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 <stdio.h>
#include <string>
#include <math.h>
#include <algorithm>
#include <vector> 
using namespace std;

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

string map[20];
int W, H;

int dfs(int y, int x){
  
  int count = 1;
  int i;
  for(i = 0;i < 4;i++){
    int next_x = x + dx[i] ,next_y = y + dy[i];
    if(0 <= next_x && next_x < W && 0 <= next_y && next_y < H && map[next_y][next_x] == '.'){
      map[next_y][next_x] = 'a';
      count += dfs(next_y, next_x);
    }
  }
  return count;
}


int main(){
  int i, j, x, y;
  while(1){
    cin >> W >> H;
    if(W == 0 && H == 0) break;
    bool start = false;
    for(i = 0;i < H;i++){
      cin >> map[i];
      
    }

    for(i = 0;i < H;i++){
      for(j = 0;j < W;j++){
        if(map[i][j] == '@'){
          x = j;
          y = i;
          start = true;
          break;
        }
      }
      if(start) break;
    }

    cout << dfs(y, x) << endl;

    
  }

  return 0;
}

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(nx < 0 || nx > m || ny < 0 || ny > n) return; 
    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++: #define B(a,x,b) ((a)<=(x)&&(x)<(b))

char T[999];
int x,y,cx,cy,i,c;

void R(a,b){
	if(B(0,a,x)&&B(0,b,y)&&T[b*20+a]){
		T[b*20+a]=0;i++;R(a-1,b);R(a+1,b);R(a,b-1);R(a,b+1);
	}
}

main(){
	for(;scanf("%d%d",&x,&y),x;){
		for(i=0;i<x*y;i++){
			int _x=i%x, _y=i/x;
			for(;(c=getchar())!='\n';);
			switch(c){
				case '@': cx=_x;cy=_y;
				case '.': T[_y*20+_x]=1; break;
				case '#': T[_y*20+_x]=0; break;
			}
		}
		i=0;
		R(cx,cy);
		printf("%d\n",i);
	}
	return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <limits.h>
#include <stdlib.h>
#include <string>
#include <vector>

#define el endl
#define fd fixed
#define INF INT_MAX/2-1
#define pb push_back

using namespace std;

bool tile[20][20];
bool visited[20][20];

void visit(int x, int y) {
  visited[y][x] = true;
  if (x > 0 && tile[y][x-1] && !visited[y][x-1]) visit(x-1, y);
  if (x < 19 && tile[y][x+1] && !visited[y][x+1]) visit(x+1, y);
  if (y > 0 && tile[y-1][x] && !visited[y-1][x]) visit(x, y-1);
  if (y < 19 && tile[y+1][x] && !visited[y+1][x]) visit(x, y+1);
}

int main() {
  int w, h, x, y, count;
  string tmp;
  while (cin >> w >> h, w | h) {
    for (int i = 0; i < 20; i++) {
      for (int j = 0; j < 20; j++) {
        visited[i][j] = false;
        tile[i][j] = false;
      }
    }
    for (int i = 0; i < h; i++) {
      cin >> tmp;
      for (int j = 0; j < w; j++) {
        if (tmp[j] == '.') {
          tile[i][j] = true;
        } else if (tmp[j] == '@') {
          x = j;
          y = i;
          tile[i][j] = true;
        }
      }
    }
    visit(x, y);
    count = 0;
    for (int i = 0; i < 20; i++) {
      for (int j = 0; j < 20; j++) {
        if (visited[i][j]) count++;
      }
    }
    cout << count << el;
  }
}