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>
#include <queue>
#include <set>
using namespace std;

const int MAX_SIZE = 20;
typedef pair<int, int> P;

int calc(char f[][MAX_SIZE + 1], queue<P> q, int W, int H) {
  set<P> s;
  while (q.size()) {
    P p = q.front();
    q.pop();
    s.insert(p);
    int di[] = {-1, 1, 0, 0};
    int dj[] = {0, 0, -1, 1};
    for (int k = 0; k < 4; ++k) {
      int i = p.first  + di[k];
      int j = p.second + dj[k];
      if (i >= 0 && j >= 0 && i < H && j < W && f[i][j] == '.') {
        pair<set<P>::iterator, bool> r = s.insert(P(i, j));
        if (r.second) q.push(*r.first);
      }
    }
  }
  return s.size();
}
int main() {
  int W, H;
  char f[MAX_SIZE + 1][MAX_SIZE + 1];
  while (true) {
    queue<P> q;
    cin >> W >> H;
    if (W == 0 && H == 0) break;
    for (int i = 0; i < H; ++i) {
      cin >> f[i];
      for (int j = 0; j < W; ++j) {
        if (f[i][j] == '@') {
          q.push(P(i, j));
        }
      }
    }
    cout << calc(f, q, W, H) << endl;
  }
}

C++: #include<iostream>
#include<string>
using namespace std;
int field[60][60];
int dx[4] = {1,-1,0,0};
int dy[4] = {0,0,1,-1};
int dfs(int x, int y);
int main(){
  int n,m;
  while(cin >>n>>m,n||m){
    for(int i=0; i<60; i++) for(int j=0; j<60; j++) field[i][j] = 1;
    int x,y;
    string s;
    for(int i=0; i<m; i++){
      cin >>s;
      for(int j=0; j<n; j++){
	if(s[j] == '.') field[i+20][j+20] = 0;
	if(s[j] == '#') field[i+20][j+20] = 1;
	if(s[j] == '@'){
	  field[i+20][j+20] = 1;
	  x = i+20; y= j+20;
	}
      }
    }
    cout <<dfs(x,y)<<endl;
  }
  return 0;
}
int dfs(int x, int y){
  int ans = 0;
  field[x][y] = 1;
  ans++;
  for(int i=0; i<4; i++){
    if(field[x+dx[i]][y+dy[i]] == 0) ans+=dfs(x+dx[i],y+dy[i]);
  }
  return ans;
}

C++: #include<iostream>
#include<queue>
#include<vector>
#define INF 999999
#define fst first
#define snd second
#define MAX 32
using namespace std;
typedef pair<int,int> P;
typedef unsigned long long ll;

int w,h,d[MAX][MAX],dx[4]={1,0,-1,0},dy[4]={0,1,0,-1},sx,sy;
char maze[MAX][MAX];
	
void bfs(){
	int i,j;
	queue<P> que;
	for(i=0;i<h;i++)for(j=0;j<w;j++)d[i][j]=0;
	que.push(P(sx,sy));
	while(que.size()){
		P p=que.front();
		que.pop();
		if(maze[p.fst][p.snd]=='#'||d[p.fst][p.snd])continue;
		d[p.fst][p.snd]=1;
		for(i=0;i<4;i++){
			int nx=p.fst+dx[i],ny=p.snd+dy[i];
			if(0<=nx&&nx<h&&0<=ny&&ny<w){
				que.push(P(nx,ny));
			}
		}
	}
}
	
int main(){
	int i,j,t;
	while(1){
		scanf("%d %d",&w,&h);
		getchar();
		if(w==0&&h==0)break;
		t=0;
		for(i=0;i<MAX;i++)for(j=0;j<MAX;j++)maze[i][j]='\0';
		for(i=0;i<h;i++){
			for(j=0;j<w;j++){
				scanf("%c",&maze[i][j]);
				if(maze[i][j]=='@'){
					sx=i;
					sy=j;
				}
			}
			getchar();
		}
		bfs();
		for(i=0;i<h;i++)for(j=0;j<w;j++)t+=d[i][j];
		printf("%d\n",t);
	}
	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) {
    num++;
    
    room[y][x] = 0;
    
    // ??????????????¢?´¢
    if (y-1 > -1 && room[y-1][x] == 0) dfs(x, y-1);
    if (y+1 < h && room[y+1][x] == 0) dfs(x, y+1);
    if (x-1 > -1 && room[y][x-1] == 0) dfs(x-1, y);
    if (x+1 < w && room[y][x+1] == 0) dfs(x+1, y);

    return;
}

int main() {
    while (cin >> w >> h, w || h) {
        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;
}

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

const int dx[] = {0, 0, -1, 1};
const int dy[] = {1, -1, 0, 0};
int W, H;
char board[30][30];

int dfs(int y, int x) {
   board[y][x] = '#';
   int ret = 1;
   for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx >= 0 && nx < W && ny >= 0 && ny < H && board[ny][nx] == '.') {
         ret += dfs(ny, nx);
      }
   }
   return ret;
}

int main() {
   while (cin >> W >> H, W) {
      for (int i = 0; i < H; i++) {
         cin >> board[i];
      }
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (board[i][j] == '@') {
               cout << dfs(i, j) << endl;
            }
         }
      }
   }

   return 0;
}