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 <cstdio>
#include <cstring>
using namespace std;

int W, H;
char map[20][20];
int cnt;
int angle[][2] = {
	{-1, 0}, {0, -1}, {1, 0}, {0, 1}
};

void dfs(int x, int y)
{
	if (map[x][y] == '#' || x < 0 || y < 0 || x >= W || y >= H){
		return;
	}
	cnt++;
	map[x][y] = '#';
	
	for (int i = 0; i < 4; i++){
		dfs(x + angle[i][0], y + angle[i][1]);
	}
}

int main()
{
	int num;
	
	while (scanf("%d %d", &W, &H), W + H){
		int x;
		int y;
		memset(map, 0, sizeof(map));
		
		for (int i = 0; i < H; i++){
			for (int j = 0; j < W; j++){
				scanf(" %c", &map[j][i]);
				if (map[j][i] == '@'){
					x = j;
					y = i;
					map[j][i] = '.';
				}
			}
		}
		
		cnt = 0;
		dfs(x, y);
		
		printf("%d\n", cnt);
	}
	
	return 0;
}

C++: #include "bits/stdc++.h"
using std::cin;
using std::cout;

int cnt = 0;
char map[21][21];

void dfs(int x, int y, int mx, int my){
    if((x < 0) || (x >= mx)){
        return;
    }
    if((y < 0) || (y >= my)){
        return;
    }
    if(map[x][y] == '#'){
        return;
    }
    map[x][y] = '#';
    cnt++;
    dfs(x+1, y, mx, my);
    dfs(x-1, y, mx, my);
    dfs(x, y+1, mx, my);
    dfs(x, y-1, mx, my);
}

int main(){
    int W, H, sx, sy;
    cin >> W >> H;

    while(W != 0 && H != 0){
        for(int i = 0; i < H; i++){
            scanf("%20s", map[i]);
            if(std::string(map[i]).find('@') != std::string::npos){
                sx = i;
                sy = std::string(map[i]).find('@');
            }
        }
        dfs(sx, sy, H, W);
        cout << cnt << std::endl;

        cin >> W >> H;
        cnt = 0;
        
    }

C++: #include <bits/stdc++.h>
#define int long long
#define double long double
#define INF 1e18
using namespace std;
// x[H][W]
// 1: accessible
// INF: not arrived
// -1: can't access
int a[20][20];
int H, W;

void dfs(int x, int y) {
  int moveX[4] = { 1, 0, -1, 0 };
  int moveY[4] = { 0, 1, 0, -1 };
  a[y][x] = 1;
  for (int i = 0; i < 4; i++) {
    int nx = moveX[i]+x;
    int ny = moveY[i]+y;
    if (a[ny][nx] == INF && nx >= 0 && nx < W && ny >= 0 && ny < H) {
      dfs(nx, ny);
    }
  }
}

signed main() {
  while (cin>>W>>H, W+H!=0) {
    int startX, startY;
    for (int i = 0LL; i < H; i++) {
      for (int j = 0LL; j < W; j++) {
        char c; cin >> c;
        if (c == '.') {
          a[i][j] = INF;
        } else if (c == '#') {
          a[i][j] = -1;
        } else {
          a[i][j] = INF;
          startX = j;
          startY = i;
        }
      }
    }
    dfs(startX, startY);
    
    int ans = 0;
    for (int i = 0; i < H; i++) {
      for (int j = 0; j < W; j++) {
        if (a[i][j] == 1) ans++;
      }
    }

    cout << ans << endl;
  }
}

C++: #include<iostream>
#include<cstdio>
#include<string>
using namespace std;
#define loop(i,a,b) for(int i = a ; i < b ; i++)
#define rep(i,a) for(int i = 0 ; i < a ; i++) 

char maze[21][21];
int dx[4] = { 0, 0,-1, 1};
int dy[4] = {-1, 1, 0, 0};
int H,W;
bool inmaze(int x,int y){
  return (0<=x && x<H) && (0<=y && y<W);
}

int dfs(int x,int y){
  if(!inmaze(x,y))return 0;
  maze[x][y] = '#';
  int res = 1;
  rep(i,4){
    if(maze[x+dx[i]][y+dy[i]] == '.'){
      res += dfs(x+dx[i],y+dy[i]);
    }
  }
  return res;
}
int main(void){
  while(cin>>W>>H,H+W){
    int sx,sy;
    rep(i,H){
      string s;
      cin>>s;
      rep(j,W){
	maze[i][j] = s[j];
	if(s[j] == '@'){
	  sx = i;sy = j;
	}
      }
    }
    maze[sx][sy] = '.';
    cout<<dfs(sx,sy)<<endl;
  }
}