# CはA+Bの約数であるから√A+Bの範囲のループで探索(A+B) % c == 0となるcとそのときのc'= (A+B) // c

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

ans = 1 << 60
for c0 in range(1,AB+1):
    if c0 * c0 > AB:
        break

    if AB % c0 != 0:
        continue

    if c0 != A and c0 != B and (A+c0) % B == 0 and (B+c0) % A == 0:
        ans = min(ans, c0)

    c1 = AB // c0
    if c1 != A and c1 != B and (A+c1) % B == 0 and (B+c1) % A == 0:
        ans = min(ans, c1)

if ans == 1 << 60:
    print(-1)
else:
    print(ans)