You always have the online version:
https://www.asus.com/us/Motherboards/Z170-A/HelpDesk_Manual/
And it says that, if you are only using two sticks of memory, you should install them into slots A2 and B2. Looking from the top, when the CPU is left from the slots and the ATX power connector is on the right, going from left to right skip the first slot, use the second, skip the third and use the last one near the ATX power connector.