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 <cstdio>
using namespace std;

int dx[] = {0,1,0,-1},dy[] = {1,0,-1,0};

int w,h,res;
char field[20][20];

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

int main(){
	int x,y;
	while(cin >> w >> h,w){
		res = 0;
		getchar();
		for(int i = 0;i < h;i++){
			for(int j = 0;j < w;j++) {
				field[i][j] = getchar();
				if(field[i][j] == '@'){
					x = i;
					y = j;
				}
			}
			getchar();
		}
		dfs(x,y);
		cout << res << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <queue>
#include <utility>

int dir[] = {0, -1, 0, 1, 0};

using namespace std;
int main()
{
    while (true) {
        int w, h;
        cin >> w >> h;
        if (w == 0 and h == 0) {
            break;
        }
        vector<vector<int>> tile(h, vector<int>(w));
        vector<vector<int>> check(h, vector<int>(w, 0));
        int starti = 0;
        int startj = 0;
        for (int i = 0; i < h; i++) {
            for (int j = 0; j < w; j++) {
                char c;
                cin >> c;
                if (c == '.') {
                    tile[i][j] = 0;

                } else if (c == '#') {
                    tile[i][j] = -1;
                } else {
                    tile[i][j] = 0;
                    starti = i;
                    startj = j;
                }
            }
        }

        queue<pair<int, int>> q;
        q.push(make_pair(starti, startj));
        while (not q.empty()) {
            const int pi = q.front().first;
            const int pj = q.front().second;
            check[pi][pj] = 1;
            q.pop();
            for (int i = 0; i < 4; i++) {
                const int i_ = pi + dir[i];
                const int j_ = pj + dir[i + 1];
                if (i_ >= 0 and i_ < h and j_ >= 0 and j_ < w and tile[i_][j_] == 0 and check[i_][j_] == 0) {
                    q.push(make_pair(i_, j_));
                }
            }
        }
        int cnt = 0;
        for (int i = 0; i < h; i++) {
            for (int j = 0; j < w; j++) {
                if (check[i][j] == 1) {
                    cnt++;
                }
            }
        }
        cout << cnt << endl;
    }
    return 0;
}

C++: #include<iostream>
#include<queue>
#include<vector>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#define INF 999999
#define fst first
#define snd second
#define MAX 32
using namespace std;
typedef pair<int,int> P;
typedef unsigned long long ll;

int w,h,d[MAX][MAX],dx[4]={1,0,-1,0},dy[4]={0,1,0,-1},sx,sy;
char maze[MAX][MAX];
	
void bfs(){
	int i,j;
	queue<P> que;
	for(i=0;i<h;i++)for(j=0;j<w;j++)d[i][j]=0;
	que.push(P(sx,sy));
	while(que.size()){
		P p=que.front();
		que.pop();
		if(maze[p.fst][p.snd]=='#'||d[p.fst][p.snd])continue;
		d[p.fst][p.snd]=1;
		for(i=0;i<4;i++){
			int nx=p.fst+dx[i],ny=p.snd+dy[i];
			if(0<=nx&&nx<h&&0<=ny&&ny<w){
				que.push(P(nx,ny));
			}
		}
	}
}
	
int main(){
	int i,j,t;
	while(1){
		scanf("%d %d",&w,&h);
		getchar();
		if(w==0&&h==0)break;
		t=0;
		for(i=0;i<MAX;i++)for(j=0;j<MAX;j++)maze[i][j]='\0';
		for(i=0;i<h;i++){
			for(j=0;j<w;j++){
				scanf("%c",&maze[i][j]);
				if(maze[i][j]=='@'){
					sx=i;
					sy=j;
				}
			}
			getchar();
		}
		bfs();
		for(i=0;i<h;i++)for(j=0;j<w;j++)t+=d[i][j];
		printf("%d\n",t);
	}
}