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 Haskell (GHC 7.10.3) (judge verdict: Runtime Error)

main = do
 s<- getLine
 putStrLn $ reverse $ f $ reverse s
f []= []
f (x:xs)
        |x=='0' ="0" ++ (f xs)
        |x=='1' ="1" ++ (f xs)
        |x=='B' && (dropWhile (\y -> y == 'B') xs)==[] = []
        |otherwise = f ((takeWhile (\y -> y == 'B') xs) ++ (tail (dropWhile (\y -> y == 'B') xs)))