a,b=gets.split.map &:to_i
d=a.gcd b
k=(d*(1.0/a+1.0/b)).to_i
c=a*(b/d*k-1)-b
c+=a*b/d while c<=0
c-=a*b/d while c-a*b/d>=1
p a!=c&&b!=c&&(a+b)%c==0&&(a+c)%b==0&&(b+c)%a==0 ? c : -1