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 <queue>
#include <cstdio>
using namespace std;
int W, H;
char c[100][101];
bool visited[100][100];

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

void computer(){
    memset(visited,false,sizeof visited);
    queue<int> que;
    int i, j;
    for(i=0;i<H;i++){
        for(j=0;j<W;j++){
            if(c[i][j]=='@'){
                que.push(j+i*W);
                visited[i][j] = true;
                goto Skip;
            }
        }
    }
Skip:;
    
    int answer = 1;
    while( !que.empty()){
        int cur = que.front(); que.pop();
        int x = cur % W, y = cur / W;
        
        for(i=0;i<4;++i){
            int nx = x + dx[i],ny = y + dy[i];
            if(!(0<=nx && nx < W && 0<=ny && ny<H))continue;
            if(c[ny][nx]=='#' || visited[ny][nx])continue;
            que.push(nx+ny*W);
            visited[ny][nx]=true;
            ++answer;
        }
        
        
        
    }
    
    cout << answer << endl;
    
    
}

// abcdd
// efgeg

int main(){
    while(cin >> W >> H, W==0&&H==0){
        int i;
        for(i=0;i<H;++i){
            //cin >> c[i][0];
            scanf(" %s",&c[i][0]);
        }
        computer();
        
    }
    
    return 0;
}

C++: import java.util.*;
import java.awt.Point;

public class Main {
	static int w,h,sx,sy;
	static int[] dx = {-1, 0, 1, 0}, dy = {0, -1, 0, 1};
	static String[] field;
	static boolean[][] visited;
	static Scanner sc = new Scanner(System.in);
	public static void main(String[] args) {
		while(read()) {
			System.out.println(solve());
		}
	}
	
	static boolean read() {
		w = sc.nextInt(); h = sc.nextInt();
		if(w == 0 && h == 0) {
			return false;
		}
		field = new String[h];
		
		for(int i = 0; i < h; i++) {
			field[i] = sc.next();
			int tmp = field[i].indexOf('@');
			if(tmp != -1) {
				sy = i; sx = tmp;
			}
		}
		return true;
	}
	
	static int solve() {
		visited = new boolean[h][w];
		Queue<Point> que = new LinkedList<Point>();
		Point s = new Point(sx, sy);
		
		que.add(s);
		visited[sy][sx] = true;
		int ans = 1;
		while(!que.isEmpty()) {
			Point p = que.poll();
			for(int i = 0; i < 4; i++) {
				Point next = new Point(p.x + dx[i], p.y + dy[i]);
				if(!inField(next) || field[next.y].charAt(next.x) == '#' || visited[next.y][next.x] == true) {
					continue;
				} else {
					que.add(next);
					visited[next.y][next.x] = true;
					ans++;
				}
			}
		}
		return ans;
	}
	
	static boolean inField(Point p) {
		if(p.x >= 0 && p.x < w && p.y >= 0 && p.y < h) {
			return true;
		} else {
			return false;
		}
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define all(x) (x).begin(),(x).end()
const int mod=1000000007,MAX=55,INF=1<<30;
int H,W;
int cnt;
int state[MAX][MAX];
vector<int> dx={1,0,-1,0},dy={0,-1,0,1};

void DFS(int us,int ut){
    for(int i=0;i<4;i++){
        if(0<=us+dx[i]&&us+dx[i]<H&&0<=ut+dy[i]&&ut+dy[i]<W){
            if(state[us+dx[i]][ut+dy[i]]==0){
                cnt++;
                state[us+dx[i]][ut+dy[i]]=1;
                DFS(us+dx[i],ut+dy[i]);
            }
        }
    }
}

int main(){
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    while(1){
        cin>>W>>H;
        if(H==0&&W==0) break;
        
        int s,t;
        cnt=1;
        memset(state,0,sizeof(state));
        
        for(int i=0;i<H;i++){
            for(int j=0;j<W;j++){
                char c;cin>>c;
                if(c=='@'){
                    s=i;
                    t=j;
                    state[i][j]=1;
                }else if(c=='#'){
                    state[i][j]=1;
                }
            }
        }
        
        DFS(s,t);
        
        cout<<cnt<<endl;
    }
}

C++: #include <iostream>
using namespace std;
char S[21][21];
int count = 0;

void a(int i, int j){
	if(S[i][j] == '.'){
		count++;
		S[i][j] = '#';
		a(i - 1, j);
		a(i + 1, j);
		a(i, j + 1);
		a(i, j - 1);
	}
}


int main(){
	while(true){
		count = 0;
		int x, y;
		cin >> x >> y;
		if(x == 0 && y == 0){
			break;
		}
		for(int i = 0; i < y; i++){
			for(int j = 0; j < x; j++){
				cin >> S[i][j];
			}
		}

		int startx, starty;
		for(int i = 0; i < y; i++){
			for(int j = 0; j < x; j++){
				if(S[i][j] == '@'){
					startx = j;
					starty = i;
					S[i][j] = '.';
					break;
				}
			}
		}
		a(starty, startx);
		cout << count << endl;
	}
}