// need
#include <iostream>
#include <algorithm>
// data structure
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#include <complex>
//#include <deque>
#include <valarray>
#include <unordered_map>
#include <unordered_set>
#include <array>
// etc
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>
#include <fstream>

// input
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest& ...rest) { std::cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int w_=0; w_<n; ++w_){MACRO_VEC_ROW_Scan(w_, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest& ...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { std::cin >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest& ...rest) { std::cin >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& R:c)for(auto& w:R)std::cin>>w;
// output
template<typename T>void MACRO_OUT(const T t) { std::cout << t; }
template<typename First, typename...Rest>void MACRO_OUT(const First first, const Rest...rest) { std::cout << first << " "; MACRO_OUT(rest...); }
#define OUT(...) MACRO_OUT(__VA_ARGS__);
#define FOUT(n, dist) std::cout<<std::fixed<<std::setprecision(n)<<(dist);
#define SOUT(n, c, dist) std::cout<<std::setw(n)<<std::setfill(c)<<(dist);
#define EOUT(...) { OUT(__VA_ARGS__)BR; exit(0); }
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define SPBR(w, n) std::cout<<(w + 1 == n ? '\n' : ' ');
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define SHOW(dist) {std::cerr << #dist << "\t: " << (dist) << "\n";}
#define SHOWVECTOR(v) {std::cerr << #v << "\t: ";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOWVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr << yyy << " ";}std::cerr << "\n";}}
#define SHOWQUEUE(a) {auto tmp(a);std::cerr << #a << "\t: ";while(!tmp.empty()){std::cerr << tmp.front() << " ";tmp.pop();}std::cerr << "\n";}
#define SHOWSTACK(a) {auto tmp(a);std::cerr << #a << "\t: ";while(!tmp.empty()){std::cerr << tmp.top() << " ";tmp.pop();}std::cerr << "\n";}
// utility
#define ALL(a) (a).begin(),(a).end()
#define FOR(w, a, n) for(int w=(a);w<(n);++w)
#define RFOR(w, a, n) for(int w=(n)-1;w>=(a);--w)
#define REP(w, n) for(int w=0;w<int(n);++w)
#define RREP(w, n) for(int w=int(n)-1;w>=0;--w)
#define IN(a, x, b) (a<=x && x<b)
template<class T> inline T CHMAX(T & a, const T b) { return a = (a < b) ? b : a; }
template<class T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
// test
template<class T> using V = std::vector<T>;
template<class T> using VV = V<V<T>>;

template<typename S, typename T>
std::ostream& operator<<(std::ostream& os, std::pair<S, T> p) {
	os << "(" << p.first << ", " << p.second << ")"; return os;
}

// type/const
#define int ll
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<int, int>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = (1 << 30) - 1;                    // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double eps = 1e-7;
constexpr int MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;
template<class T, size_t N> void FILL(T(&a)[N], const T & val) { for (auto& x : a) x = val; }
template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T & val) { for (auto& b : a) FILL(b, val); }
template<class T> void FILL(std::vector<T> & a, const T & val) { for (auto& x : a) x = val; }
template<class ARY, class T> void FILL(std::vector<std::vector<ARY>> & a, const T & val) { for (auto& b : a) FILL(b, val); }
// ------------>8------------------------------------->8------------

std::map<PAIR, int> map;

bool check(const VV<int> & g_, const std::string & s, const V<int> & d) {
	int n = s.size();
	struct Edge {
		int to, cost;
		Edge() {}
		Edge(int to, int cost) : to(to), cost(cost) {}
		bool operator<(const Edge& r) const {
			return cost < r.cost;
		}
	};
	std::vector<std::vector<Edge>> g(n + 2);
	REP(i, n) {
		for (int to : g_[i]) if (i < to) {
			int cost = map[{i, to}];
			g[i].emplace_back(to, cost);
			g[to].emplace_back(i, cost);
		}
	}

	auto g_base = g;

	std::vector<int> dist(n + 2);
	std::function<void(int)> Dijkstra = [&](int s) {
		using P = std::pair<int, int>;
		std::priority_queue<P, std::vector<P>, std::greater<P>> pq;
		std::fill(dist.begin(), dist.end(), INFLL);
		dist[s] = 0;
		pq.push(P(0, s));
		while (!pq.empty()) {
			P p = pq.top(); pq.pop();
			int v = p.second;
			if (dist[v] < p.first) continue;
			for (int i = 0; i < g[v].size(); ++i) {
				Edge e = g[v][i];
				if (dist[e.to] > dist[v] + e.cost) {
					dist[e.to] = dist[v] + e.cost;
					pq.push(P(dist[e.to], e.to));
				}
			}
		}
	};


	REP(i, n) {
		if (s[i] == 'B') {
			g[n].emplace_back(i, 0);
		}
		else {
			g[i].emplace_back(n + 1, 0);
		}
	}
	Dijkstra(n);
	REP(i, n) {
		if (s[i] == 'W') {
			if (dist[i] != d[i]) return false;
		}
	}
	g = g_base;
	REP(i, n) {
		if (s[i] == 'B') {
			g[i].emplace_back(n, 0);
		}
		else {
			g[n + 1].emplace_back(i, 0);
		}
	}
	Dijkstra(n + 1);
	REP(i, n) {
		if (s[i] == 'B') {
			if (dist[i] != d[i]) return false;
		}
	}
	return true;
};

signed main() {
	INIT;

	VAR(int, n, m);
	VEC(int, d, n);

	VEC_ROW(int, m, a, b);
	std::vector<std::vector<int>> g(n);
	REP(i, m) {
		--a[i]; --b[i];
		g[a[i]].emplace_back(b[i]);
		g[b[i]].emplace_back(a[i]);
	}

	std::string s(n, '~');

	std::priority_queue<PAIR, V<PAIR>, std::greater<PAIR>> roots;
	REP(i, n) roots.emplace(d[i], i);

	V<bool> visited(n, false);
	while (!roots.empty()) {
		PAIR pair = roots.top(); roots.pop();
		int root = pair.second;
		if (visited[root]) continue;
		visited[root] = true;
		s[root] = 'B';
		std::priority_queue<PAIR, V<PAIR>, std::greater<PAIR>> pq;
		pq.emplace(d[root], root);
		while (!pq.empty()) {
			PAIR p = pq.top(); pq.pop();
			for (int to : g[p.second]) if (!visited[to]) {
				if (d[p.second] == d[to]) {
					visited[to] = true;
					s[to] = (s[p.second] == 'B' ? 'W' : 'B');
					map[std::minmax(to, p.second)] = d[to];
					pq.emplace(d[to], to);
				}
				else if (d[p.second] < d[to]) {
					visited[to] = true;
					s[to] = s[p.second];
					map[std::minmax(to, p.second)] = d[to] - d[p.second];
					pq.emplace(d[to], to);
				}
			}
		}
	}

	REP(i, m) {
		PAIR p = std::minmax(a[i], b[i]);
		if (!map.count(p)) map[p] = 1000000000;
	}
	if (!check(g, s, d)) {
		OUT(-1)BR;
	}
	else {
		OUT(s)BR;
		REP(i, m) {
			OUT(map[std::minmax(a[i], b[i])])BR;
		}
	}

	return 0;
}

Measured memory_bytes: 57336