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;
#define N_PRIME 1000000007
#define ll int64_t

void calc(){
    int w,h;
    int board[32][32];
    bool moved[32][32];
    int px,py;
    
    cin >> w >> h;
    if( w == 0 && h == 0)exit(0);
    for(int j = 0 ; j < h ;j++ ){
        string s;
        cin >> s;
        for(int i = 0 ; i < w ; i++ ){
            moved[i][j] = false;
            
            if( s[i] == '.' ){
                board[i][j] = 1;
            }else if(s[i] == '#' ){
                board[i][j] = 2;
            }else if(s[i] == '@' ){
                px = i;
                py = j;
                board[i][j] = 1;
            }
        }
    }
    
    int count = 0;
    queue<int> xs;
    queue<int> ys;
    xs.push(px);
    ys.push(py);
    
    while(xs.size()>0){
        px = xs.front();
        py = ys.front();
        xs.pop();
        ys.pop();
        
        if( px < 0 || w <= px || py < 0 || h <= py )
            continue;
        
        if( moved[px][py] )
            continue;
        moved[px][py] = true;
        
        if( board[px][py] == 2 )
            continue;

        xs.push(px + 1 );
        ys.push(py + 0 );
        
        
        xs.push(px + 0 );
        ys.push(py + 1 );
        
        
        xs.push(px - 1 );
        ys.push(py + 0 );
        
        
        xs.push(px + 0 );
        ys.push(py - 1 );
        count++;
    }
    
    cout << count << endl;
}

int main(void){
    while(true)calc();
    return 0;
}

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

using i64 = int64_t;

char room[22][22]={0};
bool visited[22][22]={0};

int di[4]={0,1,0,-1}, dj[4]={-1,0,1,0};

void init(){
    for(int i=0;i<=20;++i){
        for(int j=0;j<=20;++j){
            visited[i][j] = false;
            room[i][j] = '#';
        }
    }
}

int main(){
    int h, w;
    while(cin >> w >> h, w > 0){
        init();
        int si, sj;
        for(int i=0;i<h;++i){
            for(int j=0;j<w;++j){
                cin >> room[i][j];
                if(room[i][j] == '@'){
                    si = i;
                    sj = j;
                }
            }
        }
        queue<pair<int, int>> que;
        que.push({si, sj});
        visited[si][sj] = true;
        int ans = 0;
        while(!que.empty()){
            auto cur = que.front(); que.pop();
            ans++;
            for(int k=0;k<4;++k){
                int ni = cur.first+di[k], nj = cur.second+dj[k];
                if(ni < 0 || ni >= h || nj < 0 || nj >= w)continue;
                if(room[ni][nj] == '#')continue;
                if(visited[ni][nj])continue;
                visited[ni][nj] = true;
                que.push({ni, nj});
            }
        }
        cout << ans << 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];
int dx[] = {-1, 1, 0, 0};
int dy[] = { 0, 0,-1, 0};

void dfs(int x, int y) {
    num++;
    room[y][x] = 0;
    
    // ??????????????¢?´¢
    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 (room[my][mx] == 0) continue;
        dfs(mx, my);
    }

    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;
}