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

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

if A in clist:
    clist.remove(A)

if B in clist:
    clist.remove(B)

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

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