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;

int vis[25][25];

void dfs(int i,int j){
  if(vis[i][j]==0 || vis[i][j]==2)return;
  vis[i][j]=0;

    dfs(i-1,j);
    dfs(i,j-1);
    dfs(i,j+1);
    dfs(i+1,j);
}

int main(){
  int w,h;
  int i,j;
  char ch;
  int cnt;

  while(1){
    cnt=0;
    cin >> w >> h;
    if(w==0 && h==0)break;

    for(i=1;i<=h;i++){
      for(j=1;j<=w;j++){
	cin >> ch;
	if(ch=='.'){
	  vis[i][j]=1;
	}else if(ch=='#'){
	  vis[i][j]=2;
	}else{
	  vis[i][j]=5;
	}
      }
    }
     for(i=1;i<=h;i++){
      for(j=1;j<=w;j++){
	if(vis[i][j]==5)dfs(i,j);
      }
     }
      for(i=1;i<=h;i++){
      for(j=1;j<=w;j++){
	if(vis[i][j]==0)cnt++;
      }
     }
      cout << cnt << endl;
  }
}

C++: #include <iostream>

using namespace std;

int w, h, ans;
char map[20][20];

void ex(int x, int y) {
  if(x < 0 || y < 0 || x >= w || y >= h) return;
  if(map[x][y] == '#') return;

  if(map[x][y] == '@' || map[x][y] == '.') {
    map[x][y] = ':';
    ans ++;
  }
  else {
    return;
  }
  
  ex(x-1, y);
  ex(x+1, y);
  ex(x, y-1);
  ex(x, y+1);
  
  return;
}

int main() {
  while(cin >> w >> h) {
    if(w == 0 && h == 0) break;
    char tmp;
    int s[2];
    for(int j = 0; j < h; j ++) {
      for(int i = 0; i < w; i ++) {
	cin >> tmp;
	map[i][j] = tmp;
	if(tmp == '@') {
	  s[0] = i;
	  s[1] = j;
	}
      }
    }
    
    ans = 0;
    //cout << s[0] << s[1] << endl;
    ex(s[0], s[1]);
    
    cout << ans << endl;
  }
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;

int main(){
    int w,h;
    while(1){
        cin>>w>>h;  if(w==0)break;
        char tile[h][w];    //タテｘヨコ
        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++)cout<<tile[i][j]<<" ";
//            cout<<endl;
//        }

        int ans=1;

        for(int a=0;a<w*h;a++){

            for(int i=0;i<h;i++){
                for(int j=0;j<w;j++){

                    if(tile[i][j]=='@'){
                        tile[i][j]='#';

                        if(j<w-1 && tile[i][j+1]=='.'){
                            ans++;
                            tile[i][j+1]='@';
                        }
                        if(j>0 && tile[i][j-1]=='.'){
                            ans++;
                            tile[i][j-1]='@';
                        }
                        if(i<h-1 && tile[i+1][j]=='.'){
                            ans++;
                            tile[i+1][j]='@';
                        }
                        if(i>0 && tile[i-1][j]=='.'){
                            ans++;
                            tile[i-1][j]='@';
                        }

//                        for(int b=0;b<h;b++){
//                            for(int c=0;c<w;c++)cout<<tile[b][c];
//                            cout<<endl;
//                        }
//                        cout<<endl;
                        break;

                    }
                }
            }
        }
        cout<<ans<<endl;
    }
    return 0;
}

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

int main(){
  int w, h;
  
  int qi[10000], qj[10000];
  for(;;){
     char m[30][30];
     for(int i=0; i<30; i++){
       for(int j=0; j<30; j++){
         m[i][j] = '#';
       }
     }
    cin >> w >> h;
    if(w==0 && h==0) break;
    for(int i=0; i<h; i++){
      string s;
      cin >> s;
      for(int j=0; j<w; j++){
        m[i+1][j+1] = s[j];
        if(s[j]=='@'){
          qi[0] = i+1;
          qj[0] = j+1;
          m[i+1][j+1] = '.';
        }
      }
    }
    int nl=0, nr=1;
    int count = 0;
    while(nr-nl>0){
      int i = qi[nl];
      int j = qj[nl];
      if(m[i][j]=='@'){
        nl++;
      }else{
        m[i][j] = '@';
        count++;
        if(m[i+1][j]=='.'){
          qi[nr] = i+1;
          qj[nr] = j;
          nr++;
        }
        if(m[i-1][j]=='.'){
          qi[nr] = i-1;
          qj[nr] = j;
          nr++;
        }
        if(m[i][j+1]=='.'){
          qi[nr] = i;
          qj[nr] = j+1;
          nr++;
        }
        if(m[i][j-1]=='.'){
          qi[nr] = i;
          qj[nr] = j-1;
          nr++;
        }
        nl++;
      }
    }
    /*
    for(int i=0; i<h+2; i++){
      for(int j=0; j<w+2; j++){
        cout << m[i][j];
      }
      cout << endl;
    }
  */
    
    cout << count << endl;
  }

	return 0;
}