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<iomanip>
#include<cstdlib>
#include<stdio.h>
#include<time.h>
#include<math.h>
#include<algorithm>
#include<string>
#include<vector>
#include<list>
#include<queue>
#include<stack>
#include<deque>
#include<set>
#include<map>
#define REP(i, N) for(ll i = 0; i < N; ++i)
#define FOR(i, a, b) for(ll i = a; i < b; ++i)
#define ALL(a) (a).begin(),(a).end()
#define pb push_back
#define INF 1000000000
#define MOD 1000000007
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
int qx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
int qy[8] = {0, 1, 1, 1, 0, -1, -1, -1};

vector<vector<bool>> d;
vector<string> m;
int w, h;

void cal(ll x, ll y) {
	REP(i, 4) {
		ll nx = x + dx[i];
		ll ny = y + dy[i];
		if(nx >= 0 && nx < h && ny >= 0 && ny < w && m[nx][ny] == '.' && !d[nx][ny]) {
			d[nx][ny] = true;
			cal(nx, ny);
		}
	}
}

int main(void) {
	while(true) {
		cin>>w>>h;
		if(w == 0 && h == 0) break;
		m.resize(h);
		d.resize(h);
		REP(i, h) {
			d[i].resize(w);
			REP(j, w) {
				d[i][j] = false;
			}
		}
		REP(i, h) cin>>m[i];
		REP(i, h) {
			REP(j, w) {
				if(m[i][j] == '@') {
					d[i][j] = true;
					cal(i, j);
					break;
				}
			}
		}
		ll ans = 0;
		REP(i, h) {
			REP(j, w) {
				if(d[i][j]) ++ ans;
			}
		}
		cout<<ans<<endl;
	}
}

C++: #include <iostream>
#include <cmath>
#include <string>
#include <iomanip>
#define GROUND 0
#define WALL 900
#define INITIAL -1

int count();
void init();
void map();
void mark_n(int,int,int,int,int);
void mark_n_all(int);
bool isValidPoint(int,int);

using namespace std;
const int MAX_W = 20;
const int MAX_H = 20;
char floor_[MAX_W][MAX_H];
//floorツつセツづ算ツ術ツ甘鳴青板づ債ャツつエツづゥ
int step[MAX_W][MAX_H];
int width,height;

int main(){
  //  char floor[MAX_W][MAX_H];
  //int step[MAX_W][MAX_H];
  
  while(cin >> width >> height && width > 0){
    string line;
    int ix,iy;
    //cout << width << " " << height << endl;
    init();
    for(int i = 0; i < height; ++i){
      cin >> line;
      for(int j=0; j < width ; ++j){
	floor_[j][i] = line[j];
	if(line[j]=='@'){
	  step[j][i]=GROUND;
	  ix = j; iy = i;
	}else if(line[j]=='#'){
	  step[j][i]=WALL;
	}
	//cout << step[j][i];
      }
      //cout << endl;
    }
    //cout << count('.') << count('#') << count('@') << endl;
    for(int k = 0; k < 400; k++){
      //map();
      mark_n_all(k);
      //mark_n(k+1,ix,iy,1,0);
      //cout << endl;
      //map();
      
      //cout << endl;
    }
    cout << count() << endl;
  }
  
}

int count(){
  int result =0;
  for(int i=0; i<height;++i){
    for(int j=0; j< width; ++j){
      if(step[j][i] >= 0 && step[j][i] < WALL){
	result++;
      }
    }
  }
  return result;
}

void init(){
  for(int i=0; i<height;++i){
    for(int j=0; j< width; ++j){
      step[j][i]=INITIAL;
    }   
  }
}

void map(){
  for(int i = 0; i < height; ++i){
    for(int j=0; j < width ; ++j){
      cout << setw(3)<< step[j][i] << " ";
    }
    cout << endl;
  }
}

void mark_n(int n,int x,int y,int dx,int dy){
  int temp;
  for(int i =0; i< 4;i++){
    temp = dx;
    dx = dy;
    dy = -temp;
    if(isValidPoint(x+dx,y+dy) && step[x+dx][y+dy]==INITIAL){
      step[x+dx][y+dy] = n;
    }
  }
}


void mark_n_all(int n){
  for(int i = 0; i < height; i++){
    for(int j = 0; j < width ; j++){
      if(step[j][i]==n){
	//	cout << j <<" " << i << "jツづツづ個値"<< endl;
	mark_n(n+1,j,i,1,0);
      }
    }
  }
}



bool isValidPoint(int x,int y){
  if(x < 0 || x >  width || y < 0 || y > height){
    return false;
  }else{
    return true;
  }
}

C++: #include<iostream>
#include<string>
#include<vector>
using namespace std;
#define loop(i,a,b) for(int i=a;i<b;i++)
#define rep(i,a) loop(i,0,a)
int x[4]={1,0,-1,0};
int y[4]={0,1,0,-1};
int func(int i,int j,int n,int m,vector<string>& in,int& co){
  co++;
  //cout<<co<<endl;
  in[i][j]='!';
  rep(k,4){
    int nx=i+x[k];
    int ny=j+y[k];
    if(nx>=0 && nx<=n-1 && ny>=0 && ny<=m-1 && in[nx][ny]!='!' && in[nx][ny]=='.')func(nx,ny,n,m,in,co);
  }
  return co;
}
int main(){
  int n,m;
  while(cin>>m>>n,n||m){
    vector<string>in(n);
    int co=0;
    rep(i,n)cin>>in[i];
    rep(i,n)rep(j,m)if(in[i][j]=='@')co=func(i,j,n,m,in,co);
    cout<<co<<endl;
  }
}

C++: #include<iostream>
using namespace std;
char map[114][514];
int w, h, ans;
void dfs(int x, int y) {
	map[x][y] = '#';
	ans++;
	if (map[x - 1][y] == '.')dfs(x - 1, y);
	if (map[x + 1][y] == '.')dfs(x + 1, y);
	if (map[x][y - 1] == '.')dfs(x, y - 1);
	if (map[x][y + 1] == '.')dfs(x, y + 1);
	return;
}
int main() {
	while (cin >> w >> h) {
		if (!w && !h)return 0;
		int x, y;
		ans = 0;
		for (int i = 1; i <= h; i++) {
			for (int j = 1; j <= w; j++) {
				cin >> map[i][j];
				if (map[i][j] == '@') {
					x = i;
					y = j;
				}
			}
		}
		dfs(x, y);
		cout << ans << endl;
	}
}