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

C = -1
for i in range(1, int((A+B)**(1/2)) + 2):
    if A!=i and B!=i and (A+i) >= B and (B+i) >= A:
        if (A+B) % i ==0 and  (A+i)%B == 0 and (B+i)%A==0:
            C = i                
            break
        
if C == -1:            
    i = A+B
    if A != i and B != i and A+i >= B and B+i >= A:
        if (A+B)%i == 0 and (A+i)%B == 0 and (B+i)%A==0:
            C = i
print(C)