[백준] 8393번 : 합 - C#

https://www.acmicpc.net/problem/8393

using System;

class Program
{
    static void Main() {
        string s=Console.ReadLine();
        int n=int.Parse(s);
        int sum=0;
        if(n%2==1){
            int m=(n+1)/2;
            sum=(1+n)*(n/2)+m;
        } 
        else sum=(1+n)*((n+1)/2);
        Console.Write(sum);
    }
}