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 (4.10.0) (judge verdict: Accepted)

Scanf.scanf "%s" (fun s ->
    let n = String.length s in

    let rec loop i =
        if i = n then "YES" else
            if n - i >= 7 && String.sub s i 7 = "dreamer" then (
                if n - i = 7 then "YES" else
                    if s.[i + 7] = 'a' then loop (i + 5) else loop (i + 7)
            ) else
            if n - i >= 6 && String.sub s i 6 = "eraser"  then loop (i + 6) else
            if n - i >= 5 && String.sub s i 5 = "dream"   then loop (i + 5) else
            if n - i >= 5 && String.sub s i 5 = "erase"   then loop (i + 5) else
                "NO"
    in
    loop 0 |> print_endline
)