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>
#include<vector>

using namespace std;

int w,h;
int ans=1;
char tile[21][21];

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

int main(void){
	int a,b; 
	while(1){
		scanf("%d %d",&w,&h);
		if(w==0 && h==0)break;
		ans=1;
		for(a=0;a<h;a++){
			scanf("%s",tile[a]);
		}
		for(a=0;a<h;a++){
			for(b=0;b<w;b++){
				if(tile[a][b]=='@')saiki(a,b);
			}
		}
		printf("%d\n",ans);
	}
	return 0;
}

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

using namespace std;

char tyle[30][30]={'0'};

int find_tyle(int i,int j){
  if(tyle[i+1][j]=='.'){
    tyle[i+1][j]='a';
    find_tyle(i+1,j);
  }
  if(tyle[i][j+1]=='.'){
    tyle[i][j+1]='a';
    find_tyle(i,j+1);
  }
  if(tyle[i-1][j]=='.'){
    tyle[i-1][j]='a';
    find_tyle(i-1,j);
  }
  if(tyle[i][j-1]=='.'){
    tyle[i][j-1]='a';
    find_tyle(i,j-1);
  }
}

int main(){
  
  int w,h;
  
  while(cin>>w>>h,w){
    int ans=0;
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
      cin>>tyle[i][j];
      }
    }
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
	if(tyle[i][j]=='@'){
	  find_tyle(i,j);
	  tyle[i][j]='a';
	}
      }
    }
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
	if(tyle[i][j]=='a'){
	  ans++;
      }
    }
  }
    cout<<ans<<endl;

   for(int i=0;i<30;++i){
      for(int j=0;j<=30;++j){
      tyle[i][j]='0';
      }
    }
    
  }
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <set>
#include <queue>
#include <stack>
#include <string>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cmath>

#define rep(i, n) for(int i = 0; i < n; i ++)
#define ALL(T) T.begin(), T.end()
#define mp make_pair
#define pb push_back
#define X first
#define Y second

using namespace std;

typedef pair<int, int> pii;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;

const int INF = 1 << 24;
const int dx[4] = {1, -1, 0, 0};
const int dy[4] = {0, 0, 1, -1};

int cnt, w, h;
char m[20][20];

void dfs(pii);

int main(void){
    while(cin >> w >> h, w){
        cnt = 0;
        pii p;

        rep(y, h){
            cin >> m[y];
            rep(x, w) if(m[y][x] == '@') m[y][x] = '.', p = mp(x, y);
        }

        dfs(p);
        cout << cnt << endl;
    }

    return 0;
}

void dfs(pii p){
    if(p.X < 0 || w <= p.X ||
            p.Y < 0 || h <= p.Y) return;

    if(m[p.Y][p.X] == '.') cnt ++;
    else return;
    m[p.Y][p.X] = '#';

    rep(i, 4){
        pii next = mp(p.X + dx[i], p.Y + dy[i]);
        dfs(next);
    }
}

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

static int cnt;

void check(int pw,int ph,char **dt,int w,int h);
void dtallput(int h,int w,char **dt);

int main(void){
	int w,h;
	int ph,pw;
	char c;

	while(1){

	//入力
	cin >> w >> h;
	if(w==0&&h==0)break;

	char **dt = new char*[h+2];
	for(int i=0; i<h+2; i++){
		dt[i] = new char[w+2];
		memset(dt[i],'#',sizeof(char)*(w+2));
	}
	
	c=getchar();
	for(int i=0; i<h; i++){
		for(int j=0; j<w; j++){
			c=getchar();
			if(c=='.'||c=='#')
				dt[i+1][j+1]=c;
			else if(c=='@'){
				dt[i+1][j+1]=c;
				ph=i+1; pw=j+1;
			}
		}
		c=getchar();
	}
	
	//測定
	cnt=1;
	check(pw,ph,dt,w,h);

	//出力
	cout << cnt << endl;

	}
	return 0;
}

void check(int pw,int ph,char **dt,int w,int h){
	int dw[]={0,0,1,-1};
	int dh[]={1,-1,0,0};

	for(int i=0; i<4; i++){
		if(dt[ph+dh[i]][pw+dw[i]]=='.'){	
			dt[ph+dh[i]][pw+dw[i]]='@';
			cnt++;
			check(pw+dw[i],ph+dh[i],dt,w,h);
		}
	}
}

void dtallput(int h,int w,char **dt){
	cout << endl;
	for(int i=0; i<h+2; i++){
		for(int j=0; j<w+2; j++){
			cout << dt[i][j];
		}
		cout << endl;
	}
	cout << endl;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#define BLACK 1
#define RED 2
#define ARUKERUYO 10
using namespace std;

void aaa(int *data, int n, int a, int b){

  data[n] = ARUKERUYO;

  if(data[n-a] == BLACK && n-a >= 0) aaa(data,n-a,a,b);
  if(data[n-1] == BLACK && n%a != 0) aaa(data,n-1,a,b);
  if(data[n+1] == BLACK && n%a != a-1) aaa(data,n+1,a,b);
  if(data[n+a] == BLACK && n+a <= a*b-1) aaa(data,n+a,a,b);

  return;
}

int main(){
  int a,b;

  while(cin>>a>>b){
    if(a==0&&b==0)
      break;
    char sym;
    int data[a*b];
    int stand;

    for(int i=0;i<a*b;i++){
      cin >> sym;
      if(sym=='.')
	data[i] = BLACK;
      else if(sym=='#')
	data[i] = RED;
      else
	stand = i;
    }
    aaa(data,stand,a,b);
    stand = 0;
    for(int i=0;i<a*b;i++)
      if(data[i] == ARUKERUYO)
	stand++;
    cout << stand << endl;
  }
  return 0;
}