a,b=map(int,input().split())
for c in range(max(a,b)-min(a,b),a+b):
    if (a+c)%b==0 and (a+b)%c==0 and (b+c)%a==0 and a!=b and b!=c and c!=a:
        print(c)
        break
    else:
        c+=1
else:
    print(-1)