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;

void checkboard(void);
void startmove(void);
int checked(int i,int j);

char board[20][20];
int H,W,now[2];

int main(void){
	while(1){
		cin >> H >> W ;
		if (!H && !W) break;
		// count=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				cin >> board[i][j];
				if(board[i][j]=='@'){
					now[0]=i;
					now[1]=j;
				}
			}
		}
		startmove();
		checkboard();
	}

}


void startmove(void){
	int n;
	do{
		n=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				if(board[i][j]=='@')
					n+=checked(i,j);
			}
		}

	}while(n);
}

int checked(int i,int j){
	int a=0;
	if(i>=0 && board[i-1][j]=='.'){
		board[i-1][j]='@';
		a=1;
	}
	if(i<W && board[i+1][j]=='.'){
		board[i+1][j]='@';
		a=1;
	}
	if(j>=0 && board[i][j-1]=='.'){
		board[i][j-1]='@';
		a=1;
	}
	if(j<H && board[i][j+1]=='.'){
		board[i][j+1]='@';
		a=1;
	}
	// checkboard();
	return a;
}
void checkboard(void){
		int count=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				if(board[i][j]=='@') count++; 
				// cout << board[i][j];
			}
		}
			cout << count << endl;

}

C++: #if 0
08-162015?????????????????°???????????????????????????
pair???
#endif

#include <iostream>
#include <queue>
#include <string.h>
#include <utility>
using namespace std;

int w,h;
int tot=1; //?????£????????????(?????????????????????)
const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1}; //??????????§??????¨
char P[30][30]; //??§?¨?????????????????????¨???

bool valid(int x,int y){ //??????????????????
	return (x>=0)&&(x<w)&&(y>=0)&&(y<h)&&(P[x][y]!='#');
}

void bfs(int x,int y){ //???????????¢?´¢
	queue<pair<int,int> > q;
	pair<int,int> p;
	p = make_pair(x,y); //pair??????????????¨??????????????????
	q.push(p);
	while(!q.empty()){
		pair<int,int> cur=q.front(); //?????¨??°?????????????????????
		q.pop();
		for(int i=0;i<4;++i){
			if(valid(cur.first+dx[i],cur.second+dy[i])){
				tot+=1;
				P[cur.first+dx[i]][cur.second+dy[i]]='#'; //???????????£?????´???????°???????
				q.push(make_pair(cur.first+dx[i],cur.second+dy[i]));
			}
		}
		P[cur.first][cur.second]='#';
	}
}

int main(){
	int x1,y1;
	while((cin>>w>>h)&&(w!=0&&h!=0)){
		for(int i=0;i<h;++i){
			for(int j=0;j<w;++j){
				cin>>P[i][j];
				if(P[i][j]=='@')
					x1=i; y1=j; 
			}
		}
		bfs(x1,y1); //??????????????°????????£??\????????¢?´¢????§?
		cout << tot << endl;
	}
}

C++: #include<iostrem>
#include<utility>
#include<queue>
using namespace std;
char room[21][21];
int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};
typedef pair<int,int> p;


int main(){
	int x,y;
	queue<p> que;
	int w,h;
	cin>>w>>h;
	if(w+h==0) return;
	for(int a=0;a<h;a++){
		for(int b=0;b<w;b++){
			cin>>room[a][b];
			if(room[a][b]=='@'){
				x=b,y=a;
				que.push(p(b,a));
				room[a][b]='#';
			}
		}
	}
	int cnt=1;
	int nx,ny;
	while(que.size()){
		p ap=que.front();
		que.pop;
		for(int i=-1;i<=1;i++){
			nx=ap.first+dx[i];
			ny=ap.second+dy[i];
			if(0<=nx&&nx<=w&&0<=ny&&ny<=h&&room[ny][nx]!='#'){
				room[ny][nx]='#';
				que.push(p(nx,ny));
				cnt++;
			}
		}
	}
}

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

using namespace std;
vector<int> ans;
int cf[20][20];
bool field[20][20];
int w,h;
void dfs(int i,int j,int count);

int main(){
	while(1){
		cin >> w >> h;
		if(w + h == 0) break;
		for(int i = 0;i < 20;i++){
			for(int j = 0;j < 20;j++){
				cf[i][j] = -1;
				field[i][j] = false;
			}
		}
		
		int px,py;
		for(int i = 0;i < h;i++){
			for(int j = 0;j < w;j++){
				char t;
				cin >> t;
				if(t == '.') field[i][j] = true;
				else if(t == '@'){ px = j; py = i;field[i][j] = true;}
			}
		}
		
		int counta = 0;
		for(int i = 0;i < h;i++){
			for(int j = 0;j < w;j++){
				if(field[i][j] && cf[i][j] == -1){
					dfs(i,j,counta);
					counta++;
				}
			}
		}
		
		int pn = cf[py][px];
		int count = 0;
		for(int i = 0;i < h;i++){
			for(int j = 0;j < w;j++){
				if(cf[i][j] == pn) count++;
			}
		}
		
		ans.push_back(count);
	}
	for(int i = 0;i < ans.size();i++) cout << ans[i] << endl;
	return 0;
}

void dfs(int i,int j,int count){
	cf[i][j] = count;
	if(i != 0 && field[i-1][j] && cf[i-1][j] == -1) dfs(i-1,j,count);
	if(i != h-1 && field[i+1][j] && cf[i+1][j] == -1) dfs(i+1,j,count);
	if(j != 0 && field[i][j-1] && cf[i][j-1] == -1) dfs(i,j-1,count);
	if(j != w-1 && field[i][j+1] && cf[i][j+1] == -1) dfs(i,j+1,count);
}