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)

def check(offset, words)
  h = {}
  border = words.size / 2.0
  words.each_with_index do |w,i|
    v = h[w]
    if v
      c = v[0] + 1
      h[w] = [c, v[1]]
    else
      c = 1
      h[w] = [c, i]
    end
    if c > border
      return "#{offset + v[1] + 1} #{offset + i + 1}"
    end
  end
  nil
end

words = gets.split('')
(words.size - 3).times do |i|
  parts = words[i, 3]
  result = check(i, parts)
  if result
    puts result
    exit 
  end
end
puts "-1 -1"