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 <bits/stdc++.h>
using namespace std;

int W, H;
char board[30][30];
bool visited[30][30];

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

void dfs(int x, int y) {
   for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= H || ny < 0 || ny >= W) continue;
      if (board[nx][ny] == '#') continue;
      if (visited[nx][ny]) continue;
      visited[nx][ny] = true;
      dfs(nx, ny);
   }
}

int main() {
   while (cin >> W >> H, W || H) {
      memset(visited, false, sizeof(visited));
      int sx = 0, sy = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            cin >> board[i][j];
            if (board[i][j] == '@') {
               sx = i, sy = j;
            }
         }
      }

      board[sx][sy] = true;
      dfs(sx, sy);

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

      cout << cnt << endl;
   }

   return 0;
}

C++: #include<iostream>
#include<string>
using namespace std;
int ans,w,h,f;
string m[25];
void solve(int x,int y){
  if(x<0 || y<0 || x>=h || y>=w) return;
  if(m[x][y]=='#') return;
  m[x][y]='#';
  ans++;
  solve(x+1,y);
  solve(x-1,y);
  solve(x,y+1);
  solve(x,y-1);
}
int main(void){
  for(;;){
    cin >> w >> h;
    if(!w && !h) break;
    for(int i=0;i<h;i++) cin >> m[i];
    ans=0;
    f=0;
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	if(m[i][j]=='@'){
	  solve(i,j);
	  f=1;
	  break;  
	}
      }
      if(f) break;
    }
    cout << ans << endl;
  }
}

C++: #include <iostream>
#include <stack>
#include <vector>
#include <map>

using namespace std;

void input_field(vector< vector< char > > &field, int w, int h);
int reach_tile_num(vector< vector< char > > &field);
pair< int, int> search_start(vector< vector< char > > &field);

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

int main(){
  int w, h;

  while(cin >> w >> h, w&&h){
    vector< vector< char > >field(h+2, vector< char >(w+2, 0));
    
    input_field(field, w+2, h+2);
    cout << reach_tile_num(field) << endl;
  }
}


void input_field(vector< vector< char > > &field, int w, int h){
  for(int y = 0; y < field.size(); y++){
    for(int x = 0; x < field[y].size(); x++){
      //cout << x << y << endl;
      if(x == 0 || y == 0
         || x == w-1 || y == h-1){
	field[y][x] = '#';
      }else{
	cin >> field[y][x];
      }
    }
  }
  //cout << "input done" << endl;
}

int reach_tile_num(vector< vector< char > > &field){
  int sum = 0;
  pair< int, int > start, cur;
  stack< pair< int, int > > s;

  start = search_start(field);
  s.push(start);

  while(!(s.empty())){
    cur = s.top();
    s.pop();
    if(field[cur.second][cur.first] == '.'
       || field[cur.second][cur.first] == '@'){
      sum++;
      for(int i = 0; i < 4; i++){
        s.push(make_pair(cur.first+dx[i], cur.second+dy[i]));
	field[cur.second][cur.first] = '#';
      }
    }
  }
  return sum;
}

pair< int, int> search_start(vector< vector< char > > &field){
  for(int y = 0; y < field.size(); y++){
    for(int x = 0; x < field[y].size(); x++){
      if(field[y][x] == '@') return make_pair(x,y);
    }
  }
}

C++: 
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<math.h>
#include<stack>
#include<queue>
#include<vector>
#include<algorithm>
#include<string>
#include<iostream>
#include<set>
#include<map>

using namespace std;
typedef long long ll;
int mod(ll a){
    int c=a%1000000007;
    if(c>=0)return c;
    else return c+1000000007;
}
typedef pair<int,int> i_i;
typedef pair<ll,ll> l_l;
#define inf 100000000/*10^8*/

/////////////////////////////////

int w,h;
int si,sj;

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

int room[25][25];
int ans=0;
int dfs(int x,int y){
    room[x][y]=2;ans++;
    for(int i=0;i<=3;i++){
        int xx=x+dx[i],yy=y+dy[i];
        if(0<=xx&&xx<=h-1&&0<=yy&&yy<=w-1&&room[xx][yy]==0)dfs(xx,yy);
    }
    return ans;
}


int main(){
    while(1){
        cin>>w>>h;
        if(w==0)break;
        ans=0;
        for(int i=0;i<=h-1;i++){
            char s[w+1];cin>>s;
            for(int j=0;j<=w-1;j++){
                if(s[j]=='.')room[i][j]=0;
                else if(s[j]=='#')room[i][j]=1;
                else if(s[j]=='@')room[i][j]=2,si=i,sj=j;
            }
        }
        cout<<dfs(si,sj)<<endl;
    }
    return 0;
}