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

if B < A:
    A,B = B,A
if B == A * 2:
    print(3 * A)
elif B == A * 3:
    print(A * 2)
elif A & 1 == 0 and B == A//2 * 3:
    print(A//2)
else:
    print(-1)