a, b = map(int, input().split())
tmp = a + b

lst = []
for i in range(2, int(tmp ** 0.5) + 1):
    if tmp % i == 0:
        lst.append(i)
        lst.append(tmp // i)
        
for num in lst:
    if a == num or b == num:
        continue
    if (a + num) % b == 0 and (b + num) % a == 0:
        print(num)
        exit()
print(-1)