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

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