Problem:
Problem B: Red and Black

There is a rectangular room, covered with square tiles. Each tile is
colored either red or black. A man is standing on a black tile.
From a tile, he can move to one of four adjacent tiles. But he can't
move on red tiles, he can move only on black tiles.

Write a program to count the number of black tiles which he can reach
by repeating the moves described above.

Input

The input consists of multiple data sets.
A data set starts with a line containing two positive integers W and H;
W and H are the numbers of tiles in the x-
and y- directions, respectively. W and H are
not more than 20.

There are H more lines in the data set, each of which
includes W characters. Each character represents the color of a
tile as follows.

'.' - a black tile

'#' - a red tile

'@' - a man on a black tile(appears exactly once in a data set)

The end of the input is indicated by a line consisting of two zeros.

Output

For each data set, your program
should output a line which contains the number of
tiles he can reach from the initial tile (including itself).

Sample Input

6 9
....#.
.....#
......
......
......
......
......
#@...#
.#..#.
11 9
.#.........
.#.#######.
.#.#.....#.
.#.#.###.#.
.#.#..@#.#.
.#.#####.#.
.#.......#.
.#########.
...........
11 6
..#..#..#..
..#..#..#..
..#..#..###
..#..#..#@.
..#..#..#..
..#..#..#..
7 7
..#.#..
..#.#..
###.###
...@...
###.###
..#.#..
..#.#..
0 0

Output for the Sample Input

45
59
6
13

Solution in C# (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using static System.Math;
using static System.Array;
using static AtCoder.Tool;

namespace AtCoder
{
    class AC
    {
        const int mod = 1000000007;
        const int INF = int.MaxValue / 2;
        const long SINF = long.MaxValue / 2;

        static int[] dx = { 0, 0, 1, -1 };
        static int[] dy = { 1, -1, 0, 0 };

        static void Main(string[] args)
        {
            //var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            //Console.SetOut(sw);
            var cin = new Scanner();

            while (true)
            {
                var input = cin.SplitRead();
                if (input[0] == 0 && input[1] == 0) { return; }

                int W = input[0];
                int H = input[1];
                char[,] tile = new char[H + 2, W + 2];
                bool[,] seen = new bool[H + 2, W + 2];
                int x0 = -1;
                int y0 = -1;
                for(var i = 0; i < H + 2; i++)
                {
                    if (i == 0 || i == H + 1)
                    {
                        for(var j = 0; j < W + 2; j++)
                        {
                            seen[i, j] = false;
                            tile[i, j] = '#';
                        }
                        continue;
                    }
                    string s = Console.ReadLine();
                    for(var j = 0; j < W + 2; j++)
                    {
                        seen[i, j] = false;
                        tile[i, j] = j == 0 || j == W + 1 ? '#' : s[j - 1];
                        if (tile[i,j] == '@')
                        {
                            x0 = i;
                            y0 = j;
                        }
                    }
                }
                Dfs(tile, seen, x0, y0);
                long ans = 0;
                for(var i = 0; i < H + 2; i++)
                {
                    for(var j = 0; j < W + 2; j++)
                    {
                        ans += seen[i, j] ? 1 : 0;
                    }
                }

                Console.WriteLine(ans);

            }
            //Console.Out.Flush();
        }

        public static void Dfs(char[,] tile, bool[,] seen, int curx, int cury)
        {
            seen[curx, cury] = true;
            for(var i = 0; i < 4; i++)
            {
                int nextx = curx + dx[i];
                int nexty = cury + dy[i];
                if (seen[nextx, nexty] || tile[nextx, nexty] == '#')  { continue; }
                Dfs(tile, seen, nextx, nexty);
            }
            return;
        }

        
    }
    public class Scanner
    {
        public int[] SplitRead()
        {
            int[] array = Console.ReadLine().Split().Select(int.Parse).ToArray();
            return array;
        }
        public long[] SplitReadL()
        {
            long[] array = Console.ReadLine().Split().Select(long.Parse).ToArray();
            return array;
        }
    }

    public static class Tool
    {
        static public void Initialize<T>(ref T[] array, T initialvalue)
        {
            for (var i = 0; i < array.Length; i++)
            {
                array[i] = initialvalue;
            }
        }
        static public long Modulo(long value, long mod)
        {
            long r = value % mod;

            if (r * mod < 0)
            {
                r += mod;
            }
            return r;
        }
    }

    

}