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 Ruby (2.3.3) (judge verdict: Wrong Answer)

input = STDIN.gets.chomp.split('')

result = []

input.each do |n|
  if n == '0'
    result << '0'
  end

  if n == '1'
    result << '1'
  end

  if n == 'B'
    if result.length != 0
      result.pop
    end
  end
end

p result.join("")