Help me in solving LB01B problem

My issue

What is the logic behind the output

Learning course: C for problem solving - 1
Problem Link: CodeChef: Practical coding for everyone

what help you need in this . reply to this ill help

The logic is if both A and C are even or odd then their average B would be (A+C)/2.
else for else case it would be -1;
So u have to pick correct options among them .