import sys
input = sys.stdin.readline

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

X=[]
def calc(x,now):
    if x==0:
        X.append(now)
        return

    else:
        calc(x-1,now+["A"])
        calc(x-1,now+["B"])
        calc(x-1,now+["C"])

calc(N,[])
    
Y=[]
for x in X:
    if "A" in x and "B" in x and "C" in x:
        Y.append(x)

Y.sort()

if S-1<len(Y):
    print("".join(Y[S-1]))
else:
    print(-1)