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>
using namespace std;

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

int dfs(int x, int y, vector<vector<bool> >& canGo){
  if(!canGo[x][y]){ return 0; }

  int ans = 1;
  canGo[x][y] = false;
  for(int i = 0; i < 4; i++)
    ans += dfs(x + dx[i], y + dy[i], canGo);
  
  return ans;
}

int main(void){
  while(true){
    int w, h;
    cin >> w >> h;
    if(w == 0 && h == 0){ break; }

    char input;
    int sx, sy;
    vector<vector<bool> > canGo(h + 2, vector<bool>(w + 2, false));
    for(int i = 1; i <= h; i++){
      for(int j = 1; j <= w; j++){
        cin >> input;
        if(input == '.' || input == '@'){ canGo[i][j] = true; }
        if(input == '@'){
          sx = i;
          sy = j;
        }
      }
    }
    
    int ans = dfs(sx, sy, canGo);
    cout << ans << endl;
    
  }
  return 0;
}

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

int walking(vector<vector<char> > &walk, int a, int b, int &count){
	walk[a][b] = '#';
	
	if(walk[a-1][b] == '.'){
		count++;
		walking(walk, a-1, b, count);
	}

	if(walk[a][b-1] == '.'){
		count++;
		walking(walk, a, b-1, count);
	}

	if(walk[a+1][b] == '.'){
		count++;
		walking(walk, a+1, b, count);
	}

	if(walk[a][b+1] == '.'){
		count++;
		walking(walk, a, b+1, count);
	}

	return count;
}

int main(){
	while(1){
		int W, H, a=0, b=0, count=0;
		string str;
		vector<vector<char> > walk;

		cin>>W>>H;
		if((W == 0) && (H == 0))
			break;

		walk.resize(H+2);
		for(int i=0;i<H+2;i++){
			walk[i].resize(W+2);

			if((i == 0) || (i == H+1)){
				for(int j=0;j<W+2;j++)
					walk[i][j] = '#';
			}else{
				walk[i][0] = '#';
				walk[i][W+1] = '#';
			}
		}

		for(int i=1;i<=H;i++){
			cin>>str;
			
			for(int j=0;j<W;j++)
				walk[i][j+1] = str[j];
		}
		
		for(int i=1;i<=H;i++){
			for(int j=1;j<=W;j++){
				if(walk[i][j] == '@'){
					a = i;
					b = j;
					count++;
				}
			}
		}
		
		cout<<walking(walk, a, b, count)<<endl;
	}

	return 0;
}

C++: #include<iostream>
using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
char fld[21][21];
bool used[20][20];
int W, H;

int rec(int i, int j){

   if(fld[i][j]=='#') return 0;
   if(used[i][j]) return 0;
   used[i][j]=true;
   int ret=0;
   for(pii dd: dxdy){
      int ni=i+dd.first;
      int nj=j+dd.second;
      if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
      ret+=rec(ni, nj);
   }

   return ret+1;
}

int main(){

   while(1){
      cin>> W>> H;
      if(!(H||W)) break;
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
               fld[i][j]='.';
            }
         }
      }

      for(int i=0; i<20; i++)for(int j=0; j<20; j++) used[i][j]=false;
      cout<< rec(sy, sx)<< endl;
   }

   return 0;
}

C++: #include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>

 
using namespace std;
 
#define INF 1000000000
#define EPS 1e-9
#define PI acos(-1)
 
typedef long long ll;

#define MAX_W 20
#define MAX_H 20
int W, H;
int sx, sy;
char maze[MAX_H][MAX_W];

vector<int> ans_list;

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

int solve(int x, int y){
  int tmp = 1;
  maze[y][x] = '#';
  for(int i = 0; i < 4; i++){
    int nx = x + dx[i];
    int ny = y + dy[i];
    if(nx >= 0 && nx < W && ny >= 0 && ny < H && maze[ny][nx] != '#'){
      tmp += solve(nx, ny);
    }
  }
  return tmp;
}

int main(){

  while(true){
    
    cin >> W >> H;
    if(W == 0 && H == 0)
      break;

    for(int i = 0; i < H; i++){
      for(int j = 0; j < W; j++){
	cin >> maze[i][j];
	if(maze[i][j] == '@'){
	  sy = i;
	  sx = j;
	}
      }
    }

    ans_list.push_back(solve(sx, sy));
    
  }

  for(int i = 0; i < ans_list.size(); i++){
    cout << ans_list[i] << endl;
  }

  return 0;

}