N=int(input())
S=int(input())
import sys
sys.setrecursionlimit(10**7)
def dfs(s):
    global A
    if len(s)==N:
        if set(s)=={*"ABC"}:
            A.append(s)
        return
    for c in "ABC":
        dfs(s+c)
A=[]
dfs("")
if S<=len(A):
    print(A[S-1])
else:
    print(-1)