Problem:
Score : 100 points

Problem Statement

There are N children in AtCoder Kindergarten. Mr. Evi will arrange the children in a line, then give 1 candy to the first child in the line, 2 candies to the second child, ..., N candies to the N-th child. How many candies will be necessary in total?

Constraints

1≦N≦100

Input

The input is given from Standard Input in the following format:

N

Output

Print the necessary number of candies in total.

Sample Input 1

3

Sample Output 1

6

The answer is 1+2+3=6.

Sample Input 2

10

Sample Output 2

55

The sum of the integers from 1 to 10 is 55.

Sample Input 3

1

Sample Output 3

1

Only one child. The answer is 1 in this case.

Solution in PHP (5.6.30) (judge verdict: Wrong Answer)

<?php
$inputs = str_split(file_get_contents("php://stdin"));
echo array_reduce($inputs, function($prev, $cur) {
  if ($cur === "B") {
    return (strlen($prev) === 0) ? "" : substr($prev, 0, -1);
  } else if ($cur === "0" || $cur === "1"){
    return $prev.$cur;
  }
}, "");