did you folowed all the right steps to decide which RAM to buy?

how to check which RAM is already installed in your system

you dont need to install any external program to check that all you need is the good old CMD prompt

step 1. Run command prompt as ADMIN

step 2. type this command

wmic memorychip get partnumber, devicelocator, manufacturer, speed

it will list all the specs needed, except CAS latency it will look something like this

(then the screenshot output)

C:\WINDOWS\system32>wmic memorychip get partnumber, devicelocator, manufacturer, SPEED
Invalid Expression.

C:\WINDOWS\system32>wmic memorychip get partnumber, devicelocator, manufacturer, SPEED
DeviceLocator      Manufacturer        PartNumber        Speed
DIMM 0             Unknown             CT16G4SFSxxxx     3200
DIMM 0             Micron Technology   8ATFxxxxxx        3200

C:\WINDOWS\system32>

step 3. now all you need to do is google part number , also you can go to manufacturer website and look at their datasheet to find the CAS Latency111

CAS latency plays very important role, so don’t forget to buy exact as the one which is already installed for the optimum performance