1602 LCD and reading out RPM fan

Yeah sorry, I'm a 15 year old dutch student, so my english is not that well.

How can I get the two outputs (pulse 1 and pulse 2) and add those numbers together.
So that the formula will be:
(Max rpm*(pulse1+pulse2))/duration.

Nevermind that.
You will have to manually calibrate the code for a specticular fan.