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

clist = []
ans = 1000000000000000001
D = int(pow(A + B, 1 / 2))
E = A + B
for i in range(1, D + 1):
    if D % i == 0:
        clist.append(i)
        clist.append(int(E / i))

if A in clist:
    clist.remove(A)

if B in clist:
    clist.remove(B)

for i in clist:
    if A != i and B != i:
        if (A + i) % B == 0:
            if (B + i) % A == 0:
                ans = min(ans, i)

if ans == 1000000000000000001:
    ans = -1
print(ans)