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<algorithm>

using namespace std;

char tyle[30][30]={'0'};

int find_tyle(int i,int j){
  if(tyle[i+1][j]=='.'){
    tyle[i+1][j]='a';
    find_tyle(i+1,j);
  }
  if(tyle[i][j+1]=='.'){
    tyle[i][j+1]='a';
    find_tyle(i,j+1);
  }
  if(tyle[i-1][j]=='.'){
    tyle[i-1][j]='a';
    find_tyle(i-1,j);
  }
  if(tyle[i][j-1]=='.'){
    tyle[i][j-1]='a';
    find_tyle(i,j-1);
  }
}

int main(){
  
  int w,h;
  
  while(cin>>w>>h,w){
    int ans=0;
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
      cin>>tyle[i][j];
      }
    }
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
	if(tyle[i][j]=='@'){
	  find_tyle(i,j);
	  tyle[i][j]='a';
	}
      }
    }
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
	if(tyle[i][j]=='a'){
	  ans++;
      }
    }
  }
    cout<<ans<<endl;

   for(int i=0;i<30;++i){
      for(int j=0;j<=30;++j){
      tyle[i][j]='0';
      }
    }
    
  }
}

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

struct Node{
    int x, y;
};

int dx[4] = {0,1,0,-1}, dy[4] = {-1,0,1,0};
char room[25][25];

int main(){
    int w, h;
    while(cin >> w >> h, w != 0){
        for(int i=0;i<=h+1;++i){
            for(int j=0;j<=w+1;++j){
                if(i == 0 || j == 0 || i == h+1 || j == w+1)
                    room[i][j] = '#';
                else
                    room[i][j] = '.';
            }
        }
        int sx = 0, sy = 0;
        for(int i=0;i<h;++i){
            for(int j=0;j<w;++j){
                scanf(" %c",&room[i+1][j+1]);
                if(room[i+1][j+1] == '@'){
                    sx = j+1;
                    sy = i+1;
                }
            }
        }
        queue<Node> que;
        que.push((Node){sx, sy});
        while(!que.empty()){
            Node curr = que.front(); que.pop();
            for(int i=0;i<4;++i){
                int nx = curr.x + dx[i], ny = curr.y + dy[i];
                if(room[ny][nx] == '.'){
                    room[ny][nx] = '@';
                    que.push((Node){nx, ny});
                }
            }
        }
        int ans = 0;
        for(int i=1;i<=h;++i)
            for(int j=1;j<=w;++j)
                if(room[i][j] == '@')ans++;
        cout << ans << endl;
    }

    return 0;
}

C++: #include <iostream>
#include <string>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <utility>
#include <iomanip>

#define ll long long int
#define pb push_back
#define mk make_pair
#define pq priority_queue

using namespace std;
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
const int inf = 1e9;
const ll linf = 1LL << 50;

int h, w;
int sx, sy;
int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, -1, 0, 1};
bool used[21][21];
int color[21][21];

void dfs(int x, int y){
		for(int i = 0; i < 4; i++){
				int nx = x + dx[i];
				int ny = y + dy[i];
				if(nx >= 0 && nx < h && ny >= 0 && ny < w && !used[nx][ny] && color[nx][ny] == 1){
						used[nx][ny] = true;
						dfs(nx, ny);
				}
		}
}

int main(int argc, char const* argv[])
{
	while(true){
			cin >> w >> h;
			if(w == 0)break;
			for(int i = 0; i < h; i++){
					string s;
					cin >> s;
					for(int j = 0; j < w; j++){
							if(s[j] == '.' || s[j] == '@')color[i][j] = 1;
							else color[i][j] = -1;
							if(s[j] == '@'){
									sx = i; sy = j;
							}
							used[i][j] = false;
					}
			}
			dfs(sx, sy);
			int res = 0;
			for(int i = 0; i < h; i++){
					for(int j = 0; j < w; j++){
							if(used[i][j] == 1)res++;
					}
			}
			cout << res << endl;
	}
	return 0;
}

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

const int MAX = 20;
int dx[4] = { 1,0,-1,0 };
int dy[4] = { 0,1,0,-1 };
int map[MAX][MAX];
int ans = 1;

void dfs(int x, int y)
{
	map[x][y] = 2;

	for (int i = 0; i < 4; i++) {
		int nx = x + dx[i];
		int ny = y + dy[i];

		if (nx >= 0 && nx < MAX&&ny >= 0 && ny < MAX) {
			if (map[nx][ny] == 1) {
				ans++;
				dfs(nx, ny);
			}
		}
	}
}

int main()
{
	while (true) {
		ans = 1;
		int W, H;
		cin >> W >> H;

		if (W == 0 && H == 0) {
			break;
		}

		for (int i = 0; i < MAX; i++) {
			for (int j = 0; j < MAX; j++) {
				map[i][j] = 0;
			}
		}

		int x, y;

		for (int i = 0; i < H; i++) {
			string str;
			cin >> str;

			for (int j = 0; j < W; j++) {
				if (str.substr(j, 1) == ".") {
					map[i][j] = 1;
				}
				else if (str.substr(j, 1) == "@") {
					map[i][j] = 2;
					x = i;
					y = j;
				}
			}
		}

		dfs(x, y);

		cout << ans << endl;
	}

	return 0;
}