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: Time Limit Exceeded)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Text;
using static System.Math;
using static Extensions;
using static MathExtensions;

public static class Program
{
    public static void Solve()
    {
        var magics = new[] { "dream", "dreamer", "erase", "eraser" };
        var s = S;
        var memo = new HashSet<int>();
        var pi = new Queue<int>();
        pi.Enqueue(0);

        while (pi.Any())
        {
            var ci = pi.Dequeue();
            magics.ForEach((magic, i) =>
            {
                var r = Parser.String(magic)(new Parser.Input(s, ci));
                if (r.Succeeded)
                {
                    var ni = r.Rest.Index;
                    if (memo.Contains(ni)) return;
                    memo.Add(ni);
                    if (ni == s.Length) { Console.WriteLine("YES"); Exit(0); }
                    else pi.Enqueue(ni);
                }
            });
        }

        Console.WriteLine("NO");
    }

    #region Scanners
    static Scanner _scanner;
    static char C => _scanner.NextChar();
    static string S => _scanner.NextString();
    static int I => _scanner.NextInt();
    static long L => _scanner.NextLong();
    static BigInteger B => _scanner.NextBigInteger();
    static double D => _scanner.NextDouble();
    static decimal M => _scanner.NextDecimal();
    #endregion

    public static void Main()
    {
        var sw = new StreamWriter(Console.OpenStandardOutput());
        sw.NewLine = "\n";
#if DEBUG
        sw.AutoFlush = true;
#else
        sw.AutoFlush = false;
#endif
        Console.SetOut(sw);
        _scanner = new Scanner(Console.OpenStandardInput());
        Solve();
        Console.Out.Flush();
    }
}

public static partial class Extensions
{
}

#region Library
public class Scanner
{
    private readonly Stream _stream;
    private const int _bufferSize = 1024;
    private readonly byte[] _buf = new byte[_bufferSize];
    private int _len, _ptr;

    public Scanner(Stream stream)
    {
        _stream = stream;
    }

    public byte ReadByte()
    {
        if (_ptr >= _len)
        {
            _len = _stream.Read(_buf, 0, _bufferSize);
            _ptr = 0;
        }
        return _buf[_ptr++];
    }

    public char ReadChar() => (char)ReadByte();

    public string ReadLine()
    {
        var r = new StringBuilder();
        if (_ptr == 0) r.Append(ReadChar());
        for (; _ptr < _len; _ptr++) r.Append((char)_buf[_ptr]);
        return r.ToString();
    }

    public char NextChar() => char.Parse(NextString());

    public string NextString()
    {
        var r = new StringBuilder();
        var b = ReadChar();
        while (b != ' ' && b != '\n')
        {
            r.Append(b);
            b = ReadChar();
        }
        return r.ToString();
    }

    public int NextInt() => (int)NextLong();

    public long NextLong()
    {
        var r = 0L;
        var b = ReadByte();
        var n = b == '-';
        if (n) b = ReadByte();
        while (b != ' ' && b != '\n')
        {
            r = r * 10 + b - '0';
            b = ReadByte();
        }
        return n ? -r : r;
    }

    public BigInteger NextBigInteger()
    {
        var r = new BigInteger();
        var b = ReadByte();
        var n = b == '-';
        if (n) b = ReadByte();
        while (b != ' ' && b != '\n')
        {
            r = r * 10 + b - '0';
            b = ReadByte();
        }
        return n ? -r : r;
    }

    public double NextDouble()
    {
        var i = 0L;
        var b = ReadByte();
        var n = b == '-';
        if (n) b = ReadByte();
        while (b != '.' && b != ' ' && b != '\n')
        {
            i = i * 10 + b - '0';
            b = ReadByte();
        }
        if (b != '.') return n ? -i : i;
        b = ReadByte();
        var f = 0L;
        var p = 0;
        while (b != ' ' && b != '\n')
        {
            f = f * 10 + b - '0';
            b = ReadByte();
            p++;
        }
        var r = i + (double)f / MathExtensions.Pow(10, p);
        return n ? -r : r;
    }

    public decimal NextDecimal() => decimal.Parse(NextString());

    public T Next<T>(Converter<string, T> parser) => parser(NextString());
}

public delegate Parser.Result<T> Parser<T>(Parser.Input input);

public static class Parser
{
    public class Input
    {
        public Input(string text, int index)
        {
            this.Text = text;
            this.Index = index;
        }

        public string Text { get; }

        public int Index { get; }
    }

    public class Result<T>
    {
        public static Result<T> Success(T value, Input rest)
            => new Result<T> { Succeeded = true, Value = value, Rest = rest };

        public static Result<T> Failure(Input input)
            => new Result<T> { Rest = input };

        public bool Succeeded { get; private set; }

        public T Value { get; private set; }

        public Input Rest { get; private set; }
    }

    public static T Parse<T>(this Parser<T> parser, string s)
    {
        var result = parser(new Input(s, 0));
        if (!result.Succeeded) throw new InvalidOperationException("Parse failed.");
        return result.Value;
    }

    public static Parser<T2> Select<T1, T2>(this Parser<T1> parser, Func<T1, T2> selector) => input =>
    {
        var result = parser(input);
        return result.Succeeded
            ? Result<T2>.Success(selector(result.Value), result.Rest)
            : Result<T2>.Failure(result.Rest);
    };

    public static Parser<T3> SelectMany<T1, T2, T3>(this Parser<T1> parser, Func<T1, Parser<T2>> selector1, Func<T1, T2, T3> selector2) => input =>
    {
        var result = parser(input);
        if (!result.Succeeded) return Result<T3>.Failure(result.Rest);

        var parser2 = selector1(result.Value);
        var result2 = parser2(result.Rest);

        return result2.Succeeded
            ? Result<T3>.Success(selector2(result.Value, result2.Value), result2.Rest)
            : Result<T3>.Failure(result.Rest);
    };

    public static Parser<IEnumerable<T>> Many<T>(this Parser<T> parser) => input =>
    {
        var result = parser(input);
        var values = new List<T>();

        while (result.Succeeded)
        {
            values.Add(result.Value);
            result = parser(result.Rest);
        }

        return Result<IEnumerable<T>>.Success(values, result.Rest);
    };

    public static Parser<T> Option<T>(this Parser<T> parser, T @default) => input =>
    {
        var result = parser(input);
        var values = result.Succeeded ? result.Value : @default;
        return Result<T>.Success(values, result.Rest);
    };

    public static Parser<IEnumerable<T>> Repeat<T>(this Parser<T> parser, int count) => input =>
    {
        var result = parser(input);
        if (!result.Succeeded) return Result<IEnumerable<T>>.Failure(input);
        var values = new List<T> { result.Value };

        for (var i = 1; i < count && result.Succeeded; i++)
        {
            result = parser(result.Rest);
            values.Add(result.Value);
        }

        return result.Succeeded
            ? Result<IEnumerable<T>>.Success(values, result.Rest)
            : Result<IEnumerable<T>>.Failure(input);
    };

    public static Parser<T2> Then<T1, T2>(this Parser<T1> first, Parser<T2> second)
        => from f in first
           from s in second
           select s;

    public static Parser<T> Or<T>(this Parser<T> first, Parser<T> second) => input =>
    {
        var result = first(input);
        return result.Succeeded ? result : second(input);
    };

    public static Parser<IEnumerable<T>> Concat<T>(
        this Parser<IEnumerable<T>> first, Parser<IEnumerable<T>> second)
        => from f in first
           from s in second
           select f.Concat(s);

    public static Parser<T3> Concat<T1, T2, T3>(
        this Parser<T1> first, Parser<T2> second, Func<T1, T2, T3> selector)
        => from f in first
           from s in second
           select selector(f, s);

    public static Parser<IEnumerable<T>> Concat<T>(this IEnumerable<Parser<T>> parsers)
        => parsers.Aggregate(Return(Enumerable.Empty<T>()), (x, y) => x.Concat(y.Repeat(1)));

    public static Parser<string> AsString(this Parser<IEnumerable<char>> parser)
        => parser.Select(cs => new string(cs.ToArray()));

    public static Parser<T> Return<T>(T value) => input => Result<T>.Success(value, input);

    public static Parser<char> Char(Predicate<char> predicate) => input =>
    {
        if (input.Index >= input.Text.Length) return Result<char>.Failure(input);
        var c = input.Text[input.Index];
        return predicate(c)
            ? Result<char>.Success(c, new Input(input.Text, input.Index + 1))
            : Result<char>.Failure(input);
    };

    public static Parser<char> Char(char c) => Char(c_ => c_ == c);

    public static Parser<string> String(string s)
        => s.Select(Char).Concat().Select(x => new string(x.ToArray()));

    public static Parser<T> Integer<T>(Converter<string, T> converter)
        => from minus in String("-").Option("")
           from digits in Char(char.IsDigit).Many().AsString()
           select converter(minus + digits);
}

[DebuggerStepThrough]
public static partial class Extensions
{
    public static void Assert(bool condition)
    {
        if (!condition) throw new Exception("Assertion failed");
    }

    public static string AsString(this IEnumerable<char> source) => new string(source.ToArray());

    public static void Exit(int exitCode)
    {
        Console.Out.Flush();
        Environment.Exit(exitCode);
    }

    public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
    {
        foreach (var item in source) action(item);
    }

    public static void ForEach<T, _>(this IEnumerable<T> source, Func<T, _> func)
    {
        foreach (var item in source) func(item);
    }

    public static void ForEach<T>(this IEnumerable<T> source, Action<T, int> action)
    {
        var i = 0;
        foreach (var item in source) action(item, i++);
    }

    public static void ForEach<T, _>(this IEnumerable<T> source, Func<T, int, _> func)
    {
        var i = 0;
        foreach (var item in source) func(item, i++);
    }

    public static T Iterate<T>(int count, T seed, Func<T, T> func)
    {
        var r = seed;
        Repeat(count, () => { r = func(r); });
        return r;
    }

    public static void Repeat(int count, Action action)
    {
        for (var i = 0; i < count; i++) action();
    }

    public static void Repeat(int count, Action<int> action)
    {
        for (var i = 0; i < count; i++) action(i);
    }

    public static IEnumerable<T> Repeat<T>(Func<T> func)
    {
        for (var i = 0; ; i++) yield return func();
    }

    public static IEnumerable<T> Repeat<T>(int count, Func<T> func)
    {
        for (var i = 0; i < count; i++) yield return func();
    }

    public static IEnumerable<T> Repeat<T>(Func<int, T> func)
    {
        for (var i = 0; ; i++) yield return func(i);
    }

    public static IEnumerable<T> Repeat<T>(int count, Func<int, T> func)
    {
        for (var i = 0; i < count; i++) yield return func(i);
    }

    public static void Swap<T>(ref T x, ref T y)
    {
        var tmp = x; x = y; y = tmp;
    }
}

[DebuggerStepThrough]
public static class MathExtensions
{
    public static int Gcd(int left, int right)
    {
        int r;
        while ((r = left % right) != 0) { left = right; right = r; }
        return right;
    }

    public static long Gcd(long left, long right)
    {
        long r;
        while ((r = left % right) != 0L) { left = right; right = r; }
        return right;
    }

    public static BigInteger Gcd(BigInteger left, BigInteger right)
        => BigInteger.GreatestCommonDivisor(left, right);

    public static int HighestOneBit(int x)
    {
        x |= x >> 01;
        x |= x >> 02;
        x |= x >> 04;
        x |= x >> 08;
        x |= x >> 16;
        return x - (x >> 1);
    }

    public static long HighestOneBit(long x)
    {
        x |= x >> 01;
        x |= x >> 02;
        x |= x >> 04;
        x |= x >> 08;
        x |= x >> 16;
        x |= x >> 32;
        return x - (x >> 1);
    }

    public static int Lcm(int left, int right) => left / Gcd(left, right) * right;

    public static long Lcm(long left, long right) => left / Gcd(left, right) * right;

    public static BigInteger Lcm(BigInteger left, BigInteger right)
        => left / Gcd(left, right) * right;

    public static int Pow(int value, int exponent)
    {
        var r = 1;
        while (exponent > 0)
        {
            if ((exponent & 1) == 1) r *= value;
            value *= value;
            exponent >>= 1;
        }
        return r;
    }

    public static long Pow(long value, int exponent)
    {
        var r = 1L;
        while (exponent > 0)
        {
            if ((exponent & 1) == 1) r *= value;
            value *= value;
            exponent >>= 1;
        }
        return r;
    }

    public static long Fact(int value)
    {
        var r = 1L;
        for (var i = 2; i <= value; i++) r *= i;
        return r;
    }
}
#endregion