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++: //Red and Black
#include<bits/stdc++.h>
using namespace std;

#define rep(i,n) for(int i=0; i<n; i++)

int W, H;
vector<string> mp;
int dir[4][2]={{0,1},{1,0},{0,-1},{-1,0}};

void solve(int x, int y){
  if(x<0 || H<=x || y<0 || W<=y || mp[x][y]!='.')return;
  mp[x][y]='A';
  rep(i,4)solve(x+dir[i][0], y+dir[i][1]);
}

int main(){
  while(true){
    cin>>W>>H;
    if(W==0&&H==0)break;
    mp.clear();
    rep(i,H){
      string s;
      cin>>s;
      mp.push_back(s);
    }
    int sx, sy;
    rep(i,H)rep(j,W)if(mp[i][j]=='@'){sx=i; sy=j; break;}
    mp[sx][sy]='.';
    solve(sx, sy);

    int cnt=0;
    rep(i,H)rep(j,W)if(mp[i][j]=='A')cnt++;
    cout<<cnt<<endl;
  }
  return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <bitset>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
using namespace std;
typedef long long ll;
typedef pair<int,int> pint;
typedef vector<int> vint;
typedef vector<pint> vpint;
#define mp make_pair
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define REP(i,n) for (int i=(n)-1;i>=0;i--)
#define reps(i,f,n) for(int i=(f);i<(n);i++)
#define REPS(i,f,n) for (int i=(f)-1;i>=(n);i--)
int dx[4] = {1, 0, 0, -1};
int dy[4] = {0, 1, -1, 0};

char board[21][21];
int w, h;
int x, y;// ?????¨?????????

void slove(void){
	int cnt = 0;
	queue<pint> q;
	q.push(make_pair(y, x));//?????¨????????????queue?????\??????

	while(!q.empty()){
		pint now = q.front(); q.pop();
		rep(i, 4){
			int nowy = now.fi + dy[i];
			int nowx = now.se + dx[i];
			if (0 <= nowy <= h - 1 && 0 <= nowx && nowx <= w - 1){
				if (board[nowy][nowx] == '.'){
					cnt++;
					board[nowy][nowx] = 0;
					q.push(make_pair(nowy, nowx));
				}
			}
		}
	}
	printf("%d\n", cnt + 1);//?§?????????´????????\??????
	return;
}

void scan(int w, int h){
	rep(i, h){
		rep(j, w){
			cin >> board[i][j];
			if (board[i][j] == '@'){
				y = i; x = j;
			}
		}
		getchar();
	}
	/*
	rep(i, h){
		rep(j, w){
			printf("%c", board[i][j]);
		}
		printf("\n");
	}
	*/
	slove();
	return;
}

int main(void){
	rep(i, 1000){
		cin >> w >> h;

		if (w == 0) break;
		scan(w, h);
	}
	return 0;
}

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

int rec(string field[], int x, int y, int w, int h){
    int count=1;
    int f=0;
    field[y][x] = ',';
    if(x+1<w && field[y][x+1] == '.'){
        count += rec(field, x+1, y, w, h);
    }
    if(y+1<h && field[y+1][x] == '.'){
        count += rec(field, x, y+1, w, h);
    }
    if(x-1>=0 && field[y][x-1] == '.'){
        count += rec(field, x-1, y, w, h);
    }
    if(y-1>=0 && field[y-1][x] == '.'){
        count += rec(field, x, y-1, w, h);
    }
    return count;
}

int main(){
    int w, h;
    string field[20];
    int x, y;
    cin >> w >> h;
    while(w || h){
        x = string::npos;
        for(int i=0;i<h;++i){
            cin >> field[i];
            if(x==string::npos){
                x = field[i].find('@');
                y = i;
            }
        }
        cout << rec(field, x, y, w, h) << endl;
        cin >> w >> h;
    }
}

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

int W, H;

void debugprint(vector<vector<char>> &data) {
    cerr << endl;
    for(int h=0; h<H; h++) {
        for(int w=0; w<W; w++) {
            cerr << data[h][w];
        }
        cerr << endl;
    }
    cerr << endl;
}

int tansaku(vector<vector<char>> &data, int w, int h)
{
    int counter = 0;
    data[h][w] = 'x';
    counter += 1;
    if(w+1<W && data[h][w+1]=='.') counter += tansaku(data, w+1, h);
    if(w-1>=0 && data[h][w-1]=='.') counter += tansaku(data, w-1, h);
    if(h-1>=0 && data[h-1][w]=='.') counter += tansaku(data, w, h-1);
    if(h+1<H && data[h+1][w]=='.') counter += tansaku(data, w, h+1);
    return counter;
}

int solve() {
    int start_pos_w, start_pos_h;
    cin >> W >> H;
    if(W==0&&H==0) return 0;

    vector<vector<char>> datas;
    datas.resize(H);

    for(int h=0; h<H; h++) {
        for(int w=0; w<W; w++) {
            char data;        
            cin>>data;
            if(data=='@') {
                start_pos_w = w;
                start_pos_h = h;
            }
            datas[h].push_back(data);
        }
    }

    int count = tansaku(datas, start_pos_w, start_pos_h);
    cout << count << endl;
    return 1;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    while(solve()){}
}