a,b = map(int,input().split())
x =[]
for i in range(1,a+b):
    if i!=a and i!=b:
        if (a+b)%i==0:
            x.append(i)
for i in x:
    if (a+i)%b==0 and (i+b)%a==0:
        print(i)
        exit()
else:
    print(-1)