from itertools import product
N = int(input())
S = int(input())
L = list()
for bit in product([0,1,2],repeat=N):
    if 0 in bit and 1 in bit and 2 in bit:
        L.append(bit)
L.sort()
if len(L)<S:
    print(-1)
    exit()
ans = ""
for i in L[S-1]:
    if i==0:
        ans += "A"
    elif i==1:
        ans += "B"
    else:
        ans += "C"
print(ans)