A, B = map(int, input().split())
divisor = [1]
ApB = A + B
for i in range(2, ApB):
    if ApB % i == 0 and i != A and i != B:
        divisor.append(i)

divisor.append(ApB)

for C in divisor:
    if (A + C) % B == 0 and (B + C) % A == 0:
        print(C)
        break
else:
    print(-1)