#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#define ll long long
#define rep2(i,a,b) for(ll i=a;i<=b;++i)
#define rep(i,n) for(ll i=0;i<n;i++)
#define rep3(i,a,b) for(ll i=a;i>=b;i--)
#define pii pair<int,int>
#define pll pair<ll,ll>
#define pq priority_queue<int>
#define pb push_back
#define eb emplace_back
#define vec vector<int>
#define vecll vector<ll>
#define vecpii vector<pii>
#define vec2 vector<vec>
#define vecll2 vector<vecll>
#define vec3 vector<vec2d>
#define vecll3 vector<vecll2d>
#define fi first
#define se second
#define endl "\n"
#define all(c) begin(c),end(c)
#define ios ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
#define lb(c,x) distance(c.begin(),lower_bound(all(c),x))
#define ub(c,x) distance(c.begin(),upper_bound(all(c),x))
using namespace std;
int in() {int x;cin>>x;return x;}
ll lin() {ll x;cin>>x;return x;}
template<class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}
template<class T> inline void print(vector<T> v){for(T e:v)cout<<e<<" ";cout<<endl;}
template<class T> inline void print(pair<T,T> p){cout<<"("<<p.first<<","<<p.second<<") ";}
template<class T> inline void print(vector<pair<T,T>> v){for(auto e:v)print(e); cout<<endl;}
void print(vector<vec> v){for(auto e:v){for(auto ee:e)cout<<ee<<" ";cout<<endl;}}
void print(map<int,int> mp){for(auto e:mp)cout<<e.first<<" "<<e.second<<endl;cout<<endl;}

const ll MOD=1e9+7;
const int N=1100000;
 
template <ll Modulus> class modint {
  using u64 = ll;
 
public:
  u64 a;
 
  constexpr modint(const u64 x = 0) noexcept : a(((x % Modulus) + Modulus)%Modulus) {}
  constexpr u64 &value() noexcept { return a; }
  constexpr const u64 &value() const noexcept { return a; }
  constexpr modint operator+(const modint rhs) const noexcept {
    return modint(*this) += rhs;
  }
  constexpr modint operator-(const modint rhs) const noexcept {
    return modint(*this) -= rhs;
  }
  constexpr modint operator*(const modint rhs) const noexcept {
    return modint(*this) *= rhs;
  }
  constexpr modint operator/(const modint rhs) const noexcept {
    return modint(*this) /= rhs;
  }
  constexpr modint &operator+=(const modint rhs) noexcept {
    a += rhs.a;
    if (a >= Modulus) {
      a -= Modulus;
    }
    return *this;
  }
  constexpr modint &operator-=(const modint rhs) noexcept {
    if (a < rhs.a) {
      a += Modulus;
    }
    a -= rhs.a;
    return *this;
  }
  constexpr modint &operator*=(const modint rhs) noexcept {
    a = a * rhs.a % Modulus;
    return *this;
  }
  constexpr modint &operator/=(modint rhs) noexcept {
    u64 exp = Modulus - 2;
    while (exp) {
      if (exp % 2) {
        *this *= rhs;
      }
      rhs *= rhs;
      exp /= 2;
    }
    return *this;
  }
};
 
#define mint modint<MOD>
mint inv[N],comb[N],prd[N],invprd[N];
void calc_inv(){
    inv[1]=1;
    rep2(i,2,N-1){
        inv[i]=inv[MOD%i]*(-MOD/i);
    }
    return;
}
void calc_product(){
    prd[0]=prd[1]=1;
    invprd[0]=invprd[1]=1;
    rep2(i,2,N-1){
        prd[i]=prd[i-1]*i;
        invprd[i]=inv[i]*invprd[i-1];
    }
    return ;
}
mint cmb(int a,int b){
    if(a<b)return 0;
    if(a<0||b<0)return 0;
    return {prd[a]*invprd[b]*invprd[a-b]};
}
mint modpow(mint x,ll n){
    if(n==0) return 1;
    mint res=modpow(x*x,n/2);
    if(n&1) res=res*x;
    return res;
}
void calc(){calc_inv();calc_product();}
using vmint = vector<mint> ;

vector<vec> g;
vec a;
vec col,ans;
bool dfs(int x,int p,int cl){
    bool flag=false;
    col[x]=cl;
    for(auto e:g[x]){
        if(a[e] == a[x] && !col[e]){
            dfs(e,x, (cl==1 ? 2: 1));
            flag=true;
        }
    }
    return flag;
}

main(){
    map<pii,int> mp;
    int n=in(),m=in();
    col.resize(n);ans.resize(m);
    g.resize(n);
    a.resize(n);
    int f=INT_MAX;
    rep(i,n)a[i]=in();
    rep(i,m){
        int x=in()-1,y=in()-1;
        g[x].pb(y);
        g[y].pb(x);
        ans[i]=max(a[x],a[y]);
    }
    rep(i,n){
      bool flag=0;
      for(auto e:g[i]){
        if(a[e]<=a[i])flag=1;
      }
      if(!flag){
        cout<<-1<<endl;return 0;
      }
    }

    vec id;
    rep(i,n)id.pb(i);
    sort(all(id),[&](int x,int y){return a[x]<a[y];});
    for(auto i:id){
        bool flag=0;
        for(auto e:g[i]){
            if(col[e]){
                if(!col[i]) col[i]= (col[e]==1? 2:1) , flag=1;
            }
        }
        if(!col[i])dfs(i,-1,1);
    }
    rep(i,n)cout<<(col[i] == 1 ? "B":"W");
    cout<<endl;
    rep(i,m){
      cout<<ans[i]<<endl;
    }
}


Measured memory_bytes: 10872