Help me in solving LBCV201A problem

My issue

My code

//Step 1: Read and understand the problem statement and sample test cases

// Click on 'Next' once you are ready to proceed.
using namespace std;
int main()
    int a, c;
    int b=(a+c)/2;

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