n = read_line.to_i
s = read_line.to_i
s -= 1
ans = Array.new(n, -1)
n.times do |i|
  ok = false
  3.times do |j|
    ans[i] = j
    used = Array.new(3) { |k| ans.includes?(k) }
    c = count(n - 1 - i, used)
    if c > s
      ok = true
      break
    else
      s -= c
    end
  end
  if !ok
    puts -1
    exit
  end
end
puts ans.map { |i| 'A' + i }.join

def count(len, used)
  return 0 if used.count(false) > len
  ret = 3 ** len
  3.times do |i|
    if !used[i]
      ret -= 2 ** len
    end
    if !used[i] && !used[i - 1]
      ret += 1
    end
  end
  ret
end