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 <fstream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <string>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <algorithm>
#include <functional>
using namespace std;

#define REP(i,n) for(int i=0; i<n; ++i)
#define FOR(i,a,b) for(int i=a; i<=b; ++i)
#define FORR(i,a,b) for (int i=a; i>=b; --i)

typedef long long ll;

int w,h;
int a[20][20];
int dx[4] = {1,-1,0,0} , dy[4] = {0,0,1,-1};
int ans;

void dfs(int i , int j){
    a[i][j]=0;
    ans++;
    REP(k,4){
        int ii=i+dx[k] , jj=j+dy[k];
        if (ii>=0 && ii<h && jj>=0 && jj<w && a[ii][jj]){
            dfs(ii,jj);
        }
    }
}

int main(void) {
    while (cin>>w>>h && w){
        int x , y;
        REP(i,h){
            REP(j,w){
                char c;
                cin >> c;
                if (c=='.')
                    a[i][j]=1;
                else if (c=='#')
                    a[i][j]=0;
                else{
                    a[i][j]=1;
                    x=i;
                    y=j;
                }
            }
        }
        ans = 0;
        dfs(x,y);
        cout << ans << endl;
    }
    
    return 0;
}

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

int W, H;
char visit[21][21];
int sum;
int  dfs(int w, int h){
    int sum = 0;
    if(0<=w && w<W && 0<=h && h<H){
    if(visit[w][h]=='.'){
        visit[w][h] = '!';
        return 1;
    }
    sum += dfs(w+1, h);
    sum +=dfs(w-1, h);
    sum +=dfs(w, h+1);
    sum +=dfs(w, h-1);
    return sum;
    }
    return sum;
}

int main(void){
    string str;
    while(cin >> W >> H){
        if(W == 0 && H == 0)    break;
        for(int i=0; i<H; i++){
            cin >> str;
            for(int j=0; j<W; j++){
                visit[i][j] = str[j];
                if(visit[i][j] == '@')
                    dfs(i, j);
            }
        }
        cout << sum <<endl;
    }
    
    return 0;
}

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


using namespace std;

#define MAX_W 50
#define MAX_H 50

int main(){

  int w,h;
  char tile[MAX_H][MAX_W];

  cin>>w>>h;
  while(w!=0 && h!=0){
    pair<int,int> s;
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin>>tile[i][j];
	if(tile[i][j] == '@'){
	  s = make_pair(i,j);
	}
      }
    }

    int ans = 0;
    queue<pair<int,int> > que;
    que.push(s);
    int next[4][2] = {{0,1},{0,-1},{1,0},{-1,0}};

    while(!que.empty()){
      pair<int,int> now = que.front();
      que.pop();

      if(tile[now.first][now.second] != '#'){
	ans++;
	tile[now.first][now.second] = '#';

	for(int i=0;i<4;i++){
	  int ny = now.first + next[i][0];
	  int nx = now.second + next[i][1];
	  if(ny>=0 && ny<h && nx>=0 && nx<w && tile[ny][nx]!='#'){
	    que.push(make_pair(ny,nx));
	  }
	}
      }
    }

    cout<<ans<<endl;
    cin>>w>>h;
  }

  return 0;
}

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

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




int main() {
	int W,H;
	int q[20][20];
	char p[20][20];
	queue<pair<int,int> > M;
	pair<int,int> X;
	while(cin >> W >> H){
		for (int i=1; i <= 21; i++){
			for (int j=1; j <= 21; j++){
				q[i][j] = 0;
				p[i][j] = ' ';
			}
		}
		if (W == 0) return 0;
		int count = 0;
		for (int i = 1; i <= H; i++){
			for (int j = 1; j <= W; j++){
				cin >> p[i][j];
				if (p[i][j] == '@'){
					M.push(make_pair(i,j));
					count = 1;
					q[i][j] = 1;
				}
			}
		}
		while(!M.empty()){
			W = M.front().first;
			H = M.front().second;
			M.pop();
			for (int i = 0; i <= 3; i++){		
				if (p[W+dx[i]][H+dy[i]] == '.' && q[W+dx[i]][H+dy[i]] == 0){
					M.push(make_pair(W + dx[i], H + dy[i]));
					q[W + dx[i]][H + dy[i]] = 1;
					count += 1;
				}
			}
		}
		cout << count << endl;
	}
}