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)

(* O(|s|) *)
let s = read_line ()
let i = ref @@ String.length s - 1
let ws = [|"dream"; "dreamer"; "erase"; "eraser"|]
let _ =
  while !i >= 0 do
    let f b w =
      if b then b
      else
        let m = String.length w in
        if String.sub s (max 0 @@ !i - m + 1) @@ m = w then (i := !i - m; true) else b in
    if not @@ Array.fold_left f false ws then (print_endline "NO"; exit 0) done;
  print_endline "YES"