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;

int w, h;
string inp[21];

const int MAX=23;
int room[MAX][MAX];
int cnt;

void init(){
    int i, j;
    for(i=0; i<MAX; i++){
        for(j=0; j<MAX; j++){
            room[i][j]=0;
        }
    }
    cnt = 0;
}

void dfs(int x, int y){
    cnt++;
    room[x][y]=0;
    if(room[x+1][y]){
        dfs(x+1,y);
    }
    if(room[x-1][y]){
        dfs(x-1,y);
    }
    if(room[x][y+1]){
        dfs(x,y+1);
    }
    if(room[x][y-1]){
        dfs(x,y-1);
    }
}

int main() {
    ios_base::sync_with_stdio(0);
    
    
    while(1){
        cin >> w >> h;
        
        if(w==0 && h==0)
            break;
        
        int i, j;
        int x=0, y=0;
        for(i=1; i<=h; i++){
            cin >> inp[i];
            for(j=0; j<w; j++){
                switch(inp[i][j]){
                    case '.':
                        room[j+1][i]=1;
                        break;
                    case '#':
                        room[j+1][i]=0;
                        break;
                    case '@':
                        room[j+1][i]=1;
                        x=j+1;
                        y=i;
                        break;
                }
            }
        }
        
        dfs(x, y);
        cout << cnt << endl;
        
        init();
    }
    return 0;
}

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

char c[21][21];
int w, h;
int sy, sx;
int dy[4] = {1, -1, 0, 0};
int dx[4] = {0, 0, 1, -1};
bool f[21][21];
int ans = 0;

void dfs(int y, int x){
    f[y][x] = true;
    ans++;
    for(int i = 0; i < 4; i++){
        int newy = y + dy[i];
        int newx = x + dx[i];
        if(0 <= newy && newy < h && 0 <= newx && newx < w && f[newy][newx] == false && c[newy][newx] != '#'){
            dfs(newy, newx);
        }
    }
    return;
}

int main(){
    while(cin >> w >> h, w, h){
        for(int i = 0; i < h; i++){
            for(int j = 0; j < w; j++){
                cin >> c[i][j];
                if(c[i][j] == '@'){
                    sy = i;
                    sx = j;
                }
                f[i][j] = false;
            }
        }
        ans = 0;
        dfs(sy, sx);
        cout << ans << endl;
    }
}