序列求和（long）

 1 import java.math.BigDecimal;
 2 import java.math.BigInteger;
 3 import java.util.*;
 4 
 5 public class Main {
 6     public static void main(String[] args) {
 7         Scanner scanner = new Scanner(System.in);
 8         long n = scanner.nextLong();
 9         long resultLong = (long) ((1+n)*n/2);
10         System.out.println(resultLong);
11     }
12 }