Different outputs !?

Question

https://code.sololearn.com/cNK7O88bC545/?ref=app

I ran this C language code first on turbo C++ 3.1 and then on Sololearn's Code Playground.. in the first case the output was 18 and then here it was 19.
I am in a fix. Help !

Answer 1

I am a beginner.. so.. I just want to understand.. why is there difference between outputs on my laptop using turbo c++ 3.1 and on my phone using sololearn's code playground. The code I typed in both places are exactly the same.. I checked.

Answer 2

Since the first one gives 18 and the second one gives 19.. my teacher also prefers the turbo c++ 3.1 answer.. that is 18.. but I used the rules of postfix and prefix increment and decrement operators and got 19.. So which one is actually correct ?