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 <cstdlib>
#include <cstring>
#include <iostream>
#include <string>
#include <vector>
using namespace std;

int w, h;
bool visited[20][20];
vector<string> field;
const int dx[4] = {0, 0, 1, -1};
const int dy[4] = {1, -1, 0, 0};

int dfs(const int x, const int y) {
	visited[y][x] = true;
	int res = 1;
	for(int i = 0; i < 4; ++i) {
		const int nx = x + dx[i], ny = y + dy[i];

		if(nx < 0 || ny < 0 || nx >= w || ny >= h)
			continue;

		if(!visited[ny][nx] && field[ny][nx] != '#')
			res += dfs(nx, ny);
	}

	return res;
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	while(cin >> w >> h, w) {
		field.clear();
		field.resize(h);
		for(int i = 0; i < h; ++i)
			cin >> field[i];

		int sx, sy;
		for(int i = 0; i < h; ++i)
			for(int j = 0; j < w; ++j)
				if(field[i][j] == '@')
					sx = j, sy = i;

		memset(visited, false, sizeof(visited));
		cout << dfs(sx, sy) << endl;
	}

	return EXIT_SUCCESS;
}

C++: #include<stdio.h>
#include<string.h>

int answer,w,h;
char c[21][21];

void move(int x,int y){
    if(c[x][y]!='#'){
        answer++;
        c[x][y]='#';
        if(x-1>=0) move(x-1,y);
        if(x+1<h) move(x+1,y);
        if(y-1>=0) move(x,y-1);
        if(y+1<w) move(x,y+1);
    }
}

int main(){
    while(1){
        int x,y;
        answer=0;
        scanf("%d %d",&w,&h);
        if(w==0&&h==0) return 0;
        getchar();
        for(int i=0;i<h;i++){
            for(int j=0;j<=w;j++){
                scanf("%c",&c[i][j]);
                if(c[i][j]=='@'){
                    x=i;
                    y=j;
                }
            }
        }
        move(x,y);
        printf("%d\n",answer);
    }
}

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

int w, h, black;
char tile[24][24];
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

void dfs(int x, int y){
    black++;
    tile[x][y] = '#';
    for(int i = 0; i < 4; i++){
        if( x+dx[i] >= 0 && x+dx[i] < h &&
           y+dy[i] >= 0 && y+dy[i] < w &&
           tile[ x+dx[i] ][ y+dy[i] ] == '.' ){
            dfs( x+dx[i], y+dy[i] );
        }
    }
}

int main(){
    int i, j;
    while( cin >> w >> h){
        if(w==0&&h==0)
            break;
        for(i = 0; i < h; i++){
            for(j=0;j<w;j++){
                cin >> tile[i][j];
            }
        }
        black = 0;
        for(j = 0; j < w && !black; j++){
            for(i = 0; i < h && !black; i++){
                if( tile[i][j] == '@' ){
                    dfs(i, j);
                }
            }
        }
        cout << black << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <string>
using namespace std;
int w,h;
int dx[4] = {0,1,0,-1},dy[4] = {1,0,-1,0};
int main(void) {
	while (1) {
		int ans = 0;
		cin >> w >> h;
		vector<vector<int> > d(h,vector<int>(w,-1));
		vector<string> s(h);
		queue<pair<int,int> > q;
		if (w == 0 && h == 0) break;
		for(int i = 0;i < h;i++){
		    cin >> s[i];
		    for(int j = 0;j < w;j++){
		        if(s[i][j] == '@'){
		            d[i][j] = 0;
		            q.push(pair<int,int>(i,j));
		        }
		    }
		}
		
		while(!q.empty()){
		    pair<int,int> cur = q.front();
		    q.pop();
		    for(int k = 0;k < 4;k++){
		        int x = cur.first+dx[k];
		        int y = cur.second+dy[k];
		        if(x < 0 || x >= h || y < 0 || y >= w) continue;
		        if(s[x][y] == '#') continue;
		        if(d[x][y] == -1){
		            d[x][y] = d[cur.first][cur.second]+1;
		            ans++;
		            q.push(pair<int,int>(x,y));
		        }
		    }
		}
		cout << ans+1 << endl;
	}
	return 0;
}