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 <algorithm>
#define MH 20
#define MW 20
using namespace std;
char f[MH][MW];
bool v[MH][MW];
int dx[4] = { 1, 0, -1, 0 };
int dy[4] = { 0, 1, 0, -1 };
int w, h;
int c = 0;

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

int main() {
	int sx, sy;

	while (cin >> w >> h, !(w == 0 && h == 0)) {
		for (int i = 0; i < h; ++i) {
			for (int j = 0; j < w; ++j) {
				cin >> f[i][j];
				if (f[i][j] == '@') {
					sx = j;
					sy = i;
				}
			}
		}
		c = 0;
		fill(v[0], v[MH], false);
		search(sx, sy);
		cout << c << endl;
		
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
char maze[21][21];
int maz[21][21];
int w,h;
int f(int x,int y){
	if(maz[y][x]||maze[y][x]=='#')return 0;
	maz[y][x]=1;
	y<h-1&&f(x,y+1);
	0<y&&f(x,y-1);
	x<w-1&&f(x+1,y);
	0<x&&f(x-1,y);
	return 0;
}
int main(){
	int x,y;
	for(;cin>>w>>h,w;){
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				cin>>maze[i][j];
				maz[i][j]=0;
				if(maze[i][j]=='@'){y=i;x=j;}
			}
		}
		f(x,y);
		int res=0;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				res+=maz[i][j];
			}
		}
		cout<<res<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cassert>
#define REP(i,l,n) for(int i=l;i<n;++i)
#define rep(i,n) REP(i,0,n)
#define MAX 20
int ans,baseX,baseY;
int w,h;
char tile[MAX][MAX];
void seek(int x,int y);
using namespace std;
int main() {
  while(1){
    cin >> w;
    cin >> h;
    if(w==0&&h==0) return 0;
    rep(i,h){
      rep(j,w){
        cin >> tile[i][j];
        if(tile[i][j] == '@'){
          baseX = i;
          baseY = j;
        }
      }
      tile[i][w] = '#';
    }
    rep(i,w) tile[h][i] = '#';
    ans = 1;
    seek(baseX,baseY);
    cout << ans << endl;
  }
  return 0;
}

void seek(int x,int y){
  if(tile[x+1][y] == '.'){
    tile[x+1][y] = '#';
    ans++;
    seek(x+1,y);
  }
  if(tile[x][y+1] == '.'){
    tile[x][y+1] = '#';
    ans++;
    seek(x,y+1);
  }
  if(tile[x-1][y] == '.'){
    tile[x-1][y] = '#';
    ans++;
    seek(x-1,y);
  }
  if(tile[x][y-1] == '.'){
    tile[x][y-1] = '#';
    ans++;
    seek(x,y-1);
  }
}

C++: #include<iostream>
#include<queue>
using namespace std;
int main(){
	int w, h; 
	while (cin >> w >> h) {
		if (!w && !h)return 0;
		char map[21][21];
		bool b[21][21];
		queue<pair<int, int> >bfs;
		for (int i = 0; i < h; i++)for (int j = 0; j < w; j++) {
			cin >> map[i][j];
			b[i][j] = false;
			if (map[i][j] == '@') {
				bfs.push(make_pair(i, j));
				b[i][j] = true;
			}
		}
		int ans = 1;
		while (bfs.size() > 0) {
			pair<int, int>p = bfs.front();
			int x = p.first;
			int y = p.second;
			map[x][y] = '#';
			if (x > 0){
				if (map[x - 1][y] == '.' && !b[x - 1][y]) {
					bfs.push(make_pair(x - 1, y));
					ans++;
					b[x - 1][y] = true;
				}
			}
			if (x < h) {
				if (map[x + 1][y] == '.' && !b[x + 1][y]) {
					bfs.push(make_pair(x + 1, y));
					ans++;
					b[x + 1][y] = true;
				}
			}
			if (y > 0) {
				if (map[x][y - 1] == '.'&& !b[x][y - 1]) {
					bfs.push(make_pair(x, y - 1));
					ans++;
					b[x][y - 1] = true;
				}
			}
			if (y < w) {
				if (map[x][y + 1] == '.' && !b[x][y + 1]) {
					bfs.push(make_pair(x, y + 1));
					ans++;
					b[x][y + 1] = true;
				}
			}
			bfs.pop();
		}
		cout << ans << endl;
	}
}

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

int dfs(int x,int y);
int h,w;
char ta[22][22];
//int cheak[22][22]={0};
int dx[4]={-1,0,1,0};
int dy[4]={0,-1,0,1};
int cou=0;

int main(){
  int a,b;
  while(1){
    cin >> w >> h;
    if(w==0 && h==0) break;
    cou=0;
    
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin >> ta[i][j];
	cheak[i][j]=0;
	if(ta[i][j] == '@'){
	  a=i,b=j;
	}
      }
    }
    //cout << a << " " << b << endl;
    cout << dfs(a,b) << endl;
    /*for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
      cout << cheak[i][j];
      }
      cout << endl;
      }*/
  }
}

void dfs(int x,int y){
  //cheak[x][y]=1;
  ta[nx][ny]='#';
  cou++;
  for(int i=0;i<4;i++){
    int nx=x+dx[i];
    int ny=y+dy[i];
    if(nx>=0 && nx<h && ny>=0 && ny<w && ta[nx][ny]=='.'){
      dfs(nx,ny);
    }
  }
}