Problem:
Score : 200 points

Problem Statement

You have an integer variable x.
Initially, x=0.

Some person gave you a string S of length N, and using the string you performed the following operation N times.
In the i-th operation, you incremented the value of x by 1 if S_i=I, and decremented the value of x by 1 if S_i=D.

Find the maximum value taken by x during the operations (including before the first operation, and after the last operation).

Constraints

1≤N≤100

|S|=N

No characters except I and D occur in S.

Input

The input is given from Standard Input in the following format:

N
S

Output

Print the maximum value taken by x during the operations.

Sample Input 1

5
IIDID

Sample Output 1

2

After each operation, the value of x becomes 1, 2, 1, 2 and 1, respectively. Thus, the output should be 2, the maximum value.

Sample Input 2

7
DDIDDII

Sample Output 2

0

The initial value x=0 is the maximum value taken by x, thus the output should be 0.

Solution in Haskell (GHC 7.10.3) (judge verdict: Accepted)

import Data.IORef
import Control.Monad

main :: IO ()
main = do
  n <- readLn :: IO Int
  s <- getLine
  mxRef <- newIORef 0
  xRef <- newIORef 0
  forM_ s $ \c -> do
    mx <- readIORef mxRef
    if c == 'I'
      then modifyIORef xRef (+ 1)
      else modifyIORef xRef (subtract 1)
    x <- readIORef xRef
    writeIORef mxRef $ max mx x

  print =<< readIORef mxRef