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 <vector>
#include <algorithm>
#include <set>
#include <queue>
#include <stack>
#include <string>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cmath>

#define rep(i, n) for(int i = 0; i < n; i ++)
#define ALL(T) T.begin(), T.end()
#define mp make_pair
#define pb push_back
#define X first
#define Y second
#define MEMSET(v, h) memset((v), h, sizeof(v))

using namespace std;

typedef pair<int, int> pii;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;

const int INF = 1 << 24;
const int dx[4] = {1, -1, 0, 0};
const int dy[4] = {0, 0, 1, -1};

int cnt, w, h;
char m[100][100];

void dfs(pii);

int main(void){
    while(cin >> w >> h, w){
        MEMSET(m, '#');

        cnt = 0;
        pii p;

        rep(y, h){
            cin >> m[y];
            rep(x, w) if(m[y][x] == '@') m[y][x] = '.', p = mp(x, y);
        }

        dfs(p);
        cout << cnt << endl;
    }

    return 0;
}

void dfs(pii p){
    if((p.X < 0 || w <= p.X) ||
       (p.Y < 0 || h <= p.Y)) return;

    if(m[p.Y][p.X] == '.') cnt ++;
    else return;
    m[p.Y][p.X] = '#';

    rep(i, 4){
        pii next = mp(p.X + dx[i], p.Y + dy[i]);
        dfs(next);
    }
}

C++: #include <iostream>
#include <queue>

using namespace std;

struct TILE {
	char tile;
	bool hantei;
};

typedef pair<int, int> P;

TILE yuka[20][20];
int w, h;

int main() {
	while (cin >> w >> h) {
		if (w == 0 && h == 0) {
			break;
		}
		int si, sj;
		int imove[4] = {1, 0, -1, 0};
		int jmove[4] = {0, 1, 0, -1};
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				cin >> (yuka[i][j]).tile;
				(yuka[i][j]).hantei = false;
				if ((yuka[i][j]).tile == '@') {
					si = i;
					sj = j;
				}
			}
		}
		queue<P> q;
		q.push(P(si, sj));
		while (!q.empty()) {
			P p = q.front();
			q.pop();
			(yuka[p.first][p.second]).hantei = true;
			for (int k = 0; k < 4; k++) {
				if (0 <= p.first+imove[k] && p.first+imove[k] < h 
					&& 0 <= p.second+jmove[k] && p.second+jmove[k] < w 
					&& (yuka[p.first+imove[k]][p.second+jmove[k]]).tile == '.' 
					&& !(yuka[p.first+imove[k]][p.second+jmove[k]]).hantei) {
						q.push(P(p.first+imove[k], p.second+jmove[k]));
				}
			}
		}
		int counter = 0;
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if ((yuka[i][j]).hantei) {
					counter++;
				}
			}
		}
		cout << counter << endl;
	}
	return 0;
}

C++: #include <iostream> // C++の入出力
#include <stdio.h> // C の入出力
#include <algorithm> // sort などのアルゴリズム
#include <string.h> // C の文字列用
#include <ctype.h> // C の文字種用
#include <string> // C++の文字列
#include <math.h>
#include <sstream>
#include <cstdio>
#include <queue>
using namespace std;
char fel[20][20];
int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
void dfs(int i,int x,int y);
int h,w,sx,sy,cnt=1;
int main(void){
    while(true){
        cin>>w>>h;
        if(h+w==0)break;
        for(int s=0;s<h;s++){
            string in;
            cin>>in;
            for(int i=0;i<w;i++){
                fel[s][i]=in[i];
                if(fel[s][i]=='@'){
                    sx=i;
                    sy=s;
                }
            }
        }
        dfs(0,sx,sy);
        cout<<cnt<<endl;
        cnt=1;
    }
    return 0;
}

void dfs(int i,int x,int y){
    fel[y][x]='#';
    for(int s=0;s<4;s++){
        int nx=x+dx[s];
        int ny=y+dy[s];
        if(nx>=0&&nx<w&&ny>=0&&ny<h&&fel[ny][nx]=='.'){
            cnt++;
            dfs(i+1,nx,ny);
        }  
    }
    return;
}

C++: #include <bits/stdc++.h>
using namespace std;

#define pb         push_back

const int INF = 100000000;

int dx[4] = {-1, 0 , 1, 0}, dy[4] = {0 , -1, 0, 1};
int w, h;
vector<string> ma;

void rec(int y, int x);

int main(void) {
	while(1){
		cin >> w >> h;
		if(w == 0 && h == 0) break;

		ma = vector<string>();

		for(int i=0; i<h; i++){
			string t;
			cin >> t;
			ma.pb(t);
		}

		int sx, sy;
		for(int i=0; i<h; i++){
			//cout << ma[i] << endl;
			for(int j=0; j<w; j++){
				if(ma[i][j] == '@'){
					ma[i][j] = '.';
					sx = j;
					sy = i;
				}
			}
		}

		rec(sy, sx);

		int ans = 0;
		for(int i=0; i<h; i++){
			for(int j=0; j<w; j++){
				if(ma[i][j] == 'o'){
					ans++;
				}
			}
		}

		cout << ans << endl;

	}
	
	return 0;
}

void rec(int y, int x){
	//cout << y << ' ' << x << endl;
	if(ma[y][x] == '.') ma[y][x] = 'o';
	else return;
	int nx, ny;
	for(int i=0; i<4; i++){
		nx = x + dx[i];
		ny = y + dy[i];

		if(nx >= 0 && nx < w && ny >= 0 && ny < h){
			rec(ny, nx);
		}
	}
}