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

from collections import deque

ans = []
q = deque(["C", "B", "A"])

while q:
    s = q.pop()
    if len(s) == N:
        if s.count("A") != 0 and s.count("B") != 0 and s.count("C") != 0:
            ans.append(s)
    else:
        q.append(s + "C")
        q.append(s + "B")
        q.append(s + "A")

if S-1 < len(ans):
    print(ans[S-1])
else:
    print(-1)