Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

type Node struct {
	ID    int
	X, Y  int
	Color Color
	Nexts []*Node
}

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])

		// Initialize map
		m := map[int]map[int]*Node{}
		for x := 1; x <= w; x++ {
			m[x] = map[int]*Node{}
			for y := 1; y <= h; y++ {
				m[x][y] = &Node{ID: x + y*w, X: x, Y: y}
			}
		}
		for x := 1; x <= w; x++ {
			for y := 1; y <= h; y++ {
				nexts := []*Node{}
				if n, ok := m[x-1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y-1]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x+1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y+1]; ok {
					nexts = append(nexts, n)
				}
				m[x][y].Nexts = nexts
			}
		}

		// Set colors
		queue := []*Node{}
		touchedIDs := map[int]struct{}{}
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					n := m[i+1][y]
					n.Color = ColorBlack
					if c == '@' {
						queue = append(queue, n)
						touchedIDs[n.ID] = struct{}{}
					}
				case '#':
					m[i+1][y].Color = ColorRed
				}
			}
		}

		// Search
		visitedIDs := map[int]struct{}{}
		for {
			if len(queue) == 0 {
				break
			}
			n := queue[0]
			queue = queue[1:]
			visitedIDs[n.ID] = struct{}{}
			for _, next := range n.Nexts {
				if next.Color == ColorRed {
					continue
				}
				if _, ok := touchedIDs[next.ID]; !ok {
					queue = append(queue, next)
					touchedIDs[n.ID] = struct{}{}
				}
			}
		}
		fmt.Println(len(visitedIDs))
	}
}

C++: //??¨??¢?´¢:??±???????????¢?´¢??????????????¢?´¢ 

//??????????????¢?????¬?????????
 
//?????°??¢??°???????????????(?????????)????????\???(?????????)
#include <stdio.h>
#include <queue>
#include <string.h>
using namespace std;	
int W,H;
char c[101][101];
bool visited[100][100];
 
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
 	
void compute();//????????????????????£?¨? 
	
int main(){
	
	while(scanf("%d %d",&W,&H),W |H){
	    int i;
		
		for(i=0;i<H;i++){
			scanf(" %s",&c[i][0]);
		}
	}
	return 0;
}

void compute(){
	
	memset(visited,false,sizeof visited);  //{????????????????????????1bit??????????????????1bit????????§?????????} 
	
	queue<int> que;
	int i,j;
	for(j=0;j<W;j++){ 
	 for(i=0;i<H;i++){
	  if(c[i][j]=='@'){
	  	que.push(j+i*H);
	  	visited[i][j]=true;
	  	 break;
	  }
	 }
	}
  int answer=1;
  while(!que.empty()){
	int cur=que.front();
	        que.pop();
	        int x=cur%H;
	        int y=cur/H;
    

  for(i=0;i<4;i++){
	int nx=x+dx[i],ny=y+dy[j];
	
  
  
  if(!(0<=nx&&nx<W&&0<=ny))continue;
  if(c[ny][nx]=='#'||visited[ny][nx])continue;

  que.push(nx+ny*H);
  visited[ny][nx]=true;

    }
   }
 
 }

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;
    }
}