a,b=map(int,input().split())
if a==2*b or b==2*a:
    print(3*min(a,b))
    exit()
elif a==3*b or b==3*a:
    print(2*min(a,b))
    exit()
elif a/b==1.5 or b/a==1.5:
    print(min(a,b)//2)
print(-1)