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 <queue>
//#include <pair>
#include <tuple>
#define range(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,b) for(int i = 0; i < (b); i++)
#define all(a) (a).begin(), (a).end()
#define show(x)  cerr << #x << " = " << (x) << endl;
using namespace std;

template <typename X, typename T>
auto vectors(X x, T a) {
	return vector<T>(x, a);
}

template <typename X, typename Y, typename Z, typename... Zs>
auto vectors(X x, Y y, Z z, Zs... zs) {
	auto cont = vectors(y, z, zs...);
	return vector<decltype(cont)>(x, cont);
}

template<typename T>
ostream& operator << (ostream& os, vector<T>& v){
	rep(i,v.size()){ os << v[i] << (i == v.size() - 1 ? "" : " "); } return os;
}
template<typename T>
istream& operator >> (istream& is, vector<T>& v){
	for(T& x: v){ is >> x; } return is;
}

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

int main(){
	int h, w;
	while(cin >> w >> h,h||w){
		vector<vector<char>> m(h, vector<char>(w));
		queue<pair<int, int>> q;
		rep(i,h){
			rep(j,w){
				cin >> m[i][j];
				if(m[i][j] == '@') q.emplace(i,j);
			}
		}

		bool u[20][20] = {};
		while(not q.empty()){
			int y, x;
			tie(y, x) = q.front(); q.pop();
			if(u[y][x]) continue;
			u[y][x] = true;
			rep(d,4){
				int ny = y + dy[d];
				int nx = x + dx[d];
				if(ny < 0 or ny >= h or nx < 0 or nx >= w) continue;
				if(m[ny][nx] == '#') continue;
				q.emplace(ny,nx);
			}
		}

		int cnt = 0;
		rep(i,h){
			rep(j,w){
				cnt += u[i][j] == true;
			}
		}
		cout << cnt << endl;
	}
}

C++: #include <iostream>
#include <vector>
#include <string>
using namespace std;
vector<int> x;
vector<int> y;
vector<int> nextx;
vector<int> nexty;
int w,h;
int count;
bool check(int a,int b){
  for (int i=0;i<nextx.size();i++){
    if (nextx[i]==a&&nexty[i]==b)
      return false;
  }
  return true;
}
int search(int **field){
  x.clear();
  y.clear();
  for (int i=0;i<nextx.size();i++){
    x.push_back(nextx[i]);
    y.push_back(nexty[i]);
  }
  nextx.clear();
  nexty.clear();
  for (int i=0;i<x.size();i++){
    field[x[i]][y[i]]=0;
    count++;
  }
  for (int i=0;i<x.size();i++){
    if (field[x[i]-1][y[i]]==1&&x[i]-1>=0&&check(x[i]-1,y[i])){
      nextx.push_back(x[i]-1);
      nexty.push_back(y[i]);
    }
    if (field[x[i]+1][y[i]]==1&&x[i]+1<h&&check(x[i]+1,y[i])){
      nextx.push_back(x[i]+1);
      nexty.push_back(y[i]);
    }
    if (field[x[i]][y[i]-1]==1&&y[i]-1>=0&&check(x[i],y[i]-1)){
      nextx.push_back(x[i]);
      nexty.push_back(y[i]-1);
    }
    if (field[x[i]][y[i]+1]==1&&y[i]+1<w&&check(x[i],y[i]+1)){
      nextx.push_back(x[i]);
      nexty.push_back(y[i]+1);
    }
  }
  if (nextx.empty()){
    return -1;
  }
  int *f[h];
  for (int i=0;i<h;i++)
    f[i]=field[i];
  return search(f);
}
int main(){
  string temp;
  while (cin>>w>>h,w|h!=0){
    count=0;
    int field[h][w];
    for (int i=0;i<h;i++){
      cin>>temp;
      for (int j=0;j<w;j++){
	if (temp[j]=='.')
	  field[i][j]=1;
	if (temp[j]=='#')
	  field[i][j]=0;
	if (temp[j]=='@'){
	  field[i][j]=1;
	  nextx.push_back(i);
	  nexty.push_back(j);
	}
      }
    }
    int *f[h];
    for (int i=0;i<h;i++)
      f[i]=field[i];
    search(f);
    cout<<count<<endl;
  }
}

C++: #include <bits/stdc++.h>
using namespace std;

const int dx[] = {1, -1, 0, 0};
const int dy[] = {0, 0, 1, -1};
int W, H;
char board[30][30];

int dfs(int y, int x) {
   board[y][x] = '#';
   int ret = 1;
   for (int i = 0; i < 4; i++) {
      int ny = y + dy[i], nx = x + dx[i];
      if (ny >= 0 && ny < H && nx >= 0 && nx < W && board[ny][nx] == '.') {
         ret += dfs(ny, nx);
      }
   }
   return ret;
}

int main() {
   while (cin >> W >> H, W) {
      for (int i = 0; i < H; i++) {
         cin >> board[i];
      }

      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (board[i][j] == '@') {
               cout << dfs(i, j) << endl;
            }
         }
      }
   }

   return 0;
}

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

char map[20][20];

int W, H, start_x, start_y, ans = 0;

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

void dfs(int x, int y)
{
  ans++;
  if(map[x][y] != '#'){
    map[x][y] = '#';
    for(int i = 0; i < 4; i++){
      int next_x = x + dx[i], next_y = y + dy[i];
      if(next_x < W && next_x >= 0 && next_y < H && next_y >= 0){
	if(map[next_x][next_y] == '#')
	  continue;
	dfs(next_x, next_y);
      }else{
	continue;
      }
    }
  }
}

int main()
{
  while(1){
    cin >> W >> H;
    if(W == 0 && H == 0){
      break;
    }else{
      for(int i = 0; i < H; i++){
	for(int j = 0; j < W; j++){
	  cin >> map[j][i];
	  if(map[j][i] == '@'){
	    start_x = j;
	    start_y = i;
	  }
	}
      }
      dfs(start_x, start_y);
      cout << ans << endl;
    }
  }
}