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
fscanf(STDIN, "%s", $initial_string);
$length = strlen($initial_string);
$result = '';
$current_count = 0;
for($i = 0; $i < $length; $i++) {
    $char = $initial_string[$i];
    if ($char == '0') {
        $result[] = $char;
    } else if ($char == 'B') {
        $current_count = count($result);
        $pre_index = $current_count - 1 ;
        if (!empty($result)) {
            unset($result[$pre_index]);
            $result = array_values($result);
        }
    } else if ($char == '1') {
        $result[] = $char;
    }
}
echo implode("", $result);
?>