Go: package main

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

var strByColor = map[Color]string{
	ColorBlack: "black",
	ColorRed:   "red",
}

func (c Color) String() string {
	return strByColor[c]
}

type Node struct {
	ID      int
	X, Y    int
	Color   Color
	NextIDs map[int]struct{}
}

func (n *Node) String() string {
	d, _ := json.Marshal(n)
	return string(d)
}

type Map struct {
	Width, Height int
	Nodes         map[int]*Node
}

func (m *Map) GetByID(id int) *Node {
	return m.Nodes[id]
}

func (m *Map) Get(x, y int) *Node {
	return m.Nodes[m.getID(x, y)]
}

func (m *Map) SetColor(x, y int, c Color) {
	m.Nodes[m.getID(x, y)].Color = c
}

func (m *Map) getID(x, y int) int {
	return x + y*m.Width
}

func (m *Map) init() {
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			id := m.getID(x, y)
			m.Nodes[id] = &Node{ID: id, X: x, Y: y}
		}
	}
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			nextIDs := map[int]struct{}{}
			if x > 1 {
				nextIDs[m.getID(x-1, y)] = struct{}{}
			}
			if x < m.Width {
				nextIDs[m.getID(x+1, y)] = struct{}{}
			}
			if y > 1 {
				nextIDs[m.getID(x, y-1)] = struct{}{}
			}
			if y < m.Height {
				nextIDs[m.getID(x, y+1)] = struct{}{}
			}
			m.Get(x, y).NextIDs = nextIDs
		}
	}
}

func NewMap(w, h int) *Map {
	m := &Map{Width: w, Height: h, Nodes: map[int]*Node{}}
	m.init()
	return m
}

type Searcher struct {
	Map        *Map
	queue      []*Node
	touchedIDs map[int]struct{}
	visitedIDs map[int]struct{}
}

func NewSearcher(m *Map) *Searcher {
	return &Searcher{
		Map:        m,
		queue:      []*Node{},
		touchedIDs: map[int]struct{}{},
		visitedIDs: map[int]struct{}{},
	}
}

func (s *Searcher) Search(start *Node) {
	s.enqueue(start)

	for {
		if len(s.queue) == 0 {
			break
		}
		for nextID := range s.dequeue().NextIDs {
			n := s.Map.GetByID(nextID)
			if n.Color == ColorRed || s.hasTouched(n) {
				continue
			}
			s.enqueue(n)
		}
	}
}

func (s *Searcher) VisitedNodeCount() int {
	return len(s.visitedIDs)
}

func (s *Searcher) hasTouched(n *Node) bool {
	_, ok := s.touchedIDs[n.ID]
	return ok
}

func (s *Searcher) enqueue(n *Node) {
	s.queue = append(s.queue, n)
	s.touchedIDs[n.ID] = struct{}{}
}

func (s *Searcher) dequeue() *Node {
	n := s.queue[0]
	s.queue = s.queue[1:]
	s.visitedIDs[n.ID] = struct{}{}
	return n
}

func main() {
	for sc.Scan() {
		if sc.Text() == "0 0" {
			break
		}
		chunks := strings.Split(sc.Text(), " ")
		w, _ := strconv.Atoi(chunks[0])
		h, _ := strconv.Atoi(chunks[1])

		m := NewMap(w, h)

		// Set colors
		var start *Node
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					m.SetColor(i+1, y, ColorBlack)
					if c == '@' {
						start = m.Get(i+1, y)
					}
				case '#':
					m.SetColor(i+1, y, ColorRed)
				}
			}
		}

		// Search
		s := NewSearcher(m)
		s.Search(start)
		fmt.Println(s.VisitedNodeCount())
	}
}

C++: #include <iostream>
#include <queue>
#include <array>

const int kSize = 20 + 2;

std::array<int, 4> ways = {-1, -kSize, +1, +kSize};

inline int xy2pos(const int x, const int y){
    return x + y * kSize;
}

std::array<bool, kSize*kSize> Bfs(const std::array<bool, kSize*kSize> &tiles, int pos){
    std::array<bool, kSize*kSize> arrived;
    std::queue<int> q;
    q.push(pos);
    std::fill(arrived.begin(), arrived.end(), false);
    while(!q.empty()){
        pos = q.front();
        q.pop();
        arrived[pos] = true;
        for(int way : ways)
            if(tiles.at(pos + way) && !arrived.at(pos + way))
                q.push(pos + way);
    }
    return arrived;
}

int main(){
    int w,h;
    std::array<bool,kSize*kSize> tiles; ///< true if black
    while((std::cin >> w >> h) && w != 0){
        std::fill(tiles.begin(), tiles.end(), false);
        int pos;
        for(int y=1; y<=h; ++y){
            for(int x=1; x<=w; ++x){
                char c;
                std::cin >> c;
                tiles[xy2pos(x,y)] = (c == '.');
                if(c == '@'){
                    pos = xy2pos(x,y);
                    tiles[pos] = true;
                }
            }
        }
        auto arrived = Bfs(tiles, pos);
        int sum = 0;
        for(bool v : arrived)
            sum += (v) ? 1 : 0;
        std::cout << sum << std::endl;
    }
}

C++: #include<iostream>
#include<algorithm>

using namespace std;

char tyle[30][30]={'0'};

int find_tyle(int i,int j){
  if(tyle[i+1][j]=='.'){
    tyle[i+1][j]='a';
    find_tyle(i+1,j);
  }
  if(tyle[i][j+1]=='.'){
    tyle[i][j+1]='a';
    find_tyle(i,j+1);
  }
  if(tyle[i-1][j]=='.'){
    tyle[i-1][j]='a';
    find_tyle(i-1,j);
  }
  if(tyle[i][j-1]=='.'){
    tyle[i][j-1]='a';
    find_tyle(i,j-1);
  }
}

int main(){
  
  int w,h;
  
  while(cin>>w>>h,w){
    int ans=0;
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
      cin>>tyle[i][j];
      }
    }
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
	if(tyle[i][j]=='@'){
	  find_tyle(i,j);
	  tyle[i][j]='a';
	}
      }
    }
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
	if(tyle[i][j]=='a'){
	  ans++;
      }
    }
  }
    cout<<ans<<endl;

   for(int i=0;i<30;++i){
      for(int j=0;j<=30;++j){
      tyle[i][j]='0';
      }
    }
    
  }
}

C++: #include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < n; i++)
using namespace std;
typedef long long ll;
char maze[25][25];
bool visited[25][25];
int dx[4] = {1,0,-1,0};
int dy[4] = {0,1,0,-1};
int W,H,ans;

void init()
{
	rep(i,25)
	{
		rep(j,25)
		{
			visited[i][j] = false;
		}
	}
}

void dfs(int x, int y)
{
	visited[x][y] = true;
	ans++;
	for (int i = 0; i < 4; ++i)
	{
		int x2 = x + dx[i];
		int y2 = y + dy[i];

		if (x2 >= 0 && x2 < H && y2 >= 0 && y2 < W)
		{
			if (!visited[x2][y2] && maze[x2][y2] != '#')
			{
				dfs(x2,y2);
			}
		}
	}
}

int main()
{
	while(1)
	{
		ans = 0;
		int si,sj;
		cin >> W >> H;
		if (W == 0 && H == 0) return 0;
		rep(i,H)
		{
			rep(j,W)
			{
				cin >> maze[i][j];
				if (maze[i][j] == '@')
				{
					si = i;
					sj = j;
				}
			}
		}

		init();

		dfs(si,sj);

		cout << ans << endl;
	}
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int mod = 1e9 + 7;
const int inf = (1 << 30) - 1;
const ll infll = (1LL << 61) - 1;
#define fast() ios::sync_with_stdio(false), cin.tie(0), cout.tie(0)
#define digit(N) cout << fixed << setprecision((N))

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

int main()
{
    while (cin >> H >> W)
    {
        if (H == 0 && W == 0)
            return 0;
        vector<string> grid(W);

        pair<int, int> s;
        for (int i = 0; i < W; i++)
        {
            cin >> grid[i];
            for (int j = 0; j < H; j++)
            {
                if (grid[i][j] == '@')
                    s = {i, j};
            }
        }

        queue<pair<int, int>> que;
        que.push(s);

        vector<vector<int>> visited(W, vector<int>(H));

        visited[s.first][s.second] = 1;

        while (!que.empty())
        {
            auto p = que.front();
            que.pop();

            visited[p.first][p.second] = 1;
            for (int i = 0; i < 4; i++)
            {
                int nx = p.first + dx[i], ny = p.second + dy[i];
                if (nx < 0 || nx >= W)
                    continue;
                if (ny < 0 || ny >= H)
                    continue;
                if (visited[nx][ny])
                    continue;
                if (grid[nx][ny] == '.')
                    que.push({nx, ny});
            }
        }

        int ans = 0;

        for (int i = 0; i < W; i++)
        {
            for (int j = 0; j < H; j++)
            {
                ans += visited[i][j];
            }
        }

        cout << ans << endl;
    }
}