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<algorithm>
#include<vector>
#include<math.h>
#include<string>
#include<limits.h>
#define ll long long
#define rep(i,n) for(int i=0;i<n;i++)
#define nrep(i,a,b) for(int i=a;i<b;i++)
#define mrep(i,a) for(int i=a;i<=0;i++)
#define INF LLONG_MAX
using namespace std;

// .:true #:false
vector<pair<ll,ll> > idou(4);
ll w, h, ans;

void dfs(vector<vector<bool> > &map,ll tate, ll yoko) {
	ans++;
	map[tate][yoko] = false;
	ll t, y;
	rep(i, 4) {
		t = tate+idou[i].first;
		y = yoko+idou[i].second;
		if (0 <= t && t < h && 0 <= y && y < w && map[t][y]) {
			dfs(map,t, y);
		}
	}
}

int main() {

	//???
	idou[0].first = -1;
	idou[0].second = 0;
	//???
	idou[1].first = 0;
	idou[1].second = 1;
	//???
	idou[2].first = 1;
	idou[2].second = 0;
	//???
	idou[3].first = 0;
	idou[3].second = -1;

	while (1) {
		ans = 0;
		cin >> w >> h;
		if (!w && !h) break;
		vector<vector<bool> > map(h, vector<bool>(w));
		ll s_t, s_y;
		rep(i, h) {
			string s; cin >> s;
			rep(j, w) {
				if (s[j] == '@') {
					s_t = i;
					s_y = j;
					map[i][j] = true;
				}
				else map[i][j] = (s[j] == '.');
			}
		}dfs(map,s_t, s_y);
		cout << ans << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <queue>
using namespace std;
int dx[] = { -1, 0, 1, 0 };
int dy[] = { 0, -1, 0, 1 };

int main(){
	int W, H;
	while( cin >> W >> H, W, H ){
		char room[20][20];
		queue< pair<int, int> > xy;
		for( int i = 0; i < H; i++ ){
			for( int j = 0; j < W; j++ ){
				cin >> room[i][j];
				if( room[i][j] == '@' ){ xy.push(pair<int, int>(i, j)); }
			}
		}
		//cout << xy.front().first << ' ' << xy.front().second << endl;
		int sum = 1;
		while( !xy.empty() ){
			for( int i = 0; i < 4; i++ ){
				int x = xy.front().second + dx[i];
				int y = xy.front().first + dy[i];
				//cout << room[y][x];
				if( room[y][x] == '.' ){
					room[y][x] = '#';
					xy.push(pair<int, int>(y, x));
					sum++;
				}
			}
			xy.pop();
		}
		cout << sum << endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
string place[20];
int dx[4]={0,1,0,-1},dy[4]={-1,0,1,0};
int serch(int n,int m,int y,int x)
{
	int ans=0;
	place[y][x]='#';
	for(int i=0,ax,ay;i<4;i++)if((ay=dy[i]+y)<m&&(ax=dx[i]+x)<n&&ay>=0&&ax>=0&&place[ay][ax]=='.')ans+=serch(n,m,ay,ax)+1;
	return ans;
}
int main() {
	int n,m;
	while(cin>>n>>m,n,m){
		for(int i=0;i<m;i++)cin>>place[i];
		for(int i=0;i<m;i++)for(int j=0;j<n;j++)if(place[i][j]=='@')cout<<serch(n,m,i,j)+1<<endl;
	}
}

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

class P {
	public:
		int x;
		int y;
		P(int x, int y) :x(x),y(y) {}
};

char map[20][20];

int bfs(int W, int H) {
	queue<P> q;
	bool ch[H][W];
	for (int i=0; i<H; i++)
	for (int j=0; j<W; j++)
		ch[i][j] = false;
	for (int i=0; i<H; i++)
	for (int j=0; j<W; j++)
		if (map[i][j] == '@') {
			q.push(P(j,i));
			ch[i][j] = true;
			break;
		}
	while(!q.empty()) {
		P c = q.front();
		q.pop();
		if (c.x+1<W && (!ch[c.y][c.x+1]) && map[c.y][c.x+1]=='.')
		{ q.push(P(c.x+1, c.y)); ch[c.y][c.x+1]=true; }
		if (c.y+1<H && (!ch[c.y+1][c.x]) && map[c.y+1][c.x]=='.')
		{ q.push(P(c.x, c.y+1)); ch[c.y+1][c.x]=true; }
		if (c.x-1>=0 && (!ch[c.y][c.x-1]) && map[c.y][c.x-1]=='.')
		{ q.push(P(c.x-1, c.y)); ch[c.y][c.x-1]=true; }
		if (c.y-1>=0 && (!ch[c.y-1][c.x]) && map[c.y-1][c.x]=='.')
		{ q.push(P(c.x, c.y-1)); ch[c.y-1][c.x]=true; }
	}
	int cnt = 0;
	for (int i=0; i<H; i++)
	for (int j=0; j<W; j++)
		if (ch[i][j]) cnt++;
	return cnt;
}

int main() {
	int n, m;
	while (cin >> n >> m, (n||m)) {
		for (int i=0; i<m; i++)
		for (int j=0; j<n; j++)
			cin >> map[i][j];
		int res = bfs(n, m);
		cout << res << endl;
	}
	return 0;
}