//#pragma comment(linker, "/stack:200000000")
//#pragma GCC optimize("Ofast")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
//#pragma GCC optimize("unroll-loops")
  
#include <iostream>
#include <stdlib.h>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <assert.h>
#include <memory.h>
#include <time.h>
#include <bitset>
 
#define uint unsigned int
#define ll long long
#define ull unsigned long long
#define ld long double
#define rep(i, l, r) for (int i = (l); i < (r); i++)
#define repb(i, r, l) for (int i = (r); i > (l); i--)
#define sz(a) (int)a.size()
#define fi first
#define se second
#define mp(a, b) make_pair(a, b)
#define rank qwertyuio
#define next dfghjk
#define prev fhsgfhjf
#define plus fsghsf
#define minus ytryr
  
using namespace std;
  
inline bool setmin(int &x, int y) { return (y < x) ? x = y, 1 : 0; }
inline bool setmax(int &x, int y) { return (y > x) ? x = y, 1 : 0; }
inline bool setmin(ll &x, ll y) { return (y < x) ? x = y, 1 : 0; }
inline bool setmax(ll &x, ll y) { return (y > x) ? x = y, 1 : 0; }
  
const int N = 100000;
const int inf = (int)1e9 + 1;
const ll big = (ll)1e18 + 1;
const int P = 239;
const int P1 = 31;
const int P2 = 57;
const int MOD = (int)1e9 + 7;
const int MOD1 = (int)1e9 + 9;
const int MOD2 = 998244353;
const ld eps = 1e-12;
const double pi = atan2(0, -1);
const int ABC = 26;

int D[N];
pair<int, int> E[N];
vector<pair<int, int> > g[N];

int p[N], sz[N];
int col[N];
vector<int> comp[N];
bool ok[N];
int w[200000];
bool was[N];

void init(int n) {
    rep(i, 0, n) {
        p[i] = i;
        sz[i] = 1;
        col[i] = 0;
        comp[i].push_back(i);
    }
}

int get(int x) {
    if (p[x] != x) {
        p[x] = get(p[x]);
    }
    return p[x];
}

void uni(int x, int y) {
    bool need = (col[x] == col[y]);
    x = get(x); y = get(y);
    assert(x != y);
    if (sz[x] >= sz[y]) {
        p[y] = x;
        sz[x] += sz[y];
        for (int v : comp[y]) {
            if (need) {
                col[v] ^= 1;
            }
            comp[x].push_back(v);
        }
        comp[y].clear();
    } else {
        p[x] = y;
        sz[y] += sz[x];
        for (int v : comp[x]) {
            if (need) {
                col[v] ^= 1;
            }
            comp[y].push_back(v);
        }
        comp[x].clear();
    }
}

int main()
{
    //freopen("a.in", "r", stdin);
    //freopen("a.out", "w", stdout);
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.precision(20);
    cout << fixed;
    //ll TL = 10.95 * CLOCKS_PER_SEC;
    //clock_t time = clock();
    int n, m;
    cin >> n >> m;
    rep(i, 0, n) {
        cin >> D[i];
        E[i] = {D[i], i};
    }
    sort(E, E + n);
    rep(i, 0, m) {
        int u, v;
        cin >> u >> v; u--; v--;
        g[u].push_back({v, i});
        g[v].push_back({u, i});
    }
    rep(i, 0, m) {
        w[i] = 1000000000;
    }
    init(n);
    rep(i, 0, n) {
        int u = E[i].se;
        was[u] = true;
        for (auto it : g[u]) {
            int v = it.fi;
            if (!was[v]) {
                continue;
            }
            if (D[v] == D[u]) {
                if (!ok[u] || !ok[v]) {
                    w[it.se] = D[u];
                    ok[u] = ok[v] = true;
                    uni(u, v);
                }
            } else {
                if (!ok[u]) {
                    w[it.se] = D[u];
                    ok[u] = true;
                    uni(u, v);
                }
            }
        }
    }
    rep(i, 0, n) {
        if (!ok[i]) {
            cout << -1 << "\n";
            return 0;
        }
    }
    rep(i, 0, n) {
        cout << (col[i] == 0 ? 'W' : 'B');
    }
    cout << "\n";
    rep(i, 0, m) {
        cout << w[i] << "\n";
    }
    return 0;
}

Measured memory_bytes: 19840