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;

char ban[21][21];
int cont = 0;

	int w, h;
	
	int MAX = 0;

void saiki(int y, int x)
{
	cont++;
	ban[y][x] = '_';
	/*
	for(int i = 0; i < h; i++){
		for(int j = 0; j < w; j++){
			cout << ban[i][j];
		}
		cout << endl;
	}
	
	cout << w << h << endl;
	
	getchar();
	
	cout << cont << endl;;
	*/
	
	if(ban[y+1][x] == '.' && y+1 < h){
		saiki(y+1,x);
	}
	if(ban[y-1][x] == '.' && y-1 > -1){
		saiki(y-1,x);
	} 
	if(ban[y][x-1] == '.' && x-1 > -1){
		saiki(y,x-1);
	}
	if(ban[y][x+1] == '.' && x+1 < w){
		saiki(y,x+1);
	}
	
	if(cont > MAX){
		MAX = cont;
	}
}

int main()
{
	for(;;){
		int st_x, st_y;
		MAX = cont = 0;
		
		cin >> w >> h;
		
		if( w == 0){
			break;
		}
		
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				ban[i][j] = '0';
			}
		}
		
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				cin >> ban[i][j];
				if(ban[i][j] == '@'){
					st_x = j;
					st_y = i;
				}
			}
		}
		
		
		saiki(st_y,st_x);
		
		cout << MAX << endl;
	}
}

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

typedef  vector< vector<int> > vector2d;

int main() {
  
    int H,W,count=0,kaisuu=0,kosuu,sum,judge;
    char map[20][20];
    vector2d nextx,nexty; //nextx[回数][個数]
    nextx.reserve(1000);
    nexty.reserve(1000);

  while(1){
    cin >>W>>H;
    if (W==0&&H==0)break;
    
    //マップの入力
    for(int cH=0;cH<H;cH++){
	for(int cW=0;cW<W;cW++){
	  cin >> map[cW][cH];
	  if (map[cW][cH]=='@'){
	    nextx[kaisuu].push_back(cW);
	    nexty[kaisuu].push_back(cH);
	  }
	}
    }
    sum = 1;
    while(1){
      judge = 0;
      for(kosuu=0;kosuu<nextx[kaisuu].size();kosuu++){
	if (nextx[kaisuu][kosuu]-1!=-1){
	  if(map[ nextx[kaisuu][kosuu]-1 ][ nexty[kaisuu][kosuu] ]=='.'){  //@の左側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]-1);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]);
	    map[ nextx[kaisuu][kosuu]-1 ][ nexty[kaisuu][kosuu] ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nexty[kaisuu][kosuu]-1!=-1){
	  if(map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]-1 ]=='.'){  //@の上側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]-1);
	    map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]-1 ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nextx[kaisuu][kosuu]+1!=W){
	  if(map[ nextx[kaisuu][kosuu]+1 ][ nexty[kaisuu][kosuu] ]=='.'){  //@の右側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]+1);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]);
	    map[ nextx[kaisuu][kosuu]+1 ][ nexty[kaisuu][kosuu] ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nexty[kaisuu][kosuu]+1!=H){
	  if(map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]+1 ]=='.'){  //@の下側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]+1);
	    map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]+1 ]='@';
	    sum++;
	    judge++;
	  }
	}
      }
      if (judge==0)break;
      kaisuu++;
    }
    kaisuu++;
    cout << sum <<endl;
  }
  
  return 0;
  
}

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

using namespace std;
vector<int> ans;
typedef pair <int, int> P;

int  main(){
  int W,H;
  cin >> W >> H;
  int tate[4] = {0,1,0,-1};
  int yoko[4] = {1,0,-1,0};
  while(W){
    int count = 0;
    queue < P > zahyou;
    vector< vector<int> > room(H, vector<int>(W));
    for(int i = 0 ; i < H ; i++){
      for(int j = 0 ; j < W ; j++){
        char a;
        cin >> a;
        if(a == '.')room[i][j] = 1;
        else if(a == '@'){
          zahyou.push( P (i,j) );
          room[i][j] = 1;
        }
        else room[i][j] = 0;
      }
    }
    while(zahyou.size()){
      pair<int, int> now = zahyou.front();
      zahyou.pop();
      if(room[now.first][now.second] == 1){
        count++;
        room[now.first][now.second] = 0;
        for(int i = 0; i < 4 ; i++){
          zahyou.push( P (now.first+tate[i], now.second+yoko[i]));
        }
      }
    }
    ans.push_back(count);
    cin >> W >> H;
  }
  for(int i = 0 ; i < ans.size(); i++)
    cout << ans[i] << endl;
}