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 <stdio.h>
#include <string.h>

int main(){
        bool check=true;
        int w, h, at_x, at_y;

        while(1){
                int total=1;
                char tile[22][22] = { { 0 } };
                scanf("%d %d", &w, &h);
                if( w == 0 && h == 0 ) break;
                for( int i=0; i<h; i++ ){
                        scanf("%s", &tile[i+1][1]);
                }
                check = true;
                while( check ){
                        check = false;
                        for( int y=1; y<=h; y++){
                                for( int x=1; x<=w; x++ ){
                                        if( tile[y][x] == '@' ){
                                                at_x = x;
                                                at_y = y;
                                                if( tile[y-1][x] == '.' ){
                                                        tile[y-1][x] = '@';
                                                        check = true;
                                                        total++;
                                                }
                                                if( tile[y+1][x] == '.' ){
                                                        tile[y+1][x] = '@';
                                                        check = true;
                                                        total++;
                                                }
                                                if( tile[y][x-1] == '.' ){
                                                        tile[y][x-1] = '@';
                                                        check = true;
                                                        total++;
                                                }
                                                if( tile[y][x+1] == '.' ){
                                                        tile[y][x+1] = '@';
                                                        check = true;
                                                        total++;
                                                }
                                        }
                                }
                        }
                }
                printf("%d\n", total);
        }
        return 0;
}

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

int dfs(int x,int y);
int h,w;
char ta[22][22];
int cheak[22][22]={0};
int dx[4]={-1,0,1,0};
int dy[4]={0,-1,0,1};
int cou=0;

int main(){
  int a,b;
  while(1){
    cin >> w >> h;
    if(w==0 && h==0) break;
    cou=0;
    
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin >> ta[i][j];
	cheak[i][j]=0;
	if(ta[i][j] == '@'){
	  a=i,b=j;
	}
      }
    }
    //cout << a << " " << b << endl;
    cout << dfs(a,b) << endl;
    /*for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
      cout << cheak[i][j];
      }
      cout << endl;
      }*/
  }
}

int dfs(int x,int y){
  cheak[x][y]=1;
  cou++;
  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 && ta[nx][ny]=='.' && cheak[nx][ny]==0){
      dfs(nx,ny);
    }
  }
  return cou;
}

C++: #include <bits/stdc++.h>

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define RITR(x,c) for(__typeof(c.rbegin()) x=c.rbegin();x!=c.rend();x++)
#define setp(n) fixed << setprecision(n)

#define lf double
#define ll long long
#define vll vector<ll>
#define vi vector<int>
#define pll pair<ll,ll>
#define pi pair<int,int>

#define all(a) (a.begin()),(a.end())
#define rall(a) (a.rbegin()),(a.rend())
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define ins insert

using namespace std;

ll w, h;
char field[100][100];
bool use[100][100];
ll ans=0;

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

void dfs(int x, int y)
{
	use[y][x] = true;
	ans++;
	rep(i, 4){
		ll tx = x+dx[i];
		ll ty = y+dy[i];
		if (!(0<=tx && tx<=w-1 && 0<=ty && ty<=h-1)) continue;
		if (use[ty][tx] || field[ty][tx]=='#') continue;
		dfs(tx, ty);
	}
	return;
}

int main(void)
{
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	while(cin>>w>>h, w!=0){
		ll sx, sy;
		ans = 0;
		memset(use, 0, sizeof(use));
		rep(i, h){
			rep(j, w){
				cin >> field[i][j];
				if (field[i][j]=='@'){
					sx = j; sy = i;
				}
			}
		}
		dfs(sx, sy);
		cout << ans << "\n";
	}
	return 0;
}