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 <iostream>
#include <math.h>
#include <algorithm>
#include <vector>
using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;

int main(){
	int w,h;
	vector<string> f(30);
	vector<vector<int>> g(30,vector<int>(30,0));
	int count = 0;
	int count2 = 0;

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

		for(int i = 0;i < h; i++){
			cin >> f[i];
		}



		for(int i = 0;i < h; i++){
			for(int j = 0;j < w;j++){
				g[i][j] = 0;
			}
		}
		count = 0;
		count2 = 0;
		while(1){
			for(int i = 0; i < h; i++){
				for(int j = 0; j < w;j++){
					if(f[i][j] == '@' && count== 0){
						g[i][j] = 1;
						count2++;
					}else if(g[i][j] == 1){
						if(i != 0){
							if(f[i-1][j] == '.'){
								if(g[i-1][j] == 0){
									count2++;
								}
								g[i-1][j] = 1;

							}
						}
						if(i != h-1){
							if(f[i+1][j] == '.'){
								if(g[i+1][j] == 0){
									count2++;
								}
								g[i+1][j] = 1;
							}
						}
						if(j != 0){
							if(f[i][j-1] == '.'){
								if(g[i][j-1] == 0){
									count2++;
								}
								g[i][j-1] = 1;
							}
						}
						if(j != w-1){
							if(f[i][j+1] == '.'){
								if(g[i][j+1] == 0){
									count2++;
								}
								g[i][j+1] = 1;
							}
						}
					}else{}
				}
			}

			if(count == count2){
				cout << count << endl;
				break;
			}
			count = count2;
		}

	}
}

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

using namespace std;

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

int func(int x, int y, vector<vector<int>> &map){

    if(map[y][x] == -1)
        return 0;

    if(map[y][x] == 1)
        return 0;

    map[y][x] = 1;

    int ret = 0;

    for(int i = 0; i < 4; i++){
        ret += func(x + dx[i], y + dy[i], map);
    }

    return ret + 1;

}

int main(){

    cin.tie(0);
    ios::sync_with_stdio(false);

    while( true ){

        int w, h;
        cin >> w >> h;
        
        if(w == 0 && h == 0)
            break;

        int x,y;
        vector<vector<int>> map(h + 2, vector<int>(w + 2, -1));
        for(int i = 0; i < h; i++){
            string s;
            cin >> s;
            for(int j = 0; j < w ; j++){
                if(s[j] == '.')
                    map[i + 1][j + 1] = 0;
                else if(s[j] == '#')
                    map[i + 1][j + 1] = -1;
                else {
                    map[i + 1][j + 1] = 0;
                    x = j + 1;
                    y = i + 1;
                }
            }
        }

        cout << func(x, y, map) << endl;
        /*
        for(auto &x:map){
            for(auto &y: x)
                cout << y << ' ';
            cout << endl;
        }
        */

    }

    return 0;

}

C++: #include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <utility>
#include <ctime>
#define INF 999999999
#define mod 1000000007

#define rep(i,n) for(int i=0;i<int(n);i++)
#define rrep(i,n) for(int i=(n);i>=0;i--)
#define REP(i,a,b) for(int i=(a);i<int(b);i++)
#define all(x) (x).begin(),x.end()
#define pb push_back
#define MOD(x) (x%(mod))
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef pair<int,int> pi;

int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};
bool debug=false;
/*---------------------------------------------------*/

char maze[22][22];
int sx,sy;

void make_maze(int w,int h){
  rep(i,22)rep(j,22)maze[i][j]='#';
  rep(i,h)rep(j,w){
    cin>>maze[i][j];
    if(maze[i][j]=='@'){sy=i;sx=j;}
  }
  //rep(i,22){rep(j,22)cout<<maze[i][j];cout<<endl;}
}

void solve(int w,int h){
  queue<int> que;
  int cnt=0;
  que.push(sx);que.push(sy);
  while(que.size()){
    int x=que.front();que.pop();
    int y=que.front();que.pop();
    rep(i,4){
      if(maze[y+dy[i]][x+dx[i]]=='.'&&0<=x+dx[i]&&x+dx[i]<w&&0<=y+dy[i]&&y+dy[i]<h){
	cnt++;
	que.push(x+dx[i]);que.push(y+dy[i]);
	maze[y+dy[i]][x+dx[i]]='#';
      }
    }
  }
  cout<<cnt+1<<endl;
}

int main(){
  int w,h;
  while(cin>>w>>h){
    if(w==0&&h==0)break;
    make_maze(w,h);
    solve(w,h);
  }
}