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 <vector>
#include <utility>

using namespace std;

#define rep(i, n) for (int i = 0; i < (int)n; i++)
#define vec vector

int dp[22][22];
int dx[4] = {1, -1, 0,  0};
int dy[4] = {0,  0, 1, -1};
int ans;
vec<string> a;

void solve(int p, int q)
{
    if (dp[p + 1][q + 1]) return;
    dp[p + 1][q + 1] = 1;
    if (a[p][q] != '#') {
        ans++;
    } else return;
    rep(i, 4) solve(p + dx[i], q + dy[i]);
}

int main(void)
{
    while(1) {
        int w, h;
        cin >> w >> h;
        if (w == 0) break;
        rep(i, h + 2) rep(j, w + 2) dp[i][j] = 0;
        rep(i, w + 2) {
            dp[0][i] = -1;
            dp[h + 1][i] = -1;
        }
        rep(i, h + 2) {
            dp[i][0] = -1;
            dp[i][w + 1] = -1;
        }
        a.clear();
        a.resize(h);
        pair<int, int> start;
        rep(i, h) {
            cin >> a[i];
            rep(j, w) if (a[i][j] == '@') start = make_pair(i, j);
        }
        //dp[start.first + 1][start.second + 1] = 1;
        ans = 0;
        solve(start.first, start.second);
        cout << ans << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <queue>

using namespace std;

int main(){
    int w,h;
    while(cin >> w >> h && w!=0){
        int t[w+2][h+2];
        int matrix[w*h][w*h];
        int start;
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                if(i==0 || j==0 || i==w || j==h)
                    t[i][j]=0;
                char tmp; cin >> tmp;
                if(tmp=='.'){
                    t[i+1][j+1] = 1;
                }
                else if(tmp=='#'){
                    t[i+1][j+1] = 0;
                }
                else{
                    t[i+1][j+1] = 1;
                    start = i*w+j;
                }
            }
        }
        for(int i=0;i<w*h;i++){
            for(int j=0;j<w*h;j++){
                matrix[i][j] = 0;
            }
        }
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                if(i==0 || j==0 || i==w || j==h)
                    continue;
                else{
                    int id = i*w+h;
                    if(t[(id-w)/w][(id-w)%w] == 1)
                        matrix[id][id-w] = 1;
                    if(t[(id+w)/w][(id+w)/w] == 1)
                        matrix[id][id+w] = 1;
                    if(t[(id-1)/w][(id-1)/w] == 1)
                        matrix[id][id-1] = 1;
                    if(t[(id+1)/w][(id+1)/w] == 1)
                        matrix[id][id+1] = 1;
                }
            }
        }
        int count = 1;
        queue<int> Q;
        int came[w*h];
        for(int i=0;i<w*h;i++){
            came[i] = 0;
        }
        Q.push(start);
        came[start] = 1;
        while(Q.size()!=0){
            int cur = Q.front();
            for(int i=0;i<w*h;i++){
                if(matrix[cur][i]==1 && came[i] == 0){
                    Q.push(i);
                    came[i] = 1;
                    count++;
                }
            }
            Q.pop();
        }
        cout << count << endl;
    }
}

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] == 1) dfs(x, y-1);
    if (y+1 < h && room[y-1][x] == 1) dfs(x, y+1);
    if (x-1 > -1 && room[y-1][x] == 1) dfs(x-1, y);
    if (x+1 < w && room[y-1][x] == 1) 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;
}