Problem Background

NOIP2015 Junior Group T1

Problem Description

The king pays his loyal knights with gold coins. On the first day, the knight receives one coin; for the next two days (the second and third days), he receives two coins each day; for the next three days (the fourth, fifth, and sixth days), he receives three coins each day; for the next four days (the seventh, eighth, ninth, and tenth days), he receives four coins each day... This payment pattern continues indefinitely: after receiving \( n \) coins for \( n \) consecutive days, the knight will receive \( n+1 \) coins for the next \( n+1 \) consecutive days.

Calculate the total number of coins the knight has received in the first \( k \) days.

Input Format

A positive integer \( k \), representing the number of days for which coins are distributed.

Output Format

A positive integer, representing the total number of coins received by the knight.

Sample Input and Output

Sample Input #1

6

Sample Output #1

14

Sample Input #2

1000

Sample Output #2

29820

Notes

[Sample 1 Explanation]

The knight receives one coin on the first day; two coins each on the second and third days; three coins each on the fourth, fifth, and sixth days. Therefore, the total number of coins received is \( 1 + 2 + 2 + 3 + 3 + 3 = 14 \).

For \( 100\% \) of the data, \( 1 \le k \le 10^4 \).

见题面 Input 部分

见题面 Output 部分