A,B=map(int,input().split())

if A==B:
    print(A)
elif max(A,B)==min(A,B)*2:
    print(min(A,B)*3)
elif max(A,B)==min(A,B)*3:
    print(min(A,B)*2)
elif max(A,B)==min(A,B)*1.5:
    print(min(A,B)//2)
else:
    print(-1)