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 <cstdio>
#include <iostream>
#include <cmath>
#include <cstring> 
#include <sstream>
#include <iostream>
#include <algorithm>
#include <cstdlib>

using namespace std;

int a[30][30];
int w, h, cnt;
int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, -1, 0, 1};

void search(int x, int y);

bool issearched[30][30];

int main()
{
	while(1){
		cin >> w >> h;
		if(w == 0 && h == 0) break;
		char c;
		int is, js;
		cnt = 1;
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				cin >> c;
				if(c == '.') a[i][j] = 1;
				else if(c == '#') a[i][j] = -1;
				else if(c == '@'){
					is = i;
					js = j;
				}
			}
		}
		search(is, js);
		cout << cnt << endl;
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				a[i][j] = 0;
				issearched[i][j] = false;
			}
		}
	}
	return 0;
}

void search(int x, int y)
{
	int nx, ny;
	for(int i = 0; i < 4; i++){
		nx = x + dx[i];
		ny = y + dy[i];
		issearched[x][y] = true;
		if(nx >= h || ny >=w || nx < 0 || ny < 0) continue;
		if(a[nx][ny] == 1 && !issearched[nx][ny]){
			cnt++;
			search(nx, ny);
		}
	}
}

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

void dfs(int x,int y);
int h,w;
char ta[22][22];
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];
	if(ta[i][j]=='@'){
	  a=i,b=j;
	}
      }
    }
    dfs(a,b);
    cout << cou << endl;
  }
}

void dfs(int x,int y){
  ta[x][y]='#';
  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]=='.'){
      dfs(nx,ny);
    }
  }
}

C++: #include <vector>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>
#include <utility>
#include <functional>
#include <sstream>
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <climits>

using namespace std;

inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v;}
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str();}

typedef vector<int> vi;
typedef vector<vi>  vvi;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;

#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define EACH(t,i,c) for(t::iretator i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)

const double EPS = 1e-10;

int field[22][22];
int num;

const int RED = 0;
const int BLACK = 1;

void dfs(int x, int y) {
	++num;
	field[y][x] = RED;


	if(field[y-1][x] == BLACK) {
		dfs(x, y-1);
	}

	
	if(field[y][x-1] == BLACK) {
		dfs(x-1, y);
	}
	
	if(field[y][x+1] == BLACK) {
		dfs(x+1, y);
	}
	
	if(field[y+1][x] == BLACK) {
		dfs(x, y+1);
	}
}

int main() {
	int w, h;

	while(cin >> w >> h, w, h) {
		REP(i, 22) {
			REP(j, 22) {
				field[i][j] = RED;
			}
		}

		int startx, starty;

		FOR(i, 1, h+1) {
			FOR(j, 1, w+1) {
				char c;
				cin >> c;
				if(c == '.') {
					field[i][j] = BLACK;
				} else if(c == '#') {
					field[i][j] = RED;
				} else {
					startx = j, starty = i;
					field[i][j] = BLACK;
				}
			}
		}

		num = 0;
		dfs(startx, starty);

		cout << num << endl;
	}
}

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

int w, h;
string field[21]; // 文字を扱うのでstring型の配列を使うと便利です。

// 4近傍(上下左右)のときは次のような配列を用意すると便利
int dx[4] = {0,0,1,-1};
int dy[4] = {1,-1,0,0};

// 現在地(x,y)
void dfs(int x, int y){
    // 今いるところを'B'に置き換える
    field[y][x] = 'B';

    // 移動する4方向をループ
    for(int i=0 ; i < 4 ; i++ ){
        // x方向にdx[i], y方向にdy[i], 移動した場所を(mx,my)とする
        int mx = x + dx[i];
        int my = y + dy[i];
        // 範囲外に出たときは次の処理へ
        if( mx < 0 || my < 0 || mx >= w || my >= h ) continue;
        // 黒いタイル'.'だったら再帰呼び出し
        if( field[my][mx] == '.' ){
            dfs( mx , my );
        }
    }
}

int main(){
    while( cin >> w >> h , w || h ){
        for(int y=0 ; y < h ; y++ ){
            cin >> field[y];
        }

        for(int y=0 ; y < h ; y++ ){
            for(int x=0 ; x < w ; x++ ){
                if( field[y][x] == '@' ){ // 人'@'だったら再帰関数を呼び出す
                    dfs( x , y );
                }
            }
        }
        int ans = 0;
        // ループで'B'の数を数える(到達可能な黒いタイルの数)
        for(int y=0 ; y < h ; y++ ){
            for(int x=0 ; x < w ; x++ ){
                if( field[y][x] == 'B' )
                    ans++;
            }
        }
        cout << ans << endl;
    }
}