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<cstdio>
#include<string>
#include<algorithm>
#include<numeric>
#include<map>
#include<math.h>

using namespace::std;

char gMap[21][21];
bool MapFlg[21][21];
int gRedCount;
void initMap(void)
{
  for(int i = 0; i < 21; i++)
    {
      for(int j = 0; j < 21; j++)
	{
	  gMap[i][j] = '#';
	  MapFlg[i][j] = false;
	}
    }
  gRedCount = 0;
}

void showMap(void)
{
  for(int i = 0; i < 21; i++)
    {
      for(int j = 0; j < 21; j++)
	{
	  cout << gMap[i][j];
	}
      cout << endl;
    } 
}

int countMap(int x , int y)
{
  int dx[] = { 0  , 1 , -1 , 0 };
  int dy[] = { -1 , 0 ,  0 , 1 };

  for(int i = 0; i < 4; i++)
    {
      int mx = x + dx[i];
      int my = y + dy[i];

      if( gMap[my][mx] != '@' && gMap[my][mx] != '#' && MapFlg[my][mx] == false)
	{
	  gRedCount++;
	  MapFlg[my][mx] = true;
	  countMap(mx , my);
	}
    }

  return gRedCount;

}

void calcAns(int sx, int sy)
{
  cout << countMap(sx,sy) + 1 << endl;
}

int main(void)
{

  int w,h;

  while( cin >> w >> h  && (w != 0 && h != 0))
    {
      int sx,sy;
      initMap();
      for(int i = 1; i <= h; i++)
	{
	  for(int j = 1; j <= w; j++)
	    {
	      cin >> gMap[i][j];
	      if( gMap[i][j] == '@')
		{
		  sy = i;
		  sx = j;
		}
	    }
	}
      calcAns(sx,sy);
    }


  return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
using namespace std;
int W, H, sx, sy, cnt;
vector<int> dxy = { 1, 0, -1, 0, 1 }; 
string field[20];

void fun(int y, int x){
    field[y][x] = '#';
    cnt++;
    for(int i=0; i<4; ++i){
        int ny = y + dxy[i+1],
            nx = x + dxy[i];
        if(ny < 0 || ny >= H ||nx < 0 || nx >= W || field[ny][nx] != '.') continue;
        fun(ny, nx);
    }
}
    
int main(){
    while(cin >> W >> H, (W||H)){
        cnt = 0;
        string str;
        cin.ignore();
        for(int i=0; i<H; ++i){
            cin >> field[i];
        }
        for(int i=0; i<H; ++i){
            for(int j=0; j<W; ++j){
                if(field[i][j] == '@'){
                    sx = j; sy = i;
                }
            }
        }
        fun(sy, sx);
        cout << cnt << endl;
    }
    return 0;
}

C++: #include <iostream>
using namespace std;
int W,H;
int x[4]={0,0,1,-1};
int y[4]={1,-1,0,0};
int i,j;
int count=0;
char room[100][100];

int dfs(int, int);

int main(){
  while(1){
    int sy,sx;
    scanf("%d %d",&W,&H);
    if(W==0&&H==0)break;
    for(i = 0 ; i < H ; i++){
      scanf("%c",&room[i][0]);
      for(j = 0 ; j < W ; j++){
	scanf("%c",&room[i][j]);
	if(room[i][j] == '@'){
	  sy = i;
	  sx = j; 
	}
      }
    }
    printf("%d\n", dfs(sx,sy));
  }
  return 0;
}

int dfs(int ax,int ay){
  if(ax<0||ay<0)return 0;
  if(ax>=W||ay>=H)return 0;
  if(room[ay][ax]=='#')return 0;
  room[ay][ax]='#';
  int cnt=1;
  int i;
  for(i=0;i<4;i++){
    int nx=ax+x[i];
    int ny=ay+y[i];
    cnt+=dfs(nx,ny);
  }
  return cnt;
}

C++: #include <iostream>
using namespace std;
#define MAX 21

string str[MAX];
int n,m,cnt,dx[4] = {1,0,-1,0},dy[4] = {0,1,0,-1};

void dfs(int x,int y){
  str[x][y] = '#';

  for(int i = 0 ; i < 4 ; i++){
    int nx = x + dx[i] , ny = y + dy[i];
    if(0 <= nx && nx < m && 0 <= ny && ny < n){
      if(str[nx][ny] == '.'){
	cnt++;
	dfs(nx,ny);
      }
    }
  }

  return;
}

int main(){
  while(cin >> n >> m , n + m){
    for(int i = 0 ; i < m ; i++)
      cin >> str[i];
    
    int x,y;
    for(int i = 0 ; i < n ; i++){
      for(int j = 0 ; j < m ; j++){
	if(str[j][i] == '@'){
	  x = j;
	  y = i;
	  break;
	}
      }
    }
    cnt = 1;
    dfs(x,y);
    cout << cnt << endl;

  }

  return 0;
}

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

struct P {
  int x, y;
  P(int x, int y) : x(x), y(y) { }
};

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

int main() {
  for(int W, H; cin >> W >> H && W; ) {
    vector<string> M;
    queue<P> que;

    for(int y = 0; y < H; y++) {
      string line;
      cin >> line;

      M.push_back(line);

      for(int x = 0; x < W; x++) {
        if(M[y][x] == '@') {
          que.push(P(x, y));
        }
      }
    }

    int answer = 0;
    for(; !que.empty(); que.pop()) {
      P p = que.front();
      answer++;

      for(int i = 0; i < 4; i++) {
        int nx = p.x + dx[i], ny = p.y + dy[i];

        if(0 <= nx && nx < W && 0 <= ny && ny < H && M[ny][nx] == '.') {
          M[ny][nx] = '#';
          que.push(P(nx, ny));
        }
      }
    }

    cout << answer << endl;
  }
}