Oh, thank you a lot!
The current ram memories - since you asked - are the one which existed during the purchase of the laptop, that is, August of 2016. You mean I have to choose the same brand of ram (Sasmung 8GB x2 2133Mhz latency I don't remember since I am at work now), right? Yes, the same brand is difficult along with same latency and frequency.
I didnt know though that it will not work if you haven't everything same as far as the memory slots are concerned. So, thanks for your important advice!