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>

using namespace std;

#define MAX_H 22
#define MAX_W 22

int W, H, X, Y, A;

char tile[MAX_H][MAX_W];

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

void dfs(int x, int y)
{
	tile[y][x] = '@';

	for (int i = 0; i < 4; i++)
	{
		if (0 <= x + dx[i] && x + dx[i] <= W - 1 && 0 <= y + dy[i] && y + dy[i] <= H - 1)
		{
			if (tile[y + dy[i]][x + dx[i]] == '.')
			{
				dfs(x + dx[i], y + dy[i]);
			}
		}
	}
}

int main()
{
	while (true)
	{
		cin >> W >> H;

		if (W == 0 && H == 0) { break; }

		for (int i = 0; i < H; i++)
		{
			for (int j = 0; j < W; j++)
			{
				cin >> tile[i][j];

				if (tile[i][j] == '@')
				{
					X = j;
					Y = i;
				}
			}
		}

		dfs(X, Y);

		A = 0;

		for (int i = 0; i < H; i++)
		{
			for (int j = 0; j < W; j++)
			{
				if (tile[i][j] == '@')
				{
					A++;
				}
			}
		}

		cout << A << endl;
	}

	return 0;
}

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

int main(){
	int W=0,H=0;
	int posW=0,posH=0;
	int ans=0;
	int fposW=0,fposH=0;
	int n=0;

	while(1){
	cin >> W >> H;

	if(H==0&&W==0) return 0;

	char tile[20][20];

	for(int i=0;i<H;i++){
		for(int j=0;j<W;j++){
			cin >> tile[i][j];
		}
	}

	for(int i=0;i<H;i++){
		for(int j=0;j<W;j++){
			if(tile[i][j] == '@'){
				fposH = i;
				fposW = j;
			}
		}
	}
	posH = fposH;
	posW = fposW;

	for(int n=0;n<1000*W*H;n++){
		if(tile[posH][posW-1]=='.'){
			ans++;
			tile[posH][posW-1]='@';
			posW--;
		}
		else if(tile[posH-1][posW]=='.'){
			ans++;
			tile[posH-1][posW]='@';
			posH--;
		}
		else if(tile[posH][posW+1]=='.'){
			ans++;
			tile[posH][posW+1]='@';
			posW++;
		}
		else if(tile[posH+1][posW]=='.'){
			ans++;
			tile[posH+1][posW]='@';
			posH++;
		}
		else if(tile[posH+1][posW]=='@'&&n%4==0){
			posH++;
		}
		else if(tile[posH][posW+1]=='@'&&n%4==1){
			posW++;
		}
		else if(tile[posH-1][posW]=='@'&&n%4==2){
			posH--;
		}
		else if(tile[posH][posW-1]=='@'&&n%4==3){
			posW--;
		}
		if(n==(1000*H*W)/4){
			posH=fposH;
			posW=fposW;
		}
		if(n==(1000*H*W)/2){
			posH=fposH;
			posW=fposW;
		}if(n==(1000*H*W*3)/4){
			posH=fposH;
			posW=fposW;
		}
		
		}

	cout << ans+1 << endl;
	ans = 0;
	for(int i=0;i<H;i++){
		for(int j=0;j<W;j++){
			tile[i][j]='#';
		}
	}
	posH=0;
	posW=0;
	fposH=0;
	fposW=0;
	
	}

	return 0;
}

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

using namespace std;

int tile[22][22];
int w, h;
int cnt;

void fill(int x, int y) {
	tile[x][y] = 0;
	cnt++;
	if (tile[x - 1][y] == 1) fill(x - 1, y);
	if (tile[x][y - 1] == 1) fill(x, y - 1);
	if (tile[x][y + 1] == 1) fill(x, y + 1);
	if (tile[x + 1][y] == 1) fill(x + 1, y);
}

int main() {
	int x = 0, y = 0;
	while (cin >> w >> h && (w || h)) {
		cnt = 0;
		for (int i = 0; i < 22; ++i) for (int j = 0; j < 22; ++j) tile[i][j] = 0;
		for (int i = 1; i <= h; ++i){
			for (int j = 1; j <= w; ++j) {
				char tmp;
				cin >> tmp;
				if (tmp == '.') tile[i][j] = 1;
				else if (tmp == '#') tile[i][j] = 0;
				else x = i, y = j, tile[i][j] = 1;
			}
		}
		fill(x, y);
		cout << cnt << endl;
	}

	return 0;
}

C++: #include<iostream>
#include<cstdio>
using namespace std;
int tile(int i,int j);
char memo[22][22];
int w,h;
int ii,jj;

int main(){
  while(1){
    cin >> w >> h;
    if(w==0&&h==0)break;
    for(int i=0;i<h;i++){
      cin >> memo[i];
      for(int j=0;j<w;j++){
	if(memo[i][j]=='@'){
	  ii=i;
	  jj=j;
	}
      }
    }
    int c=tile(ii,jj);
    cout << c << endl;
  }
}

int tile(int i,int j){
  if(i>=h||j>=w||i<0||j<0)return 0;
  if(memo[i][j]=='#') return 0;
  memo[i][j]='#';
  int ret=0;
  ret+=tile(i,j-1);
  ret+=tile(i,j+1);
  ret+=tile(i-1,j);
  ret+=tile(i+1,j);
  return ret+1;
}