#1078588の亜種
from itertools import combinations

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

L=[i for i in range(1,N+1)]
res=[]
for a in range(1,N-1):
    for b in range(1,N-1):
        c = N - a - b
        if c < 1:
            continue
        else:
            for l in combinations(L,a):
                n=[]
                t=1
                for i in l:
                    for u in range(t,i):
                        n+=[u]
                    t=i+1
                else:
                    for u in range(t,N+1):
                        n+=[u]
                for m in combinations(list(range(N-a)),b):
                    nn=[]
                    t=0
                    for i in m:
                        for u in range(t,i):
                            nn+=[n[u]]
                        t=i+1
                    else:
                        for u in range(t,N-a):
                            nn+=[n[u]]
                    
                    ans=[-1]*N
                    for i in l:
                        ans[i-1] = "A"
                    for i in m:
                        ans[n[i]-1] = "B"
                    for i in nn:
                        ans[i-1] = "C"
                    
                    res.append("".join(ans))

res=sorted(res)
try:
    print(res[S-1])
except:
    print(-1)