0. Use DFS to find all vertices reachable from the start and exclude others from further steps.
1. Use O(m + n) algo.
2. [For C# coders] do not use List or SortedList or even Dictionary! And do not use foreach!
3. Use the fastest input parser you have. For example my parser looks like this:

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Text;

namespace _1450__acm.timus.ru_
{
    class SerialInput
    {
        private static Dictionary<int, int> separators =
            new Dictionary<int, int> { { ' ', 0 }, { '\n', 0 }, { '\r', 0 } };

        private StreamReader reader;

        public SerialInput(StreamReader reader)
        {
            this.reader = reader;
        }
        public int ReadInt()
        {
            while (separators.ContainsKey(reader.Peek()))
                reader.Read();
            int symbol = reader.Read();
            int value = 0;
            while ((symbol != -1) && !separators.ContainsKey(symbol))
            {
                value = value * 10 + (symbol - '0');
                symbol = reader.Read();
            }
            return value;
        }
    }
    class Program
    {
        static void Main()
        {
            const int InputBufferSize = 65536;
            const int OutputBufferSize = 65536;

#if (ONLINE_JUDGE)
            var input = new SerialInput(
                new StreamReader(Console.OpenStandardInput(InputBufferSize)));
            var output = new StreamWriter(Console.OpenStandardOutput(OutputBufferSize));
#else
            var input = new SerialInput(
                new StreamReader(new FileStream(@"..\..\input.txt",
                    FileMode.Open, FileAccess.Read, FileShare.Read,
                    InputBufferSize)));
            var output = new StreamWriter(new FileStream(@"..\..\output.txt",
                    FileMode.Create, FileAccess.Write, FileShare.Write,
                    OutputBufferSize));
#endif

            int n = input.ReadInt(), m = input.ReadInt();

            // write your code here

            output.Flush();
        }
    }
}