n = int(input())
s = int(input())

ans = []
for i in range(3**n):
	t = []
	ii = i
	for j in range(n):
		t.append(ii%3)
		ii//=3
	if len(set(t)) <= 2:
		continue
	si = []
	for k in t:
		if k == 0:
			si.append("A")
		elif k == 1:
			si.append("B")
		else:
			si.append("C")
	ans.append("".join(si))
ans.sort()
if len(ans) >= s:
	print(ans[s-1])
else:
	print(-1)