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
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }

//math
//-------------------------------------------
template<class T> inline T sqr(T x) { return x*x; }

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI = acos(-1.0);

//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;
int done[20][20];
char tile[20][20];
int mw,mh;
string temp;
int loopchk(int h,int w,int v);
int check(int h,int w);
int check(int h,int w){
	int posw=114514;
	int posh=114514;
	for(int i=0;i<h;i++){
		for(int j=0;j<w;j++){
			if(tile[i][j] == '@'){
				posh = i;
				posw = j;
				break;
			}
		}
		if(posw!=114514){break;}
	}
	return 1+loopchk(posh,posw,0);
}
int loopchk(int h,int w,int v){
	int value=v;
	if(done[h][w] == 1){
		return value-1;
	}else{
		done[h][w] = 1;
	}
	for(int i=-1;i<=1;i++){
		for(int j=-1;j<=1;j++){
			if(w + j < 0 || w + j > mw - 1){continue;}
			if(h + i < 0 || h + i > mh - 1){continue;}
			if(i == 0 && j == 0){continue;}
			if(i == -1 && j == -1 || i == -1 && j == 1 || i == 1 && j == -1 || i == 1 && j == 1){continue;} 
			if(tile[h+i][w+j] == '.'){
				value = 1 + loopchk(h + i, w + j,value);
			}
		}
	}
	return value;
}
int main(){
	int w,h;
	while(cin >> w >> h, w != 0 || h != 0){
		mw = w;
		mh = h;
		for(int i=0;i<20;i++){
			for(int j=0;j<20;j++){
				done[i][j] = 0;
				tile[i][j] = '#';
			}
		}
		for(int i=0;i<h;i++){
			cin >> temp;
			for(int j=0;j<w;j++){
				tile[i][j] = temp[j];
			}
		}
		cout << check(h,w) << endl;
	}
	return 0;
}

C++: #include<iostream>

using namespace std;

char ban[21][21];
int cont = 0;

	int w, h;
	
	int MAX = 0;

void saiki(int y, int x)
{
	cont++;
	ban[y][x] = '_';
	/*
	for(int i = 0; i < h; i++){
		for(int j = 0; j < w; j++){
			cout << ban[i][j];
		}
		cout << endl;
	}
	
	cout << w << h << endl;
	
	getchar();
	
	cout << cont << endl;;
	*/
	
	if(ban[y+1][x] == '.'){
		saiki(y+1,x);
	}
	if(ban[y-1][x] == '.' && y-1 > -1){
		saiki(y-1,x);
	} 
	if(ban[y][x-1] == '.' && x-1 > -1){
		saiki(y,x-1);
	}
	if(ban[y][x+1] == '.'){
		saiki(y,x+1);
	}
	
	if(cont > MAX){
		MAX = cont;
	}
}

int main()
{
	for(;;){
		int st_x, st_y;
		MAX = cont = 0;
		
		cin >> w >> h;
		
		if( w == 0){
			break;
		}
		
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				ban[i][j] = '0';
			}
		}
		
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				cin >> ban[i][j];
				if(ban[i][j] == '@'){
					st_x = j;
					st_y = i;
				}
			}
		}
		
		
		saiki(st_y,st_x);
		
		cout << MAX << endl;
	}
}

C++: #include<bits/stdc++.h>
#define rep(i,n) for(int i=0; i<n; i++)
using namespace std;
char field[21][21];
char visit[21][21];
int dx[]={1,-1,0,0};
int dy[]={0,0,1,-1};
int W,H;
void dfs(int y, int x){
  visit[y][x] = 1;
  rep(i,4){
    int ny = y+dy[i];
    int nx = x+dx[i];
    if(field[ny][nx] == '.'
       && visit[ny][nx] == 0
       && nx >= 0
       && nx < W
       && ny >= 0
       && ny < H){
          dfs(ny,nx);
        }
  }
}
int main(){
  while(cin >> W >> H && W){
    int y,x;
    rep(i,H){
      rep(j,W){
        cin >> field[i][j];
        if(field[i][j] == '@') y=i,x=j;
      }
    }
    memset(visit,0,sizeof(visit));
    dfs(y,x);
    int ans = 0;
    rep(i,H){
      rep(j,W){
        ans += visit[i][j];
      }
    }
    cout << ans << endl;
  }
  return 0;
}