Problem:
Score : 300 points

Problem Statement

You are given a string S consisting of lowercase English letters.
Another string T is initially empty.
Determine whether it is possible to obtain S = T by performing the following operation an arbitrary number of times:

Append one of the following at the end of T: dream, dreamer, erase and eraser.

Constraints

1≦|S|≦10^5

S consists of lowercase English letters.

Input

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

S

Output

If it is possible to obtain S = T, print YES. Otherwise, print NO.

Sample Input 1

erasedream

Sample Output 1

YES

Append erase and dream at the end of T in this order, to obtain S = T.

Sample Input 2

dreameraser

Sample Output 2

YES

Append dream and eraser at the end of T in this order, to obtain S = T.

Sample Input 3

dreamerer

Sample Output 3

NO

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

import Control.Applicative
import Control.Monad
import Data.Monoid
import Data.Maybe
import Data.List

main :: IO ()
main = do
    s <- getLine :: IO String
    putStrLn $ if isJust $ parse s then "YES" else "NO"

parse :: String -> Maybe String
parse "" = Just ""
parse s = let
    a = if take (length "dream")   s == "dream"   then Just $ drop (length "dream")   s else Nothing
    b = if take (length "dreamer") s == "dreamer" then Just $ drop (length "dreamer") s else Nothing
    c = if take (length "erase")   s == "erase"   then Just $ drop (length "erase")   s else Nothing
    d = if take (length "eraser")  s == "eraser"  then Just $ drop (length "eraser")  s else Nothing
    in (parse =<< a) <|> (parse =<< b) <|> (parse =<< c) <|> (parse =<< d)