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;
int a=0;
int box[21][21]={{}};
int  kazoe(int p,int P,int H,int W){
  int i,I;
  a++;
  box[p][P]=0;
  if(box[p][P+1]==1) kazoe(p,P+1,H,W);
  if(box[p][P-1]==1) kazoe(p,P-1,H,W);
  if(box[p-1][P]==1) kazoe(p-1,P,H,W);
  if(box[p+1][P]==1) kazoe(p+1,P,H,W);
  else {
  return a;
  }
}
int main(){
  int i,I,H,W,p,P;
  
  char v;
  while(1){
  cin>>W>>H;
  if(W==0&&H==0) break;
  for(i=0;i<H;i++){
    for(I=0;I<W;I++){
      cin>>v;
      if(v=='.') box[i][I]=1;
      if(v=='#') box[i][I]=2;
      if(v=='@') {
	box[i][I]=0;
	p=i;
	P=I;
      }
    }
  }
  cout<<kazoe(p,P,H,W)<<endl;
  a=0;
 
  
 
  }
  return 0;
}

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

using namespace std;

struct locate{
  int x;
  int y;
};
 
char graph[20][20];
int cou = 0;
int H,W;

void bfs(locate z){
  queue<locate> Q;
  locate u;
  locate v;
  Q.push(z);
 

  while(!Q.empty()){

    u = Q.front();Q.pop();

    if(graph[u.y + 1][u.x] == '.' && graph[u.y + 1][u.x] != 'N'){
      graph[u.y + 1][u.x] = 'N';
      cou++;
      v.y = u.y + 1;
      v.x = u.x;
      Q.push(v);
    }

    if(graph[u.y][u.x + 1] == '.' && graph[u.y][u.x + 1] != 'N'){
      graph[u.y][u.x + 1] = 'N';
      cou++;
      v.x  = u.x + 1;
      v.y = u.y;
      Q.push(v);
    }

    if(graph[u.y - 1][u.x] == '.' && graph[u.y - 1][u.x] != 'N'){
      graph[u.y - 1][u.x] = 'N';
      cou++;
      v.x = u.x;
      v.y = u.y - 1;
      Q.push(v);
    }

    if(graph[u.y][u.x - 1] == '.' && graph[u.y][u.x - 1] != 'N'){
      graph[u.y][u.x - 1] = 'N';
      cou++;
      v.x = u.x - 1;
      v.y = u.y;
      Q.push(v);
    }
    
  }
  cout << endl;
    for(int i = 0; i< H ;i++){
      for(int j = 0; j < W; j++){
	cout << graph[i][j];
      }
      cout << endl;
      }
}

int main(){

  int a1,a2;
  locate a;
  while(1){

    cin >> W >> H;

    if(W == 0 && H == 0) break; 


    for(int i = 0; i< H ;i++){
      for(int j = 0; j < W; j++){
	cin >> graph[i][j];
	if(graph[i][j] == '@'){
	  a1 = i;
	  a2 = j;
	}
      }
    }

    /*
      for(int i = 0; i< H ;i++){
      for(int j = 0; j < W; j++){
      cout << graph[i][j];
      }
      cout << endl;
      }
    */

    a.y = a1;
    a.x = a2;
    bfs(a);

    cout << cou+1 << endl;

    cou = 0;
    for(int i = 0;i < H ;i++){
      for(int j = 0;j < W; j++){
	graph[i][j] = 'N';
      }
    }
    cou = 0;
  }
  return 0;
}

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

using namespace std;

#define rep(i,n) for((i)=0;(i)<(int)(n);(i)++)
#define each(c,itr) for(__typeof((c).begin()) itr=(c).begin();itr!=(c).end();itr++)

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

int w,h,res;
char field[30][30];

void dfs(int y, int x){
  int i;

  res++;
  rep(i,4){
    int my = y + dy[i], mx = x + dx[i];
    if(0 <= my && 0 <= mx && my < h && mx < w && field[my][mx] != '#' && field[my][mx] != '@'){
      field[my][mx] = '#';
      dfs(my,mx);
    }
  }
}

int main(){
  int i,j;

  while(true){
    scanf("%d %d",&w,&h);
    if(w == 0 && h == 0) break;
    res = 0;
    rep(i,h) scanf("%s",field[i]);
    rep(i,h) rep(j,w) if(field[i][j] == '@') dfs(i,j);
    printf("%d\n",res);
  }

  return 0;
}

C++: #include <iostream>
#include <queue>
#include <vector>
#include <string>
#include <utility>
#include <algorithm>

using namespace std;

#define RED false
#define BLACK true

const int dx[] = { -1, 0, 1, 0 }, dy[] = { 0, 1, 0, -1 };
int w,h;

bool valid(int x, int y, const vector<vector<bool> >& area){
  return 0 <= x && x < w && 0 <= y && y < h && area[x][y];
}

int main(){
  while(true){
    cin >> w >> h;
    if(w == 0 && h == 0)
      return 0;

    vector<vector<bool> > area(w, vector<bool>(h, BLACK));
    pair<int, int> hito;
    for(int i = 0; i < h; i++){
      string line;
      cin >> line;
      for(int j = 0; j < w; j++){
        if(line[j] == '#'){
          area[j][i] = RED;
        } else if(line[j] == '@'){
          hito = { j, i };
        }
      }
    }

    queue<pair<int, int> > qu;
    qu.push(hito);

    vector<vector<bool> > visit(w, vector<bool>(h, false));
    visit[hito.first][hito.second] = true;

    int ans = 1;

    while(!qu.empty()){
      auto now = qu.front();
      // cout << "now: " << now.first << ", " << now.second << endl;
      qu.pop();
      for(int i = 0; i < 4; i++){
        auto dst = make_pair( now.first + dx[i], now.second + dy[i] );
        if(valid(dst.first, dst.second, area) && !visit[dst.first][dst.second]){
          visit[dst.first][dst.second] = true;
          ans++;
          qu.push(dst);
        }
      }
    }
    cout << ans << endl;
  }
}