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<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;
}

C++: //??¨??¢?´¢:??±???????????¢?´¢??????????????¢?´¢ 

//??????????????¢?????¬?????????
 
//?????°??¢??°???????????????(?????????)????????\???(?????????)
#include <stdio.h>
#include <queue>
#include <string.h>
using namespace std;	
int W,H;
char c[101][101];
bool visited[100][100];
 
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
 	
void compute();//????????????????????£?¨? 
	
int main(){
	
	while(scanf("%d %d",&W,&H),W |H){
	    int i;
		
		for(i=0;i<H;i++){
			scanf(" %s",&c[i][0]);
		}
compute();
	}
	return 0;
}

void compute(){
	
	memset(visited,false,sizeof visited);  //{????????????????????????1bit??????????????????1bit????????§?????????} 
	
	queue<int> que;
	int i,j;
	for(j=0;j<W;j++){ 
	 for(i=0;i<H;i++){
	  if(c[i][j]=='@'){
	  	que.push(j+i*H);
	  	visited[i][j]=true;
	  	 break;
	  }
	 }
	}
  int answer=1;
  while(!que.empty()){
	int cur=que.front();
	        que.pop();
	        int x=cur%H;
	        int y=cur/H;
    

  for(i=0;i<4;i++){
	int nx=x+dx[i],ny=y+dy[i];
	
  
  
  if(!(0<=nx&&nx<W&&0<=ny&&ny<H))continue;
  if(c[ny][nx]=='#'||visited[ny][nx])continue;

  que.push(nx+ny*H);
  visited[ny][nx]=true;
++answer;
    }
   }
 //cout << answer << endl;
 printf("%d\n",answer);
 }

C++: #include<iostream>
#include<cstdio>
using namespace std;
char field[22][22];
int cnt=0;
void tansaku(int y,int x){
	if(field[y][x+1]=='.'){
		field[y][x+1]='#';
		cnt++;
		tansaku(y,x+1);
	}
	if(field[y][x-1]=='.'){
		field[y][x-1]='#';
		cnt++;
		tansaku(y,x-1);
	}
	if(field[y+1][x]=='.'){
		field[y+1][x]='#';
		cnt++;
		tansaku(y+1,x);
	}
	if(field[y-1][x]=='.'){
		field[y-1][x]='#';
		cnt++;
		tansaku(y-1,x);
	}
}
int main(){
	while(1){
		int w,h;
		cnt=0;
		cin >> w >> h;
		if(!w&&!h)	break;
		for(int i=1;i<=h;i++)
			scanf("%s",(field[i]+1));
		for(int i=0;i<=w+1;i++){
			field[0][i]='#';
			field[h+1][i]='#';
		}
		for(int i=0;i<=h+1;i++){
			field[i][0]='#';
			field[i][w+1]='#';
		}
		for(int y=1;y<=h;y++){
			for(int x=1;x<=w;x++){
				if(field[y][x]=='@'){
					tansaku(y,x);
					break;
				}
			}
		}
		cout << cnt+1 << endl;
	}
}

C++: #include <iostream>
using namespace std;
void func(int x,int y);
int h,w;
char a[111][111];
int dx[4]={0,-1,0,1};
int dy[4]={-1,0,1,0};
int nx,ny;
int cnt=0;
int main(){
  int fx,fy;
  while(1){
    cin >> w >> h;
    if(h==0 && w==0)break;
    for(int i=1;i<=h;i++){
      for(int j=1;j<=w;j++){
	cin >> a[i][j];
	if(a[i][j]=='@'){
	  fx=i;
	  fy=j;
	}
      }
    }
    func(fx,fy);
    cout << cnt << endl;
    cnt=0;
  }
}


void func(int x,int y){
  cnt++;
  a[x][y]='0';
  for(int i=0;i<4;i++){
    nx=dx[i]+x;
    ny=dy[i]+y;
    if(nx<=h && nx>=1 && ny<=w && ny>=1 && a[nx][ny]!='#' && a[nx][ny]!='0'){
      func(nx,ny);
    }
  }
}

C++: 
#include <iostream>

using namespace std;

int w, h;
string f[20];
int cnt;

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

void rec(int x, int y) {
  f[y][x] = '#';
  cnt++;

  for (int i=0; i<4; i++) {
    int mx = x + dx[i];
    int my = y + dy[i];

    if (mx < 0 || w <= mx) { continue; }
    if (my < 0 || h <= my) { continue; }
    if (f[my][mx] != '.') { continue; }

    rec(mx, my);
  }
}

int main() {
  while (cin >> w >> h, (w||h)) {
    for (int i=0; i<h; i++) {
      cin >> f[i];
    }

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

    cout << cnt << endl;
  }

  return 0;
}