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: Runtime Error)

n = gets.chomp.to_i
ary = gets.chomp.split(" ").map(&:to_i)[0..n-1]

dist = {}
ary.each do |num|
  dist[num.to_s] = dist[num.to_s].to_i + 1
end

dist_case_num = dist.sort_by {|k, v| -v}.first[0].to_i
avg_case_num = (ary.sum.to_f/ary.count).round
avg_case_sum = ary.sum {|num| (avg_case_num - num) ** 2}
dist_case_sum = ary.sum {|num| (dist_case_num - num) ** 2}

puts avg_case_sum > dist_case_sum ? dist_case_sum : avg_case_sum