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++: /*
 * http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=1130&lang=jp
 */


#include <iostream>
#include <limits>

constexpr size_t MAX_HEIGHT = 20;
const char START_TILE = '@';
const char PASSABLE_TILE = '.';
const char PROHIBITED_TILE = '#';
const char PASSED_TILE = 'P';

bool is_wall(std::string world[MAX_HEIGHT], size_t x, size_t y) {
	char tile = world[y][x];
	return tile == PROHIBITED_TILE || tile == PASSED_TILE;
}

bool valid(std::string world[MAX_HEIGHT], size_t width, size_t height, size_t x, size_t y) {
	return x < width && y < height // x and y is unsigned
		   && !is_wall(world, x, y);
}

int walk(std::string world[MAX_HEIGHT], size_t width, size_t height, size_t x, size_t y) {
	if (!valid(world, width, height, x, y)) {
		return 0; // ????????????????????\?????????????£?or??¢?´¢??????
	}
	int total = 1; // ????????????????????\?????????1?????¨??????????????????
	world[y][x] = PASSED_TILE; // ??????????????¨????????????????????¨?????????????????????????????????
	// ?????????????????§??????????????????valid???????????????walk????????§
	total += walk(world, width, height, x - 1, y);
	total += walk(world, width, height, x + 1, y);
	total += walk(world, width, height, x, y - 1);
	total += walk(world, width, height, x, y + 1);
	return total;
}

void process(size_t width, size_t height) {
	std::string world[MAX_HEIGHT];
	size_t x = std::numeric_limits<size_t>::max(), y = std::numeric_limits<size_t>::max();
	for (size_t i = 0; i < height; ++i) {
		std::cin >> world[i];
		size_t start_index = world[i].find(START_TILE);
		if (start_index != std::string::npos) {
			x = start_index;
			y = i;
		}
	}

	std::cout << walk(world, width, height, x, y);
}

int main() {
	size_t width, height;
	while (true) {
		std::cin >> width >> height;
		if (width == 0 || height == 0) {
			break;
		}
		else {
			process(width, height);
		}
	}
}

C++: #include <iostream>

#define MAX_WH 20

using namespace std;

void solve();
void dfs(int x, int y);

int W,H,count;
char field[MAX_WH][MAX_WH+1];

int main()
{
    while (1) {
        cin>>W>>H;
        if (W==0&&H==0) break;
        for (int i=0; i<H; i++) cin>>field[i];
        solve();
    }
    return 0;
}

void solve()
{
    for (int i=0; i<H; i++) {
        for (int j=0; j<W; j++) {
            if (field[i][j]=='@') {
                count=1;
                dfs(i,j);
                cout<<count<<endl;
                return ;
            }
        }
    }
}

void dfs(int x, int y)
{
    field[x][y]='#';
    
    
    if (x-1>=0&&field[x-1][y]=='.') {
        count++;
        dfs(x-1,y);
    }
    if (y-1>=0&&field[x][y-1]=='.') {
        count++;
        dfs(x,y-1);
    }
    if (x+1<H&&field[x+1][y]=='.') {
        count++;
        dfs(x+1,y);
    }
    if (y+1<W&&field[x][y+1]=='.') {
    	count++;
        dfs(x,y+1);
    }
}

C++: #include <iostream>
using namespace std;
int a=0;
int box[20][20];
int  kazoe(int p,int P){
  a++;
  box[p][P]=0;
  if(box[p][P+1]==1) kazoe(p,P+1);
  if(box[p][P-1]==1) kazoe(p,P-1);
  if(box[p-1][P]==1) kazoe(p-1,P);
  if(box[p+1][P]==1) kazoe(p+1,P);
  return a;
}
int main(){
  int i,I,H,W,p,P;
  
  char v;
  while(1){
  cin>>W>>H;
  if(W==0&&H==0) break;
  for(i=0;i<H;i++){
    for(I=0;I<W;I++){
      cin>>v;
      if(v=='.') box[i][I]=1;
      if(v=='#') box[i][I]=2;
      if(v=='@') {
	box[i][I]=0;
	p=i;
	P=I;
      }
    }
  }
  cout<<kazoe(p,P)<<endl;
  a=0;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
typedef std::pair<int, int> P;

int W = 0;
int H = 0;

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

void display(std::string *tile){
  for(int i = 0; i < H; i++){
    for(int j = 0; j < W; j++){
      std::cout << tile[i][j] << " ";
    }
    std::cout << std::endl;
  }
  std::cout << std::endl;
}

P getStart(std::string *tile){
  P start;
  bool flag = false;
  for(int i = 0; i < H; i++){
    for(int j = 0; j < W; j++){
      if(tile[i][j] == '@'){
        start.first = i;
        start.second = j;
        flag = true;
        break;
      }
      if(flag) break;
    }
  }
  return start;
}

int main(){
  while(1){
    std::cin >> W >> H;
    if(W + H == 0) break;
    std::vector<std::string> tile(H);
    for(int i = 0; i < H; i++){
      std::cin >> tile[i];
    }

    //display(tile.data());
    P s = getStart(tile.data());

    std::vector<std::vector<bool> > visited(H, std::vector<bool>(W, false));

    std::queue<P> que;
    que.push(s);
    int ans = 1;
    while(que.size()){
      P now = que.front();que.pop();
      for(int i = 0; i < 4; i++){
        int nx = now.second + dx[i], ny = now.first + dy[i];
        if(0 <= nx && nx < W && 0 <= ny && ny < H && !visited[ny][nx] && tile[ny][nx] == '.'){
          que.push(P(ny, nx));
          visited[ny][nx] = true;
          ans++;
        }
      }
    }
    
    std::cout << ans << std::endl;
    
  }
  
  return 0;
}