n=int(input())
t=int(input())
q=[[]]
ans=set()
while q:
  p=q.pop()
  if len(p)==n:
    if set(p)=={*'ABC'}:
      ans.add(''.join(p))
    continue
  for nxt in'ABC':
    q+=p+[nxt],
ans=sorted(ans)
if t<=len(ans):
  print(ans[t-1])
else:
  print(-1)