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<cstdio>
using namespace std;

int w, h;
char d[20][25];
bool wasOn[20][20];
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

int solve(int x, int y) {
  if (x < 0 || y < 0 || x >= w || y >= h) return 0;
  if (d[y][x] == '#') return 0;
  if (wasOn[y][x]) return 0;
  wasOn[y][x] = true;
  int ans = 1;
  for (int i = 0; i < 4; i++) ans += solve(x + dx[i], y + dy[i]);
  return ans;
}

int main() {
  while (1) {
    scanf("%d%d", &w, &h);
    if (w == 0) break;
    for (int i = 0; i < h; i++) scanf("%s", d[i]);
    int x, y;
    for (int i = 0; i < h; i++) {
      for (int j = 0; j < w; j++) {
        wasOn[i][j] = false;
        if (d[i][j] == '@') y = i, x = j;
      }
    }
    printf("%d\n", solve(x, y));
  }
}

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

char S[22][22];
int count = 0;

int dx[4] = {0,0,-1,1};
int dy[4] = {-1,1,0,0};

void a(int i, int j){
	if(S[i][j] == '.'){
		count++;
		S[i][j] = '#';
		for(int k = 0; k < 4; k++){
			a(i + dy[k],j + dx[k]);
		}
		//a(i - 1, j);
		//a(i + 1, j);
		//a(i, j + 1);
		//a(i, j - 1);
	}
}


int main(){
	while(true){
		memset(S,'#',sizeof(S));
		count = 0;
		int x, y;
		cin >> x >> y;
		if(x == 0 && y == 0){
			break;
		}
		for(int i = 1; i < y + 1; i++){
			for(int j = 1; j < x + 1; j++){
				cin >> S[i][j];
			}
		}

		int startx, starty;
		for(int i = 0; i < y; i++){
			for(int j = 0; j < x; j++){
				if(S[i][j] == '@'){
					startx = j;
					starty = i;
					S[i][j] = '.';
					break;
				}
			}
		}
		a(starty, startx);
		cout << count << endl;
	}
}

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

int n, m;
char map[20][20];
int ans = 0;

int dfs(int x, int y){
	if(0 <= x && x < m && 0 <= y && y < n){
		if(map[x][y] != '.') return 0;
		map[x][y] = '@';
		dfs(x-1, y);
		dfs(x+1, y);
		dfs(x, y-1);
		dfs(x, y+1);
	}
	return 0;
}

int main(){
	while(1){
		for(int i=0; i<20; i++){
			for(int j=0; j<20; j++){
				map[i][j] = '#';
			}
		}
		scanf("%d %d", &n, &m);
		if(n == 0 && m == 0) break;
		for(int i=0; i<m; i++){
			scanf("%s", map[i]);
		}
		int x, y;
		for(int i=0; i<m; i++){
			for(int j=0; j<n; j++){
				if(map[i][j] == '@'){
					x = i;
					y = j;
				}
			}
		}
		dfs(x-1, y);
		dfs(x+1, y);
		dfs(x, y-1);
		dfs(x, y+1);
		for(int i=0; i<m; i++){
			for(int j=0; j<n; j++){
				if(map[i][j] == '@') ans++;
			}
		}
		printf("%d\n", ans);
		ans = 0;
	}
	return 0;
}

C++: #include <iostream>
#include <queue>
#include <utility>
using namespace std;
typedef pair<int, int> P;

int res,W,H,tile[20][20];
P start;

int bfs()
{
	int res = 1;
	queue<P> a;
	a.push(start);
	while (!a.empty()) {
		P temp = a.front();
		a.pop();
		if (temp.first + 1 < W && tile[temp.first + 1][temp.second]) {
			tile[temp.first + 1][temp.second] = 0;
			res++;
			a.push(P(temp.first + 1, temp.second));
		}
		if (temp.first - 1 >= 0 && tile[temp.first - 1][temp.second]) {
			tile[temp.first - 1][temp.second] = 0;
			res++;
			a.push(P(temp.first - 1, temp.second));
		}
		if (temp.second + 1 < H && tile[temp.first][temp.second + 1]) {
			tile[temp.first][temp.second + 1] = 0;
			res++;
			a.push(P(temp.first, temp.second + 1));
		}
		if (temp.second - 1 >= 0 && tile[temp.first][temp.second - 1]) {
			tile[temp.first][temp.second - 1] = 0;
			res++;
			a.push(P(temp.first, temp.second - 1));
		}
	}
	return res;
}

int main()
{
	char ch;
	while (cin >> W >> H, W || H) {
		for (int i = 0; i < H; i++) {
			for (int j = 0; j < W; j++) {
				cin >> ch;
				if (ch == '.') {
					tile[j][i] = 1;
				}
				else if (ch == '#') {
					tile[j][i] = 0;
				}
				else {
					tile[j][i] = 0;
					start = P(j,i);
				}
			}
		}
		cout << bfs() << endl;
	}
	return 0;
}

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

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

void f(int x, int y){
	tile[x][y] = '#';
	cnt++;
	int X = 0;
	int Y = 0;
	for (int i = 0; i < 4; i++){
		if (tile[x + dx[i]][y + dy[i]] == '.' && 0 <= x && x < w && 0 <= y && y < h){
			X = x + dx[i];
			Y = y + dy[i];
			f(X, Y);
		}
	}
}

int main(){

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

		for (int y = 0; y < h; y++){
			for (int x = 0; x < w; x++){
				if (tile[x][y] == '@') f(x, y);
			}
		}

		cout << cnt << endl;
		cnt = 0;
	}

	return 0;
}