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 <cstdio>
#include <cmath>
#include <vector>
#include <cctype>
#include <algorithm>
#include <string>
#include <queue>
#include <cstring>
using namespace std;

char map[21][21];
int w, h;
int tile = 0;

void dfs(int x, int y){
    if (!(0 <= x && x < w && 0 <= y && y < h)) return;
    tile++;
    map[y][x] = '#';
    if (map[y-1][x] == '.') dfs(x, y-1);
    if (map[y+1][x] == '.') dfs(x, y+1);
    if (map[y][x-1] == '.') dfs(x-1, y);
    if (map[y][x+1] == '.') dfs(x+1, y);
}

int main(){
    int sx, sy;
    while (1){
	scanf("%d %d", &w, &h);
	if (w == 0 && h == 0) break;
	memset(map, '#', sizeof(map));
	tile = 0;
	for (int i = 0; i < h; i++){
	    cin >> map[i];
	    for (int p = 0; p < w; p++){
		if (map[i][p] == '@'){
		    sx = p;
		    sy = i;
		}
	    }
	}
	dfs(sx, sy);
	printf("%d\n", tile);
    }
    
    return 0;
}

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

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

std::string field[20];
int dx[4] = {0,-1,0,1};	//右、上、左、下
int dy[4] = {1,0,-1,0};
int h,w;
int cnt;
void rec(int y,int x){
	field[y][x] = '#';
	cnt++;
	int xi, yi;
	rep(i,4){
		xi = x, yi = y;
		xi += dx[i];
		yi += dy[i];

		if(xi < 0 || xi >= w)continue;
		if(yi < 0 || yi >= h)continue;
		if(field[yi][xi] != '.')continue;
		rec(yi,xi);
	}
}

int main(){

	while(std::cin >> w >> h, (h||w)){

		cnt = 0;

		rep(i,h){
			std::cin >> field[i];
		}
		rep(i,h){
			rep(j,w){
				if(field[i][j] == '@'){
					rec(i,j);
				}
			}
		}
		std::cout << cnt << std::endl;
	}
	return 0;
}

C++: #include<iostream>
#include<string>
#define REP(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;

char field[30][30];
int w, h;
int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, 1, 0, -1};

void init() {
  REP(i, w) {
    REP(j, h) {
      field[i][j] = '?';
    }
  }
}

void trample(int x, int y) {
  if (x < 0 or w <= x or 
      y < 0 or h <= y or 
      field[x][y] != '@') {
    return;
  }

  REP(d, 4) {
    int nx = x + dx[d], ny = y + dy[d];
    if (field[nx][ny] == '.') {
      field[nx][ny] = '@';
      trample(nx, ny);
    }
  }
}

int main() {
  while (cin >> w >> h and w) {
    int sx, sy;
    init();
    REP(y, h) {
      string row;
      cin >> row;
      REP(x, w) {
        field[x][y] = row[x];
        if (row[x] == '@') {
          sx = x, sy = y;
        }
      }
    }

    trample(sx, sy);

    int accessible = 0;
    REP(y, h) {
      REP(x, w) {
        if (field[x][y] == '@') {
          accessible++;
        }
      }
    }
    cout << accessible << endl;
  }
}

C++: import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);

		while (true) {
			int w = 1;
			int h = 1;
			int moveCan = 0;

			w = scan.nextInt();
			h = scan.nextInt();

			if (w == 0 && h == 0)
				break;

			char[][] table = new char[h][w];
			// int[][] num = new int[h][w];

			for (int i = 0; i < h; i++) {
				// for (int j = 0; j < w; j++) {
				table[i]/* [j] */= scan.next().toCharArray();
				// }
			}

			moveCan = AtCheck(table, w, h);
			System.out.println(moveCan);
		}

		scan.close();
	}

	public static int AtCheck(char table[][], int w, int h) {
		int befAtNum = 0;
		int aftAtNum = 0;

		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (table[i][j] == '@')
					befAtNum++;
			}
		}

		int[][] atPlace = new int[h][w];
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (table[i][j] == '@') {
					atPlace[i][j] = 1;
					// if (i != (h - 1) && j != (w - 1) && i != 0 && j != 0
					// && table[i - 1][j - 1]=='.')
					// atPlace[i - 1][j - 1] = 1;
					if (j != 0 && table[i][j - 1] == '.')
						atPlace[i][j - 1] = 1;
					// if (i != (h - 1) && j != (w - 1) && i != 0 && j != 0
					// && table[i + 1][j - 1]=='.')
					// atPlace[i + 1][j - 1] = 1;
					if (i != 0 && table[i - 1][j] == '.')
						atPlace[i - 1][j] = 1;
					if (i != (h - 1) && table[i + 1][j] == '.')
						atPlace[i + 1][j] = 1;
					// if (i != (h - 1) && j != (w - 1) && i != 0 && j != 0
					// && table[i - 1][j + 1]=='.')
					// atPlace[i - 1][j + 1] = 1;
					if (j != (w - 1) && table[i][j + 1] == '.')
						atPlace[i][j + 1] = 1;
					// if (i != (h - 1) && j != (w - 1) && i != 0 && j != 0
					// && table[i + 1][j + 1]=='.')
					// atPlace[i + 1][j + 1] = 1;
				}
			}
		}

		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (atPlace[i][j] == 1)
					table[i][j] = '@';
			}
		}
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (table[i][j] == '@')
					aftAtNum++;
			}
		}
		if (befAtNum != aftAtNum) {
			return AtCheck(table, w, h);
		} else {
			return aftAtNum;
		}
	}

}