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>
#define rep(i,n) for(int i=0; i<(n); i++)
#define FOR(i,a,n) for(int i=(a); i<(n); i++)
#define all(v) v.begin(), v.end()
using namespace std;
typedef long long ll;
const int INF = 1e9;
const int MOD = 1e9+7;

vector<string> mat;
vector<vector<bool>> seen;
int h, w;

const int dh[4] = {-1, 0, 1, 0};
const int dw[4] = {0, -1, 0, 1};

void dfs(int sh, int sw) {
    seen[sh][sw] = true;
    rep(i, 4) {
        int nh = sh+dh[i], nw = sw+dw[i];
        if (nh<0 || nh>h-1 || nw<0 || nw>w-1) continue;
        if (seen[nh][nw]) continue;
        if (mat[nh][nw] == '#') continue;
        dfs(nh, nw);
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    
    while(true) {
        cin >> w >> h;
        if (w+h == 0) break;
        mat.clear();
        rep(i, h) {
            string s; cin >> s;
            mat.push_back(s);
        }

        int sh, sw;
        rep(i, h) rep(j, w) if (mat[i][j]=='@') sh=i, sw=j;

        seen.assign(h, vector<bool>(w, false));
        dfs(sh, sw);

        int ans = 0;
        rep(i, h) rep(j, w) if(seen[i][j]) ans++;
        cout << ans << endl;
    }
}

C++: #include<iostream>
#include<vector>
#include<queue>
using namespace std;
int dx[4]={1,-1,0,0};
int dy[4]={0,0,1,-1};
int dfs(int sx ,int sy, vector<vector<bool> > &map){
	int t=0;
if(map[sx][sy]){
	map[sx][sy]=false;
	t++;
for(int i=0;i<4;i++){
int x=sx+dx[i];
int y=sy+dy[i];
if(map[x][y])	t+=dfs(x,y,map);
}
}
return t;
}
int main(void){
while(1){
int h,w,sx,sy;
cin>>w>>h;
if(h==0)break;
vector<vector<bool> > map(h+2,vector<bool>(w+2,false));
for(int i=1;i<=h;i++){
	for(int j=1;j<=w;j++){
		char a;
		cin>>a;
		if(a=='.')map[i][j]=true;
		if(a=='@'){
			map[i][j]=true;
			sx=i;
			sy=j;
		}
	}
}
//cout<<sx<<' '<<sy<<endl;
/*
for(int i=1;i<=h;i++){
	for(int j=1;j<=w;j++)if(map[i][j])cout<<1;
	else cout<<0;
	cout<<endl;
}*/
cout<<dfs(sx,sy,map)<<endl;
}


return 0;
}

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

int main(){
  int W, H;
  while(cin >> W >> H && W>0 && H>0){
    int map[400];
    for(int i=0;i<400;i++)
      map[i]=0;
    int at[2];

    for(int i=0;i<H;i++){
      string s;
      cin >> s;
      for(int j=0;j<W;j++){
        map[20*i+j]=(s[j]=='.'?1:(s[j]=='@'?2:0));
        if(s[j]=='@'){
          at[0]=i; at[1]=j;
        }
      }
    }

    vector<int> list[400];  //??£??\?????????
    for(int i=0;i<400;i++){
      if(i>=20) //???
        if(map[i-20]>0) list[i].push_back(i-20);
      if(i%20>0) //???
        if(map[i-1]>0) list[i].push_back(i-1);
      if(i%20<19) //???
        if(map[i+1]>0) list[i].push_back(i+1);
      if(i<380) //???
        if(map[i+20]>0) list[i].push_back(i+20);
    }

    bool d[400];
    for(int i=0;i<400;i++) d[i]=false;

    stack<int> cur;
    cur.push(at[0]*20+at[1]);
    d[at[0]*20+at[1]]=true;

    while(!cur.empty()){
      bool flg=false;
      for(vector<int>::iterator it=list[cur.top()].begin();it!=list[cur.top()].end();it++){
        if(!d[*it]){
          flg=true;
          cur.push(*it);
          d[*it]=true;
          break;
        }
      }
      if(flg) continue;
      cur.pop();
    }

    int cnt=0;
    for(int i=0;i<400;i++)
      if(d[i]) cnt++;

    cout << cnt << endl;
  }

  return 0;  
}

C++: //
//  RedAndBlack.cpp
//  
//
//  Created by ??°????????? on 2015/10/30.
//
//

#include <iostream>
using namespace std;

int w, h, num = 0, room[20][20];

void dfs(int x, int y) {
    if (room[y][x] == 0) return;
    
    num++;
    room[y][x] = 0;
    
    // ??????????????¢?´¢
    if (y-1 > -1) dfs(x, y-1);
    if (y+1 < h) dfs(x, y+1);
    if (x-1 > -1) dfs(x-1, y);
    if (x+1 < w) dfs(x+1, y);

    return;
}

int main() {
    while (cin >> w >> h) {
        if (w == 0 && h == 0) break;
        
        int manX, manY;
        
        for (int i = 0; i < h; i++) {
            string s;
            cin >> s;
            for (int j = 0; j < w; j++) {
                if (s[j] == '.') {
                    room[i][j] = 1;
                } else if (s[j] == '#') {
                    room[i][j] = 0;
                } else if (s[j] == '@') {
                    room[i][j] = 1;
                    manX = j;
                    manY = i;
                }
            }
        }
        
        dfs(manX, manY);
        
        cout << num << endl;
        
        num = 0;
    }
    
    return 0;
}