def main():
    import sys
    input = sys.stdin.readline

    # MAX = 10 ** 9

    N, M = map(int, input().split())
    D = list(map(int, input().split()))

    v_color = [-1] * N
    check = [1] * N #0のとき条件を満たしているとする
    G = [[] for _ in range(N)]
    for i in range(M):
        U, V = map(int, input().split())
        U -= 1
        V -= 1
        G[U].append([V, i])
        G[V].append([U, i])

    #ここまで入力

    v_color[0] = 1 #最初の頂点を白色(1)にする
    stack = [0] #スタートする点　深さ優先探索で調べる
    weight = [-1] * M
    while stack:
        tmp = stack.pop()
        tmp_color = v_color[tmp]
        for lst in G[tmp]:
            next_ = lst[0]
            i = lst[1]
            if v_color[next_] == -1: #初めて訪れるとき
                #違う色にする
                if D[tmp] == D[next_]: #距離が同じなら両方OKにする
                    check[tmp] = 0
                    check[next_] = 0
                elif D[tmp] < D[next_]:
                    check[next_] = 0
                else:
                    check[tmp] = 0
                stack.append(next_)
                v_color[next_] = 1 - tmp_color
                weight[i] = max(D[tmp], D[next_])
            else:
                if tmp_color == v_color[next_]: #同じ色のとき #回り道してOkできるようにしておく
                    weight[i] = max(abs(D[tmp] - D[next_]), 1)
                    if D[tmp] < D[next_] and check[tmp] == 0:
                        check[next_] = 0
                    elif D[next_] < D[tmp] and check[next_] == 0:
                        check[tmp] = 0
                else:
                    if D[tmp] >= D[next_]:
                        check[tmp] = 0
                    if D[tmp] <= D[next_]:
                        check[next_] = 0
                    weight[i] = max(D[tmp], D[next_])

    # print (v_color)
    # print (weight)
    # print (check)

    if max(check) == 1:
        print (-1)
        exit()

    for i in v_color:
        if i:
            print ('W', end = '')
        else:
            print ('B', end = '')
    print ()

    print (*weight, sep = '\n')

if __name__ == '__main__':
    main()