Basic Problem (Codeforces B)

Problem:
https://codeforces.com/problemset/problem/909/B

How is this solution actually working for this problem?

In short:
cin>>n;
for(int i=1;i<=n;i++){
res+=min(i,n-i+1);
}
cout<<res<<endl;

Solution:
https://codeforces.com/contest/909/submission/88639922