题解 | #小乐乐求和#

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class Main{
    public static void main(String[] args) throws IOException{
        BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
        String str = bf.readLine();
        long n = Integer.parseInt(str);
        long sum = n*(n+1)/2;
        System.out.println(sum);
    }
}