from itertools import product

N = int(input())
S = int(input())

li = []

for s in product(["A", "B", "C"], repeat=N):
    if not set(s) == {"A", "B", "C"}:
        continue
    s = "".join(s)
    li.append(s)
    
    if len(li) >= S:
        break

print(li[S-1] if len(li) >= S else -1)