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 OCaml (4.02.3) (judge verdict: Accepted)

let hd_opt t =
  match t with
  | [] -> None
  | x :: _ -> Some x

open Batteries
let () =
  let s = Scanf.scanf "%s " (fun a -> a) in
  let rev_s = String.rev s in

  let dict = ["dream";"dreamer";"erase";"eraser"] in
  let rev_dict = List.map (fun x -> String.rev x) dict in

  let rec aux s = match s with
      "" -> "YES"
    | s -> 
      let res = List.map (fun dict_s -> 
          let len = String.length dict_s in
          let target = String.slice ~last:len s in
          if dict_s = target then len else 0
        ) rev_dict |> List.filter (fun x -> x <> 0) in

      match (hd_opt res) with
      Some x -> aux (String.slice ~first:x s)
      | None -> "NO"
  in

  Printf.printf "%s\n" @@
  aux rev_s