N, M = list(map(int, input().split()))
D = list(map(int, input().split()))
UB = [list(map(int, input().split())) for _ in range(M)]

R = {}
for i in range(M):
  a, b = UB[i]
  a -= 1
  b -= 1
  if a not in R: R[a] = [[b, i]]
  else: R[a].append([b, i])
  if b not in R: R[b] = [[a, i]]
  else: R[b].append([a, i])

DD = [[0] * 2 for _ in range(N)]
for i in range(N):
  DD[i][0] = D[i]
  DD[i][1] = i
DD.sort()
L = [0] * N
H = [0] * M
LL = [[-1] * 2 for _ in range(N)]
for i in range(N):
  h, x = DD[i]
  if L[x] == 0:
    if LL[x][0] == -1:
      for n, m in R[x]:
        LL[n] = [x, m]
        if D[n] == D[x] or L[n] != 0:
          H[m] = h
          if L[n] == 1:
            L[n] = 1
            L[x] = 2
          else:
            L[n] = 2
            L[x] = 1
          break
      else:
        print(-1)
        exit()
    else:
      n, m = LL[x]
      H[m] = h
      if L[n] == 1:
        L[n] = 1
        L[x] = 2
      else:
        L[n] = 2
        L[x] = 1

t = ""
for i in range(N):
  if L[i] == 1:
    t += "B"
  else:
    t += "W"
print(t)
for i in range(M):
  if H[i] == 0:
    print(10 ** 9)
  else:
    print(H[i])
