a,b=map(int,input().split())
if a>b:
    a,b=b,a

if b//a==3 and b%a==0:
    print(a*2)

elif b//a==2 and b%a==0:
    print(a*3)

elif a*3==b*2:
    print(a//2)

else:
    print(-1)