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

s = set()
for i in range(1, int((a+b)**0.5) + 1):
    if (a + b) % i == 0:
        s.add(i)
        s.add((a + b) // i)

l = list(s - {a} - {b})

for i in l:
    if (a + i) % b == 0 and (b + i) % a == 0:
        print(i)
        break
else:
    print(-1)