#include <iostream>
#include <map>
#include <set>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>
#include <fstream>
#include <bitset>
#include <queue>
#include <stack>
#include <deque>
#include <complex>
#include <iomanip>
#include <stdio.h>
#include <string.h>
#include <random>
#include <functional>

using std::cin;
using std::cout;
using std::cerr;
using std::endl;

using std::map;
using std::set;
using std::bitset;
using std::vector;
using std::string;
using std::multimap;
using std::multiset;
using std::deque;
using std::queue;
using std::stack;
using std::pair;
using std::iterator;

using std::sort;
using std::stable_sort;
using std::reverse;
using std::max_element;
using std::min_element;
using std::unique;
using std::ios_base;
using std::swap;
using std::fill;

using std::setprecision;
using std::fixed;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef string S;

ll min(ll a, ll b) {return a < b ? a : b;}
ll min(int a, ll b) {return a < b ? a : b;}
ll min(ll a, int b) {return a < b ? a : b;}
ll min(int a, int b) {return a < b ? a : b;}

ll max(ll a, ll b) {return a > b ? a : b;}
ll max(int a, ll b) {return a > b ? a : b;}
ll max(ll a, int b) {return a > b ? a : b;}
ll max(int a, int b) {return a > b ? a : b;}

namespace MySpace{

};

#define F(i, n) for (int (i) = 0; (i) != (n); (i)++)
#define fi first
#define se second
#define re return
#define all(x) (x).begin(), (x).end()
#define int long long

int n, m;
int d[600000];
int ans[600000];
map<pii, int> id;
vector<int> graph[600000];
int p[600000];
int color[600000];
int mark[600000];
vector<int> go[600000];

signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    cin >> n >> m;
    for (int i = 0; i < n; i++)
    {
        cin >> d[i];
        color[i] = -1;
    }
    for (int i = 0; i < m; i++)
    {
        ans[i] = 1e9;
        int v, u;
        cin >> v >> u, v--, u--;
        id[{v, u}] = i;
        id[{u, v}] = i;
        graph[v].push_back(u);
        graph[u].push_back(v);
    }
    for (int v = 0; v < n; v++)
    {
        pii t = {-1, -1};
        pii x = {-1, -1};
        for (int i = 0; i < graph[v].size(); i++)
        {
            int u = graph[v][i];
            if (d[u] <= d[v])
            {
                if (t == x) t = {d[u], u};
                t = min(t, {d[u], u});
            }
        }
        if (t == x)
        {
            cout << -1;
            return 0;
        }
        p[v] = t.se;
        go[t.se].push_back(v);
    }

    vector<int> q;

    for (int v = 0; v < n; v++)
    {
        if (mark[v] == 0)
        {
            vector<int> path;
            int u = v, t = 0;
            while (1)
            {
                path.push_back(u);
                mark[u] = 1;
                if (p[u] == v)
                {
                    t = 1;
                    break;
                }
                if (mark[p[u]])
                {
                    break;
                }
                if (d[u] != d[p[u]]) break;
                u = p[u];
            }
            if (t)
            {
                for (int i = 0; i < path.size(); i++)
                {
                    color[path[i]] = i % 2;
                    ans[id[{path[i], path[(i + 1) % path.size()]}]] = d[v];
                    q.push_back(path[i]);
                }
            }
        }
    }

    for (int i = 0; i < q.size(); i++)
    {
        int v = q[i];
        for (int j = 0; j < go[v].size(); j++)
        {
            int u = go[v][j];
            if (color[u] == -1)
            {
                if (d[u] > d[v])
                {
                    color[u] = color[v];
                    ans[id[{v, u}]] = d[u] - d[v];
                }
                else
                {
                    color[u] = 1 - color[v];
                    ans[id[{v, u}]] = d[u];
                }
                q.push_back(u);
            }
        }
    }


    F(i, n)
    {
        if (color[i] == 0) cout << "B";
        else cout << "W";
    }
    cout << "\n";
    F(i, m) cout << ans[i] << "\n";
}


Measured memory_bytes: 78712