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 <string>
#include <vector>

using namespace std;

char room[25][25] = {'#'};

int walk(int x, int y) {
    int ret = 1;
    room[y][x] = '#';
    if (room[y-1][x] == '.') ret += walk(x, y-1);
    if (room[y+1][x] == '.') ret += walk(x, y+1);
    if (room[y][x-1] == '.') ret += walk(x-1, y);
    if (room[y][x+1] == '.') ret += walk(x+1, y);
    return ret;
}

int main() {
    int W, H;
    while (cin >> W >> H, W || H) {
        int x, y;
        for (int i=0; i<25; ++i) {
            for (int j=0; j<25; ++j) {
                room[i][j] = '#';
            }
        }
        for (int i=1; i<=H; ++i) {
            for (int j=1; j<=W; ++j) {
                cin >> room[i][j];
                if (room[i][j] == '@') {
                    room[i][j] = '.';
                    y = i, x = j;
                }
            }
        }
        cout << walk(x, y) << endl;
    }
    return 0;
}

C++: #include <iostream>
#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++;
    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 (cin >> w >> h, w || h){
	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);

	cout << tile << endl;
    }
    
    return 0;
}

C++: #include<stdio.h>

char map[32][32];
int ans;
int W,H;

void func(int x,int y){
	map[x][y] = '0';
	ans++;
	if(map[x][y+1] == '.' && y+1<32)
		func(x,y+1);
	if(map[x][y-1] == '.' && y-1>-1)
		func(x,y-1);
	if(map[x+1][y] == '.' && x+1<32)
		func(x+1,y);
	if(map[x-1][y] == '.' && x-1>-1)
		func(x-1,y);
}


int main(void){
	while(scanf("%d%d",&W, &H) && H){
		for(int i = 0; i < H; i++){
			scanf("%s",map[i]);
		}
		for(int i = 0; i < H; i++){
			for(int j = 0; j < W; j++){
				if(map[i][j] == '@'){
					func(i,j);
				}
			}
		}
		printf("%d\n",ans);
		ans=0;
		for(int i = 0; i < 32; i++){
			for(int j = 0; j < 32; j++){
				map[i][j] = 0;
			}
		}
	}
	return 0;
}

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

int dx[4] = { -1, 1, 0, 0 };
int dy[4] = { 0, 0, -1, 1 };

int search(vector<string> v) {

  pair<int,int> init;
  for(int i=0; i<v.size(); i++) {
    for(int j=0; j<v[i].length(); j++) {
      if( v[i][j] == '@' )
	init.second = i,init.first = j;
    }
  }

  int res = 0;

  queue< pair<int,int> > que;
  que.push(init);
  while(!que.empty()) {
    pair<int,int> q = que.front();
    que.pop();

    for(int i=0; i<4; i++) {
      int x=q.first+dx[i];
      int y=q.second+dy[i];
      if( x >= 0 && x < v[q.second].length() ) {
	if( y >= 0 && y < v.size() ) {
	  if( v[y][x] == '.') {
	    v[y][x] = '@';
	    pair<int,int> p;
	    p.first = x;
	    p.second = y;
	    que.push(p);
	  }
	}
      }
    }

  }

  for(int i=0; i<v.size(); i++) {
    for(int j=0; j<v[i].length(); j++) {
      //cout<<v[i][j];
      if(v[i][j] == '@') res++;
    }
    //cout<<endl;
  }
  return res;

}

int main() {

  int w,h;
  while(true) {

    cin>>w>>h;
    if(!w&&!h) break;
    vector<string> v;
    string s;

    for(int i=0; i<h; i++) {
      cin>>s;
      v.push_back(s);
    }

    cout<<search(v)<<endl;
  }
}

C++: #include<stdio.h>

char room[21][21];
int count,W,H;

void root(int W_n,int H_n){
	if((H_n+1)<H&&room[H_n+1][W_n]=='.'){
		room[H_n+1][W_n]='#';
		count++;
		root(W_n,H_n+1);
	}
	if(W_n>0&&room[H_n][W_n-1]=='.'){
		room[H_n][W_n-1]='#';
		count++;
		root(W_n-1,H_n);
	}
	if(H_n>0&&room[H_n-1][W_n]=='.'){
		room[H_n-1][W_n]='#';
		count++;
		root(W_n,H_n-1);
	}
	if((W_n+1)<W&&room[H_n][W_n+1]=='.'){
		room[H_n][W_n+1]='#';
		count++;
		root(W_n+1,H_n);
	}

}


int main(void){
	
	scanf("%d%d",&W,&H);
	while(W!=0&&H!=0){
		count=1;
		for(int i=0;i<H;i++){
			scanf("%s",room[i]);
		}
		for(int i=0;i<H;i++){
			for(int j=0;j<W;j++){
				if(room[i][j]=='@'){
					root(j,i);
				}
			}
		}
		printf("%d\n",count);
		scanf("%d%d",&W,&H);
	}
	return 0;
}