https://www.acmicpc.net/problem/8393
import java.util.Scanner;
class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int n = sc.nextInt();
int sum = 0;
for (int i = 1; i <= n; i++) {
sum += i;
}
System.out.println(sum);
}
}