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 Data.List

main = do
  s <- getLine
  putStrLn . showF $ f s
  
showF :: Bool -> String
showF True  = "YES"
showF False = "NO"
  
f :: String -> Bool
f [] = True
f s
  | "dream" `isPrefixOf` s && "dreamer" `isPrefixOf` s = f (drop 5 s) || f (drop 7 s)
  | "dream" `isPrefixOf` s = f (drop 5 s)
  | "dreamer" `isPrefixOf` s = f (drop 7 s)
  | "erase" `isPrefixOf` s && "eraser" `isPrefixOf` s = f (drop 5 s) || f (drop 6 s)
  | "erase" `isPrefixOf` s = f (drop 5 s)
  | "eraser" `isPrefixOf` s = f (drop 6 s)
  | otherwise = False