[백준] 8393번 : 합 - Dart(다트)

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

import 'dart:io';
void main(){
    int? n=int.parse(stdin.readLineSync()!);
    var sum=0;
    if(n%2==1){
        var m=(n+1)~/2;
        sum=(1+n)*(n~/2)+m;
    }
    else sum=(1+n)*((n+1)~/2);
    print(sum);
}