def divisor_list_calc(n):
    res_list = []
    for i in range(1, int(n ** 0.5) + 1):
        if n % i == 0:
            res_list.append(i)
            if n // i != i:
                res_list.append(n)
    return res_list
            
a, b = map(int,input().split())
c_list = divisor_list_calc(a + b)
res_list = []

for i in range(len(c_list)):
    cond_list = []
    
    cond_list.append((b + c_list[i]) % a == 0)
    cond_list.append((a + c_list[i]) % b == 0)
    cond_list.append(a != c_list[i])
    cond_list.append(b != c_list[i])
    
    if all(cond_list):
        res_list.append(c_list[i])
        break
        
if len(res_list) == 0:
    res = -1
else:
    res = min(res_list)
    
print(res)