from itertools import product
N = int(input())
S = int(input())

cnt = 0
# 3**N 通り
for pat in product(['A', 'B', 'C'], repeat=N):
    if 'A' in pat and 'B' in pat and 'C' in pat:
        cnt += 1
        if cnt == S:
            exit(print(''.join(pat)))
print(-1)