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 <stdio.h>

int main()
{
	int W,H,ans;
	char map[100][100];
	scanf("%d %d",&W,&H);
	while(W != 0 && H != 0){
		for(int i = 0; i < H; i++){
			scanf("%s",map[i]);
		}
		for(int i = 0; i < 1000; i++){
			for(int j = 0; j < H - 1; j++){
				for(int k = 0; k < W; k++){
					if((map[j][k] == '.' && map[j + 1][k] == '@') || (map[j][k] == '@' && map[j + 1][k] == '.')){
						map[j][k] = '@';
						map[j + 1][k] = '@';
					}
				}
			}
			for(int j = 0; j < H; j++){
				for(int k = 0; k < W - 1; k++){
					if((map[j][k] == '.' && map[j][k + 1] == '@') || (map[j][k] == '@' && map[j][k + 1] == '.')){
						map[j][k] = '@';
						map[j][k + 1] = '@';
					}
				}
			}
		}

		ans = 0;

		for(int j = 0; j < H; j++){
			for(int k = 0; k < W; k++){
				if(map[j][k] == '@'){
					ans++;
				}
			}
		}

		printf("%d\n",ans);
		scanf("%d %d",&W,&H);
	}
	return 0;
}

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

int h, w;

int main (void){
	while(true){
		cin >> w >> h;
		if(!h&&!w)break;
		vector<string> s(h);
		for(int i = 0; i < h; ++i){
			cin >> s[i];
		}
		queue<pair<int, int>> q;
		for(int i = 0; i < h; ++i){
			for(int j = 0; j < w; ++j){
				if(s[i][j] == '@'){
					q.push(make_pair(i,j));
				}
			}
		}
		int dx[] = {0,1,0,-1};
		int dy[] = {1,0,-1,0};
		int cnt = 1;
		while(!q.empty()){
			int x = q.front().first;
			int y = q.front().second;
			q.pop();
			for(int dir = 0; dir < 4; ++dir){
				int nx = x + dx[dir];
				int ny = y + dy[dir];
				if(nx<0||nx>=h||ny<0||ny>=w) continue;
				if(s[nx][ny]=='.'){
					q.push(make_pair(nx,ny));
					s[nx][ny] = '!';
					cnt++;
				}
			}
		}
		cout << cnt << endl;
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
int w,h;
char tail[21][21];
int a[]={-1,0,1,0};
int b[]={0,1,0,-1};
int cnt=0;
void func(int x,int y);
main(){
  int sx,sy;
  while(1){
    cin>> w>> h
      if(w==0&&h==0) break;
    cnt=0;
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin>> tail[i][j];
	if(tail[i][j]=='@'){
	  sx=j;
	  sy=i;
	}
      }
    }
    func(sx,sy);
    cout<< cnt<< endl;
  }
}

void func(int x,int y){
  tail[y][x]='#';
  cnt++;
  for(int i=0;i<4;i++){
    int nx=x+a[i];
    int ny=y+b[i];
    if(nx>=0 && nx<w && ny>=0 && ny<h && tail[ny][nx]!='#'){
      func(nx,ny);
    }
  }
}

C++: #include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
#define REP(i,a) for(int i=0;i<(a);i++)
#define MOD 1000000007

int main(){
  while(1){
    int h,w;
    cin>>w>>h;
    if(h==0&&w==0) break;
    char a[h][w];
    queue<pair<int,int>> q;
    bool used[h][w];
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
        cin>>a[i][j];
        used[i][j]=0;
        if(a[i][j]=='@') q.push(make_pair(i,j));
      }
    }
    int ans=0;
    while(!q.empty()){
      pair<int,int> p=q.front();
      q.pop();
      int x=p.first, y=p.second;
      if(used[x][y]) continue;
      used[x][y]=1;
      ans++;
      if(x>0){
        if(!used[x-1][y]&&a[x-1][y]=='.'){
          q.push(make_pair(x-1,y));
        }
      }
      if(x<h-1){
        if(!used[x+1][y]&&a[x+1][y]=='.'){
          q.push(make_pair(x+1,y));
        }
      }
      if(y>0){
        if(!used[x][y-1]&&a[x][y-1]=='.'){
          q.push(make_pair(x,y-1));
        }
      }
      if(y<w-1){
        if(!used[x][y+1]&&a[x][y+1]=='.'){
          q.push(make_pair(x,y+1));
        }
      }
    }
    cout<<ans<<endl;
  }
  return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <cstring>
using namespace std;
int w,h;
char tbl[30][30];
int dx[] = {0,0,1,-1};
int dy[] = {1,-1,0,0};
bool visited[30][30];
int ans;

void dfs(int x,int y){
  visited[y][x] = 1;
  ++ans;
  for(int i=0;i<4;++i){
    int mx = x+dx[i];
    int my = y+dy[i];
    if(mx>=w || my>=h || mx<0 || my<0) continue;
    if(visited[my][mx] || tbl[my][mx]=='#') continue;
    dfs(mx,my);
  }
}

int main(){
  while(cin>>w>>h,w||h){
    memset(visited,0,sizeof(visited));
    ans = 0;
    for(int i=0;i<h;++i) cin >> tbl[i];
    int sx=-1,sy=-1;
    for(int i=0;i<h;++i)
      for(int j=0;j<w;++j)
	if(tbl[i][j]=='@')
	  sy=i,sx=j;

    dfs(sx,sy);
    cout << ans << endl;
}
  return 0;
}