Problem:
Score : 300 points

Problem Statement

In a flower bed, there are N flowers, numbered 1,2,......,N. Initially, the heights of all flowers are 0.
You are given a sequence h=\{h_1,h_2,h_3,......\} as input. You would like to change the height of Flower k to h_k for all k (1 \leq k \leq N), by repeating the following "watering" operation:

Specify integers l and r. Increase the height of Flower x by 1 for all x such that l \leq x \leq r.

Find the minimum number of watering operations required to satisfy the condition.

Constraints

1 \leq N \leq 100

0 \leq h_i \leq 100

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
h_1 h_2 h_3 ...... h_N

Output

Print the minimum number of watering operations required to satisfy the condition.

Sample Input 1

4
1 2 2 1

Sample Output 1

2

The minimum number of watering operations required is 2.
One way to achieve it is:

Perform the operation with (l,r)=(1,3).

Perform the operation with (l,r)=(2,4).

Sample Input 2

5
3 1 2 3 1

Sample Output 2

5

Sample Input 3

8
4 23 75 0 23 96 50 100

Sample Output 3

221

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

import Control.Monad
import qualified Data.ByteString.Char8 as B  
import qualified Data.Vector.Unboxed as VU
import Data.Char

solve :: VU.Vector Int -> Int -> Int
solve xs acc
    | VU.null xs || VU.all (==0) xs = acc 
    | otherwise = solve ((VU.map (subtract 1) ts) VU.++ ds') (acc+1)
    where
        ds = VU.dropWhile (<=0) xs 
        ts = VU.takeWhile (>0) ds
        ds' = VU.drop (VU.length ts) ds

main = do
    n <- readLn :: IO Int
    hs <- VU.unfoldrN n (B.readInt . B.dropWhile isSpace) <$> B.getLine
    print $ solve hs 0