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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    static class CDaydream
    {        
        static void Main(string[] args)
        {
            const string DREAM = "dream";
            const string DREAMER = "dreamer";
            const string ERASE = "erase";
            const string ERASER = "eraser";
            
            var s = Console.ReadLine();
            string reverseS = s.ReverseString();

            while (reverseS.Length > 0)
            {
                if (reverseS.SafeSubstring(0, DREAM.Length) == DREAM.ReverseString())
                {
                    reverseS = reverseS.Substring(DREAM.Length);
                    continue;
                }
                if (reverseS.SafeSubstring(0, DREAMER.Length) == DREAMER.ReverseString())
                {
                    reverseS = reverseS.Substring(DREAMER.Length);
                    continue;
                }
                if (reverseS.SafeSubstring(0, ERASE.Length) == ERASE.ReverseString())
                {
                    reverseS = reverseS.Substring(ERASE.Length);
                    continue;
                }
                if (reverseS.SafeSubstring(0, ERASER.Length) == ERASER.ReverseString())
                {
                    reverseS = reverseS.Substring(ERASER.Length);
                    continue;
                }
                Console.WriteLine("NO");
                Console.ReadLine();
                return;
            }

            Console.WriteLine("YES");
            Console.ReadLine();
        }

        static string ReverseString(this string s)
        {
            return string.Join("", s.Reverse());
        }

        static string SafeSubstring(this string s, int start, int len)
        {
            if (s.Length < len)
            {
                return s;
            }
            return s.Substring(0, len);
        }

    }
}