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 <iostream>
#include <string>
using namespace std;

string line[30];
int w, h, x, y, sum;

void countup(int x, int y){
	if(line[y][x] != '#'){
		line[y][x] = '#';
		countup(x,y-1);
		countup(x,y+1);
		countup(x+1,y);
		countup(x-1,y);
		sum++;
	}
	return;
}

int main(){
	while(cin >> w >> h){
		if (!w && !h) break;
		sum = 0;
		for(int i = 1;i<=h;i++){
			cin >> line[i];
			line[i] = "#" + line[i] + "#";
			for(int j = 0;j<=w+1;j++){
				if(line[i][j] == '@'){
					x = j;
					y = i;
				}
			}
		}
		line[h + 1] = line[0] = "#####################";
		countup(x, y);
		cout << sum << endl;
	}
	return 0;
}

C++: 
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <vector>
#include <string>
#include <queue>
#include <algorithm>
#include <math.h>
#include <utility>
#include <set>
#define INF 100000
using namespace std;

char G[20][20];
int used[20][20];
int ans;

void dfs(int x, int y, int H, int W){
    if (x<0 || x>= W || y<0 || y>= H || G[y][x]== '#') {
        return;
    }
    if (G[y][x]=='.') {
        ans++;
    }
    used[y][x]=0;
    if (used[y-1][x]) dfs(x, y-1, H, W);
    if (used[y+1][x]) dfs(x, y+1, H, W);
    if (used[y][x-1]) dfs(x-1, y, H, W);
    if (used[y][x+1]) dfs(x+1, y, H, W);
    
    return;
}


int main(){
    for (; ;) {
        int H, W;
        cin >> W>> H;
        if (H==0 && W==0) return 0;
        for (int i=0; i<H; i++) {
            for (int j=0; j<W; j++) {
                used[i][j]=1;
                G[i][j]= '#';
            }
        }
        ans = 0;
        int sx=0,sy=0;
        for (int i=0; i<H; i++) {
            for(int j=0; j<W; j++){
                char p;
                cin >> p;
                G[i][j] = p;
                if (p=='@') {
                    sx = j;
                    sy = i;
                    G[i][j]= '.';
                }
            }
        }
        dfs(sx, sy, H, W);
       /* for (int i=0; i<H; i++) {
            for (int j=0; j<W; j++) {
                cout << used[i][j]<<"|";
            }
            cout << endl;
        }
        for (int i=0; i<H; i++) {
            for (int j=0; j<W; j++) {
                cout << G[i][j]<<"|";
            }
            cout << endl;
        }*/
        cout << ans<< endl;
    }
}

C++: #include<iostream>

using namespace std;

char ban[21][21];
int cont = 0;

	int w, h;
	
	int MAX = 0;

void saiki(int y, int x)
{
	cont++;
	ban[y][x] = '_';
	/*
	for(int i = 0; i < h; i++){
		for(int j = 0; j < w; j++){
			cout << ban[i][j];
		}
		cout << endl;
	}
	
	cout << w << h << endl;
	
	getchar();
	
	cout << cont << endl;;
	*/
	
	if(ban[y+1][x] == '.' && y+1 < h){
		saiki(y+1,x);
	}
	if(ban[y-1][x] == '.' && y-1 > -1){
		saiki(y-1,x);
	} 
	if(ban[y][x-1] == '.' && x-1 > -1){
		saiki(y,x-1);
	}
	if(ban[y][x+1] == '.' && x+1 < w){
		saiki(y,x+1);
	}
	
	if(cont > MAX){
		MAX = cont;
	}
}

int main()
{
	for(;;){
		int st_x, st_y;
		MAX = cont = 0;
		
		cin >> w >> h;
		
		if( w == 0){
			break;
		}
		
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				ban[i][j] = '0';
			}
		}
		
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				cin >> ban[i][j];
				if(ban[i][j] == '@'){
					st_x = j;
					st_y = i;
				}
			}
		}
		
		
		saiki(st_y,st_x);
		
		cout << MAX << endl;
	}
}

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

typedef  vector< vector<int> > vector2d;

int main() {
  
    int H,W,count=0,kaisuu=0,kosuu,sum,judge;
    char map[20][20];
    vector2d nextx,nexty; //nextx[回数][個数]
    nextx.reserve(1000);
    nexty.reserve(1000);

  while(1){
    cin >>W>>H;
    if (W==0&&H==0)break;
    
    //マップの入力
    for(int cH=0;cH<H;cH++){
	for(int cW=0;cW<W;cW++){
	  cin >> map[cW][cH];
	  if (map[cW][cH]=='@'){
	    nextx[kaisuu].push_back(cW);
	    nexty[kaisuu].push_back(cH);
	  }
	}
    }
    sum = 1;
    while(1){
      judge = 0;
      for(kosuu=0;kosuu<nextx[kaisuu].size();kosuu++){
	if (nextx[kaisuu][kosuu]-1!=-1){
	  if(map[ nextx[kaisuu][kosuu]-1 ][ nexty[kaisuu][kosuu] ]=='.'){  //@の左側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]-1);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]);
	    map[ nextx[kaisuu][kosuu]-1 ][ nexty[kaisuu][kosuu] ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nexty[kaisuu][kosuu]-1!=-1){
	  if(map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]-1 ]=='.'){  //@の上側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]-1);
	    map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]-1 ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nextx[kaisuu][kosuu]+1!=W){
	  if(map[ nextx[kaisuu][kosuu]+1 ][ nexty[kaisuu][kosuu] ]=='.'){  //@の右側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]+1);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]);
	    map[ nextx[kaisuu][kosuu]+1 ][ nexty[kaisuu][kosuu] ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nexty[kaisuu][kosuu]+1!=H){
	  if(map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]+1 ]=='.'){  //@の下側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]+1);
	    map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]+1 ]='@';
	    sum++;
	    judge++;
	  }
	}
      }
      if (judge==0)break;
      kaisuu++;
    }
    kaisuu++;
    cout << sum <<endl;
  }
  
  return 0;
  
}