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 <queue>
#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>
#include <fstream>
#include <complex>
#include <math.h>

using namespace std;

//conversion
//------------------------------------------
inline long toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
inline long toLong(string s) {long 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 pair<long, long> PLL;
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
//--------------------------------------------

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

const long double EPS = 1e-12;
const int INF = 10000;

char mp[22][22];

int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
bool flag[22][22];

int bfs(int s,int t){
	int sum=0;
	queue<PII>pi;
	pi.push(MP(s,t));
	flag[s][t]=true;
	while(!pi.empty()){
		PII p=pi.front();pi.pop();
		
		sum++;
		REP(i,4){
			int y=p.first+dy[i],x=p.second+dx[i];
			if(mp[y][x]!='#'&&!flag[y][x]){
				flag[y][x]=true;
				pi.push(MP(y,x));
			}
		}
	}
	return sum;
}
int main(){
	int W,H;
	while(cin>>W>>H){
		if(W==0&&H==0)break;
		REP(i,22)REP(j,22){
			mp[i][j]='#';
		}
		CLR(flag);
		int y,x;
		REP(i,H)REP(j,W){
			cin>>mp[i+1][j+1];
			if(mp[i+1][j+1]=='@'){
				mp[i+1][j+1]='.';
				y=i+1;x=j+1;
			}
		}
		
		//cout<<"!"<<y<<" "<<x<<endl;
		cout<<bfs(y,x)<<endl;
	}
	return 0;
}

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

int point(int x,int y,int w,int h){
  int s;
  s= x*w*y*h;
  return s;
}

int point_x(int x,int w){
  int s;
  s=x/w;
  return s;
}

int point_y(int x,int w){
  int s;
  s=x%w;
  return s;
}

int main(){
  int w,h;
  cin>>w>>h;
  while(w!=0){
    int a[21][21]={};
    int sx,sy;
    for(int i=0;i<w;i++){
      for(int j=0;j<h;j++){
	char s;
	cin>>s;
	if(s=='.'){
	  a[i][j]=1;
	}else if(s=='#'){
	  a[i][j]=-1;
	}else{
	  a[i][j]=2;
	  sx=i;
	  sy=j;
	}
      }
    }

    int d[21][21];
    for(int i=0;i<21;i++){
      for(int j=0;j<21;j++){
	d[i][j]=-1;
      }
    }
    queue<int>q;
    d[sx][sy]=0;
    int str;
    str=point(sx,sy,w,h);
    q.push(str);
    while(! q.empty()){
      int cur =q.front();
      q.pop();
      int x,y;
      x=point_x(cur,w);
      y=point_y(cur,w);
      if(x>=1){
	if(a[x-1][y]==1&&d[x-1][y]==-1){
	  d[x-1][y]=0;
	  q.push(point(x-1,y,w,h));
	}
      }
      if(x<=w-2){
	if(a[x+1][y]==1&&d[x+1][y]==-1){
	  d[x+1][y]=0;
	  q.push(point(x+1,y,w,h));
	}
      }
      if(y>=1){
	if(a[x][y-1]==1&&d[x][y-1]==-1){
	  d[x][y-1]=0;
	  q.push(point(x,y-w,w,h));
	}
      }
      if(y<=h-2){
	if(a[x][y+1]==1&&d[x][y+1]==-1){
	  d[x][y-1]=0;
	  q.push(point(x,y+w,w,h));
	}
      }
    }

    int t=0;
    for(int i=0;i<w;i++){
      for(int j=0;j<h;j++){
	if(d[i][j]==0){
	  t+=1;
	}
      }
    }
    cout << t<< endl;
    cin>>w>>h;
  }
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <cctype>
#include <algorithm>
#include <string>
#include <queue>
#include <cstring>
using namespace std;

char map[21][21];
int w, h;
int tile = 0;

void dfs(int x, int y){
    if (!(0 <= x && x < w && 0 <= y < h)) return;
    tile++;
    if (map[y][x] == '@') tile--;
    map[y][x] = '#';
    if (map[y-1][x] == '.') dfs(x, y-1);
    if (map[y+1][x] == '.') dfs(x, y+1);
    if (map[y][x-1] == '.') dfs(x-1, y);
    if (map[y][x+1] == '.') dfs(x+1, y);
}

int main(){
    int sx, sy;
    while (1){
	scanf("%d %d", &w, &h);
	if (w == 0 && h == 0) break;
	memset(map, '#', sizeof(map));
	tile = 0;
	for (int i = 0; i < h; i++){
	    cin >> map[i];
	    for (int p = 0; p < w; p++){
		if (map[i][p] == '@'){
		    sx = p;
		    sy = i;
		}
	    }
	}
	dfs(sx, sy);
	printf("%d\n", tile);
    }
    
    return 0;
}