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<bits/stdc++.h>

using namespace std;

int w,h;
int sum;
vector<string> tile(20);

void search(int x,int y);

int main(){

  int now_x,now_y;
  
  while(1){

	sum=0;
	cin>>w>>h;

	if(w==0 && h==0)
	  break;

	tile.clear();

	for(int i=0;i<h;++i)
	  cin>>tile[i];
		
	for(int i=0;i<h;++i){
	  for(int j=0;j<w;++j){
		if(tile[i][j]=='@'){
		  now_x=j;
		  now_y=i;
		  break;
		}
	  }
	}
	
	search(now_x,now_y);
	cout<<sum<<endl;
	
  }

  return 0;
  
}
  
void search(int x,int y){

  int i;
  tile[y][x]='#';
  sum++;
  
  if(x+1 < w){
	if(tile[y][x+1]=='.')
	  search(x+1,y);
  }
  
  if(x-1 >= 0){
	if(tile[y][x-1]=='.')
	  search(x-1,y);
  }
  
  if(y+1 < h){
	if(tile[y+1][x]=='.')
	  search(x,y+1);
  }
  
  if(y-1 >= 0){
	if(tile[y-1][x]=='.')
	  search(x,y-1);
  }
  
}

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

#define ll long long

using namespace std;


int W,H;

char r[1000][1000];

int sh,sw;
int bnum=0;

int solve(int i,int j){
    int ans=0;

    if(i <0 || H <= i || j < 0 || W <= j)
        return 0;
    if(r[i][j] == '#'){
        return 0;
    }
    ans = 1;
    r[i][j] = '#';
    ans += solve(i-1,j);
    ans += solve(i+1,j);
    ans += solve(i,j-1);
    ans += solve(i,j+1);

    return ans;
}


int main(){

    while(1){
        scanf("%d %d",&W,&H);
        if(W == 0 && H == 0) break;

        for(int i=0;i<H;i++){
            for(int j=0;j<W;j++){
                cin >> r[i][j];
                if(r[i][j] == '@'){
                    sh = i;
                    sw = j;
                    r[i][j] = '.';
                }
            }
        }
        cout << solve(sh,sw) << endl;
    }

}

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

int h,w;
char s[20][20];

void dfs(int y,int x){
	if((x<0)||(w<=x)||(y<0)||(h<=y)){
		return;
	}
	if((s[y][x]=='.')||(s[y][x]=='@')){
		s[y][x]='x';
	}else{
		return;
	}
	dfs(y,x+1);
	dfs(y,x-1);
	dfs(y+1,x);
	dfs(y-1,x);
}

int main(){
	while(cin >> w >> h,h){
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				cin >> s[i][j];
			}
		}
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(s[i][j]!='@');
				else{
					dfs(i,j);
					j=w;
					i=h;
				}
			}
		}
		int count=0;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(s[i][j]=='x')
					count++;
			}
		}
		cout << count << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <vector>
using namespace std;

int w, h, ans;
int d[4][2] = { { 1, 0 }, { -1, 0 }, {0, 1}, {0, -1} };
char map[20][20];

void search_dfs(int x, int y){
	if( x < 0 || x >= w || y < 0 || y >= h ) return;
	if( map[y][x] != '.' ) return;
	ans ++;
	map[y][x] = '*';
	for(int i = 0; i < 4; i++)
		search_dfs( x + d[i][0], y + d[i][1] );
}

int main(void){

	while( 1 ) {
		cin >> w >> h;
		if( w == 0 && h == 0 ) break;
		ans = 0;

		for(int i = 0; i < h; i++)
			cin >> map[i];

		for(int i = 0; i < h; i++)
			for(int j = 0; j < w; j++)
				if( map[i][j] == '@' ) {
					map[i][j] = '.';
					search_dfs(j, i);
				}

		cout << ans << endl;
	}

	return 0;
}