<"hi\n"
#define mmst(x, v) memset((x), v, sizeof ((x)))
#define siz(x) ll(x.size())
#define all(x) (x).begin(), (x).end()
#define lbd(x,y) (lower_bound(all(x),y)-x.begin())
#define ubd(x,y) (upper_bound(all(x),y)-x.begin())
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());    //can be used by calling rng() or shuffle(A, A+n, rng)
inline long long rand(long long x, long long y) { return rng() % (y+1-x) + x; } //inclusivesss
string inline to_string(char c) {string s(1,c);return s;} template<typename T> inline T gcd(T a,T b){ return a==0?llabs(b):gcd(b%a,a); }

using ll=long long; 
using ld=long double;
#define FOR(i,s,e) for(ll i=s;i<=ll(e);++i)
#define DEC(i,s,e) for(ll i=s;i>=ll(e);--i)
using pi=pair<ll,ll>; using spi=pair<ll,pi>; using dpi=pair<pi,pi>; 

#define LLINF ((long long)1e18)
#define INF int(1e9+1e6)
#define MAXN (100006)
ll n,m,A[MAXN],colour[MAXN],cost[MAXN];
bitset<MAXN>vis;
vector<pi>v[MAXN];
int main(){
	FAST
	cin>>n>>m;
	FOR(i,1,n)cin>>A[i];
	FOR(i,1,m){
		ll a,b;cin>>a>>b;
		v[a].eb(b,i),v[b].eb(a,i);
	}
	FOR(i,1,n){ll mi=LLINF;for(auto j:v[i])mi=min(mi,A[j.f]);if(mi>A[i]){cout<<"-1\n";return 0;}}
	vector<pi> D;
	FOR(i,1,n)D.eb(A[i],i);
	sort(all(D),greater<>());
	int co=0;
	while(D.size()){
		ll deal=D.back().f;
		vector<int> cur;
		cur.eb(D.back().s),D.pop_back();
		while(D.size() && D.back().f == deal) cur.eb(D.back().s), D.pop_back();
		function<void(ll)>dfs=[&](ll x){
			if(vis[x])return;
			vis[x]=1;
			for(auto i:v[x]) if(cost[i.s]==0) {
				if(colour[i.f]){
					assert(vis[i.f]);
					if(colour[i.f]==colour[x]) cost[i.s]=A[x]-A[i.f];
					else cost[i.s]=A[x];
				}else if(A[i.f] == deal){
					assert(!vis[i.f]);//since colour==0
					colour[i.f]=3-colour[x];
					cost[i.s]=A[x];
					dfs(i.f);
				}
			}
		};
		for(auto&i:cur)if(!vis[i])colour[i]=(co&1)+1,dfs(i);
		++ co;
	}
	FOR(i,1,n)if(colour[i]==1)cout<<'W';else cout<<'B';
	cout<<'\n';
	FOR(i,1,m)cout<<cost[i]<<'\n';
}
