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;

#define endll '\n'
#define ll long long

const int crow = 25;
bool visited [crow][crow];
string tiles[crow];
int row, col, cnt;

void dfs (int posx, int posy) {
    if (posx < 0 || posy < 0 || posx >= col || posy >= row || visited [posy][posx] || tiles[posy][posx] == '#')
        return;
    visited [posy][posx] = true;
    cnt++;
    dfs (posx-1, posy);
    dfs (posx+1, posy);
    dfs (posx, posy-1);
    dfs (posx, posy+1);
}


int main () {

    cin  >> col >> row;
    while (row && col) {
        int posx = 0, posy = 0;
        for (int j = 0; j < row; j++) {
            cin >> tiles[j];
            for (int i = 0; i < col; i++) {
                visited [j][i] = false;
                if (tiles[j][i] == '@') {
                    posx = i;
                    posy = j;
                    //visited [j][i] = true;
                }
            }
        }
        cnt = 0;
        dfs (posx, posy);
        cout << cnt << endll;
        cin  >> col >> row;
    }
}

C++: #include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (int)n; ++i)
#define REP(i, a, b) for (int i = (int)a; i < (int)b; ++i)
#define rer(i, a, b) for (int i = (int)a; i <= (int)b; ++i)
#define each(i,c) for(__typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define all(v) v.begin(), v.end()
#define mset(a, n) memset(a, n, sizeof(a))

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<vector<int> > vvi;
typedef vector<pair<int, int> > vpii;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;

const int inf = 1000000000;
const int mod = 1000000007;
const double eps = 1e-9;
const int dx[] = { -1, 0, 1, 0};
const int dy[] = { 0, -1, 0, 1};

int W, H;
int ans = 0;
string field[100];

void dfs(int y, int x) {
	if (x < 0 || y < 0 || x == W || y == H) return;
	if (field[y][x] == '#') return;
	ans++;
	field[y][x] = '#';
	rep(i, 4) dfs(y + dy[i], x + dx[i]);
}

int main() {
	while (cin >> W >> H, W) {
		rep(i, H) cin >> s[i];
		rep(i, H) rep(j, W) {
			if (s[i][j] == '@') solve(i, j);
		}
	}
	return 0;
}

C++: #include<iostream>
#include<vector>
using namespace std;
int count=0;
void move(int x,int y,vector<vector<char> > &room,int w,int h){
    count++;
    if(x>0){
        if(room[x-1][y]=='.'){
            room[x-1][y]='@';
            move(x-1,y,room,w,h);
        }
    }
    if(x<h-1){
        if(room[x+1][y]=='.'){
            room[x+1][y]='@';
            move(x+1,y,room,w,h);
        }
    }
    if(y<w-1){
        if(room[x][y+1]=='.'){
            room[x][y+1]='@';
            move(x,y+1,room,w,h);
        }
    }
    if(y>0){
        if(room[x][y-1]=='.'){
            room[x][y-1]='@';
            move(x,y-1,room,w,h);
        }
    }
}

int main(void){
    int w=1,h=1,x,y;
    vector<vector<char> > room;
    while(w|h){
        count=0;
        room.clear();
        cin>>w;
        cin>>h;
        if((w|h)==0)break;
        room.resize(h);
        for(int i=0;i<h;i++){
            room[i].resize(w);
        }
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                cin>>room[i][j];
                if(room[i][j]=='@'){
                    x=i;y=j;
                }
            }
        }
        move(x,y,room,w,h);
        cout<<count<<endl;
    }
    return 0;
}

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

using namespace std;

int field[52][52];

void DFS(int x, int y){
    if(field[x][y]==1/*||field[x][y]==-1*/){
        field[x][y]=-1;
        DFS(x,y-1);
        DFS(x-1,y);
        DFS(x+1,y);
        DFS(x,y+1);
    }
}


int main(){
    int w,h;
    while(cin>>w>>h){
        if(w==0&&h==0)break;
        for(int i=0; i<50; i++){
            for(int j=0; j<50; j++){
                field[i][j]=0;
            }
        }
        
        int start_x;
        int start_y;
        for(int i=1; i<=h; i++){
            for(int j=1; j<=w; j++){
                char temp;
                cin>>temp;
                if(temp=='#')field[i][j]=0;
                else field[i][j]=1;
                
                if(temp=='@'){
                    start_x=i;
                    start_y=j;
                }
            }
        }
        
        DFS(start_x,start_y);
        
        int sum=0;
        for(int i=1; i<=h; i++){
            for(int j=1; j<=w; j++){
                if(field[i][j]==-1){
                    sum++;
                }
            }
        }
        cout<<sum<<endl;
    }
}