//#pragma GCC optimize("Ofast")
//#pragma GCC target("avx")
//#undef LOCAL




#include <algorithm>

#include <array>

#include <bitset>

#include <cassert>

#include <complex>

#include <cstdio>

#include <cstring>

#include <iostream>

#include <map>

#include <numeric>

#include <queue>

#include <set>

#include <string>

#include <unordered_map>

#include <unordered_set>

#include <vector>

using namespace std;

using uint = unsigned int;
using ll = long long;
using ull = unsigned long long;
constexpr ll TEN(int n) { return (n == 0) ? 1 : 10 * TEN(n - 1); }
template <class T> using V = vector<T>;
template <class T> using VV = V<V<T>>;

struct Scanner {
    FILE* fp = nullptr;
    char line[(1 << 15) + 1];
    size_t st = 0, ed = 0;
    void reread() {
        memmove(line, line + st, ed - st);
        ed -= st;
        st = 0;
        ed += fread(line + ed, 1, (1 << 15) - ed, fp);
        line[ed] = '\0';
    }
    bool succ() {
        while (true) {
            if (st == ed) {
                reread();
                if (st == ed) return false;
            }
            while (st != ed && isspace(line[st])) st++;
            if (st != ed) break;
        }
        if (ed - st <= 50) reread();
        return true;
    }
    template <class T, enable_if_t<is_same<T, string>::value, int> = 0>
    bool read_single(T& ref) {
        if (!succ()) return false;
        while (true) {
            size_t sz = 0;
            while (st + sz < ed && !isspace(line[st + sz])) sz++;
            ref.append(line + st, sz);
            st += sz;
            if (!sz || st != ed) break;
            reread();
        }
        return true;
    }
    template <class T, enable_if_t<is_integral<T>::value, int> = 0>
    bool read_single(T& ref) {
        if (!succ()) return false;
        bool neg = false;
        if (line[st] == '-') {
            neg = true;
            st++;
        }
        ref = T(0);
        while (isdigit(line[st])) {
            ref = 10 * ref + (line[st++] - '0');
        }
        if (neg) ref = -ref;
        return true;
    }
    template <class T> bool read_single(V<T>& ref) {
        for (auto& d : ref) {
            if (!read_single(d)) return false;
        }
        return true;
    }
    void read() {}
    template <class H, class... T> void read(H& h, T&... t) {
        bool f = read_single(h);
        assert(f);
        read(t...);
    }
    Scanner(FILE* _fp) : fp(_fp) {}
};

struct Printer {
  public:
    template <bool F = false> void write() {}
    template <bool F = false, class H, class... T>
    void write(const H& h, const T&... t) {
        if (F) write_single(' ');
        write_single(h);
        write<true>(t...);
    }
    template <class... T> void writeln(const T&... t) {
        write(t...);
        write_single('\n');
    }

    Printer(FILE* _fp) : fp(_fp) {}
    ~Printer() { flush(); }

  private:
    static constexpr size_t SIZE = 1 << 15;
    FILE* fp;
    char line[SIZE], small[50];
    size_t pos = 0;
    void flush() {
        fwrite(line, 1, pos, fp);
        pos = 0;
    }
    void write_single(const char& val) {
        if (pos == SIZE) flush();
        line[pos++] = val;
    }
    template <class T, enable_if_t<is_integral<T>::value, int> = 0>
    void write_single(T val) {
        if (pos > (1 << 15) - 50) flush();
        if (val == 0) {
            write_single('0');
            return;
        }
        if (val < 0) {
            write_single('-');
            val = -val; // todo min
        }
        size_t len = 0;
        while (val) {
            small[len++] = char('0' + (val % 10));
            val /= 10;
        }
        reverse(small, small + len);
        memcpy(line + pos, small, len);
        pos += len;
    }
    void write_single(const string& s) {
        for (char c : s) write_single(c);
    }
    void write_single(const char* s) {
        size_t len = strlen(s);
        for (size_t i = 0; i < len; i++) write_single(s[i]);
    }
    template <class T> void write_single(const V<T>& val) {
        auto n = val.size();
        for (size_t i = 0; i < n; i++) {
            if (i) write_single(' ');
            write_single(val[i]);
        }
    }
};
struct UnionFind {
    V<int> p, r;
    int gn;
    UnionFind(int n = 0) : p(n, -1), r(n, 1), gn(n) {}
    void merge(int a, int b) {
        int x = group(a), y = group(b);
        if (x == y) return; // same
        gn--;
        if (r[x] < r[y]) {
            p[x] = y;
        } else {
            p[y] = x;
            if (r[x] == r[y]) r[x]++;
        }
    }
    int group(int a) {
        if (p[a] == -1) return a;
        return p[a] = group(p[a]);
    }
    bool same(int a, int b) { return group(a) == group(b); }
};

Scanner sc = Scanner(stdin);
Printer pr = Printer(stdout);

int main() {
    int n, m;
    sc.read(n, m);
    V<ll> d(n);
    for (int i = 0; i < n; i++) {
        sc.read(d[i]);
    }
    V<int> idx(n);
    iota(idx.begin(), idx.end(), 0);
    sort(idx.begin(), idx.end(), [&](int a, int b) {
        return d[a] < d[b];
    });

    VV<int> g(n);
    V<ll> ans(m, TEN(9));
    for (int i = 0; i < m; i++) {
        int a, b;
        sc.read(a, b); a--; b--;
        g[a].push_back(b);
        g[b].push_back(a);
        ans[i] = max(d[a], d[b]);
    }

    VV<int> tr(n);
    auto uf = UnionFind(n);
    for (int i: idx) {
        bool ok = false;
        for (int j: g[i]) {
            if (d[j] <= d[i]) ok = true;
            if (d[j] <= d[i] && !uf.same(i, j)) {
                tr[i].push_back(j);
                tr[j].push_back(i);
                uf.merge(i, j);
            }
        }
        if (!ok) {
            pr.writeln(-1);
            return 0;
        }
    }
    V<int> col(n, -1);

    auto dfs = [&](auto self, int p) -> void {
        for (int d: tr[p]) {
            if (col[d] != -1) continue;
            col[d] = 1 - col[p];
            self(self, d);
        }
    };

    for (int i = 0; i < n; i++) {
        if (col[i] == -1) {
            col[i] = 0;
            dfs(dfs, i);
        }
    }

    string s;
    for (int i = 0; i < n; i++) {
        if (col[i]) s += 'B';
        else s += 'W';
    }
    pr.writeln(s);
    for (auto x: ans) pr.writeln(x);
    return 0;
}


Measured memory_bytes: 18176