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

ans = -1

for i in range(1, int((A + B) ** 0.5) + 1):
    
    if (A + B) % i == 0:
        if (A + i) % B == 0 and (B + i) % A == 0 and i != A and i != B:
            print(i)
            exit()
        i = (A + B) // i
        if (A + i) % B == 0 and (B + i) % A == 0 and i != A and i != B:
            ans = i
            
print(ans)