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 <map>

using namespace std;

typedef pair<int,int> P;
typedef queue<P> Q;

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

#define mp make_pair

int w,h;
char fie[21][21];

void bfs(int x, int y){
    Q q;
    q.push(mp(x,y));
    while(!q.empty()){
        for(int i=0; i<4; i++){
            int xx = q.front().first + dx[i];
            int yy = q.front().second + dy[i];
            if(0 <= xx && xx < w && 0 <= yy && yy < h && fie[yy][xx] == '.'){
                fie[yy][xx] = '@';
                q.push(mp(xx,yy));
            }
        }
        q.pop();
    }
}

int main(){
    while(cin >> w >> h , w||h){
        int sx,sy;
        for(int i=0; i<h; i++){
            cin >> fie[i];
            for(int j=0; j<w; j++){
                if(fie[i][j] == '@'){
                    sx = j; sy = i;
                }
            }
        }
        bfs(sx,sy);
        int ans = 0;
        for(int i=0; i<h; i++){
            for(int j=0; j<w; j++){
                if(fie[i][j] == '@') ans++;
            }
        }
        cout << ans << endl;
    }
}

C++: #include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);i++)
#define nrep(i,n) for(int i=1;i<=(n);i++)
#define FST first
#define SEC second
using namespace std;

typedef pair <int,int> pxy;
#define WMAX 20
#define HMAX 20
int dd[5]={0,1,0,-1,0};
int W,H;

void print(char map[HMAX][WMAX]){
  putchar('\n');
  rep(i,H){
    rep(j,W){
      cout << map[i][j];
    }
    putchar('\n');
  }
}

int dfs(char map[HMAX][WMAX]){
  stack <pxy> s;
  int n=0;
  rep(i,H){
    if(n) break;
    rep(j,W){
      if(map[i][j]=='@'){
	s.push(pxy(j,i));
	n=1;
	break;
      }
    }
  }

  while(!s.empty()){
    int cx=s.top().FST, cy=s.top().SEC;
    s.pop();
    if(map[cy][cx]=='.')
      n++;
    map[cy][cx]='#';
    rep(r,4){
      int nx=cx+dd[r], ny=cy+dd[r+1];
      if(nx<0||W<=nx||ny<0||H<=ny||map[ny][nx]=='#')
	continue;
      s.push(pxy(nx,ny));
    }
  }
  return n;
}


int main(){
  for(;;){
    scanf("%d %d",&W,&H);
    if(!(W+H)) break;
    
    char map[HMAX][WMAX];
    rep(i,H){
      string s;
      cin >> s;
      strcpy(map[i],s.c_str());
    }
    printf("%d\n",dfs(map));
  }
}

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

struct tile
{
    int row;
    int col;
    bool is_visited;
    bool is_black;
};

tile start;
tile map[20][20];

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

bool valid(int row, int col)
{
    return row >= 0 && row < H && col >= 0 && col < W;
}

int bfs(tile map[20][20])
{
    queue<tile> Q;
    Q.push(start);
    int ans = 0;

    while (!Q.empty())
    {
        ans += 1;
        tile src = Q.front();
        Q.pop();

        for (int i = 0; i < 4; i++)
        {
            int row = src.row + dx[i];
            int col = src.col + dy[i];

            // map の範囲外の場合
            if (!valid(row, col))
            {
                continue;
            }

            if (map[row][col].is_black && !map[row][col].is_visited)
            {
                map[row][col].is_visited = 1;
                Q.push(map[row][col]);
            }
        }
    }

    return ans;
}

int main()
{
    while (cin >> W >> H && W)
    {
        // 入力
        for (int row = 0; row < H; row++)
        {
            for (int col = 0; col < W; col++)
            {
                char c;
                cin >> c;

                if (c == '.')
                {
                    map[row][col] = {row, col, 0, 1};
                }
                else if (c == '@')
                {
                    map[row][col] = {row, col, 1, 1};
                    start = map[row][col];
                }
                else
                {
                    map[row][col] = {row, col, 0, 0};
                }
            }
        }

        // BFS
        cout << bfs(map) << endl;
    }
}