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 <string>
#include <iostream>

using namespace std;

int w, h;
bool b[20][20];
bool ok[20][20];

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

void dfs(int x, int y){
	for(int i=0;i<4;i++){
		if(0<=x+dx[i]&&x+dx[i]<w&&0<=y+dy[i]&&y+dy[i]<h){
			if(!b[y+dy[i]][x+dx[i]]) continue;
			if(ok[y+dy[i]][x+dx[i]]) continue;
			ok[y+dy[i]][x+dx[i]] = true;
			dfs(x+dx[i],y+dy[i]);
		}
	}
}

int main(){
	while(cin >> w >> h){
		if(!w) break;
		int x, y;
		string s;
		memset(b,false,sizeof(b));
		for(int i=0;i<h;i++){
			cin >> s;
			for(int j=0;j<w;j++){
				b[i][j] = (s[j] !='#');
				if(s[j]=='@') x = j, y = i;
			}
		}
		memset(ok,false,sizeof(ok));
		ok[y][x] = true;
		dfs(x,y);
		int count = 0;
		for(int i=0;i<h;i++)
			for(int j=0;j<w;j++)
				count += ok[i][j];
		cout << count << endl;
	}
}

C++: #include <iostream>
using namespace std;

int ans;
int w, h;
char map[20][20];
int dx[4] = { 1, 0, -1, 0 };
int dy[4] = { 0, 1, 0, -1 };

void solve(int x, int y)
{
	ans++;
	map[x][y] = '#';
	for (int i = 0; i < 4; i++)
	{
		int X = x + dx[i];
		int Y = y + dy[i];
		if (0 <= X && X < w && 0 <= Y && Y < h && map[X][Y] != '#')
		{
			solve(X, Y);
		}
	}
}

int main()
{
	while (cin >> w >> h, w + h)
	{
		ans = 0;
		for (int y = 0; y < h; y++)
		{
			for (int x = 0; x < w; x++)
			{
				cin >> map[x][y];
			}
		}

		for (int y = 0; y < h; y++)
		{
			for (int x = 0; x < w; x++)
			{
				if (map[x][y] == '@') solve(x, y);
			}
		}
		cout << ans << endl;
	}

	return 0;
}

C++: #include <iostream>
using namespace std;

const int NUM = 7;

void grooping(int y, int x, int w, int h, char** room, unsigned long*** group, unsigned long* next)
{
	if (room[y][x] != '#') {
		for (int i = y - 1; i <= y + 1; i++) {
			for (int j = x - 1; j <= x + 1; j++) {
				if (i < 0 || j < 0 || i >= h || j >= w) {
					continue;
				}
				if ((i == y && j != x) || (i != y && j == x)) {
					for (int k = 0; k < NUM; k++) {
						group[y][x][k] |= group[i][j][k];
					}
				}
			}
		}
		for (int i = 0; i < NUM; i++) {
			if (group[y][x][i] == 0UL && next[i] != 0UL) {
				group[y][x][i] |= next[i];
				next[i] <<= 1;
				break;
			}
		}
	}
}

int main()
{
	int w, h;
	while (true) {
		cin >> w >> h;
		if (w == 0 && h == 0) {
			break;
		}

		unsigned long next[NUM] = { 1UL, 1UL, 1UL, 1UL, 1UL, 1UL, 1UL };
		char** room = new char*[h];
		unsigned long*** group = new unsigned long**[h];
		int x, y;
		for (int i = 0; i < h; i++) {
			room[i] = new char[w];
			group[i] = new unsigned long*[w];
			for (int j = 0; j < w; j++) {
				cin >> room[i][j];
				if (room[i][j] == '@') {
					x = j;
					y = i;
				}
				group[i][j] = new unsigned long[NUM];
				for (int k = 0; k < NUM; k++) {
					group[i][j][k] = 0UL;
				}
			}
		}

		for (int k = 0; k < 500; k++) {
			for (int i = 0; i < h; i++) {
				for (int j = 0; j < w; j++) {
					grooping(i, j, w, h, room, group, next);
				}
			}
			for (int i = h - 1; i >= 0; i--) {
				for (int j = w - 1; j >= 0; j--) {
					grooping(i, j, w, h, room, group, next);
				}
			}
			for (int j = 0; j < w; j++) {
				for (int i = 0; i < h; i++) {
					grooping(i, j, w, h, room, group, next);
				}
			}
			for (int j = w - 1; j >= 0; j--) {
				for (int i = h - 1; i >= 0; i--) {
					grooping(i, j, w, h, room, group, next);
				}
			}
		}

		int count = 0;
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				for (int k = 0; k < NUM; k++) {
					if ((group[i][j][k] & group[y][x][k]) != 0UL) {
						count++;
						break;
					}
				}
			}
		}

		cout << count << '\n';

		for (int i = 0; i < h; i++) {
			delete[] room[i];
			for (int j = 0; j < w; j++) {
				delete[] group[i][j];
			}
			delete[] group[i];
		}
		delete[] room;
		delete[] group;
	}

	return 0;
}