kod 55

x = int(input())
y = int(input())
if x == y == 1:
    print(1)
elif x == y:
    print(-1)
else:
    k = 1
    while True:
        left = y * k
        right = x * (k + 1)
        #print(left, right)
        if right - left == 1 or right - left == 0:
            print(x * k)
            break
        k += 1