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 <algorithm>
#include <vector>
#include <stack>
#include <queue>
#include <map>
#include <cmath>
#include <string>
#include <sstream>
#include <iomanip>
#include <complex>
using namespace std;
 
#define ll long long
#define vvi vector< vector<int> >
#define vi vector<int>
#define All(X) X.begin(),X.end()
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define REP(i,n) for(int i=0;i<(int)(n);i++)
#define pb push_back 
#define pii pair<int,int>
#define mp make_pair
#define pi 3.14159265359
#define shosu(X) fixed << setprecision(X)
ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
ll lcm(ll a,ll b){return a/gcd(a,b)*b;}

char grid[100][100];
int w,h;
int ans=0;
int dx[4]={0,1,0,-1};
int dy[4]={1,0,-1,0};
bool searched[100][100];
int a=0,b=0;

bool inrange(int x,int y){
  if(x<0||y<0||x>=h||y>=w) return false;
  else return true;
}

void search(int x,int y){
  if((grid[x][y]=='.'||grid[x][y]=='@')&&searched[x][y]==false&&inrange(x,y)){
	searched[x][y]=true;
	ans++;
	//	cout <<x <<' '<<y << endl;
	  REP(i,4) {
	  search(x+dx[i],y+dy[i]);
	}
  }
}

int main(){
  while(1){
	cin >> w >> h;
	if(w==0&&h==0) break;
	ans = 0;
	REP(i,h){
	  REP(j,w){
		cin >> grid[i][j];
		searched[i][j]=false;
		if(grid[i][j]=='@'){
		  a=i;
		  b=j;
		}
	  }
	}
	search(a,b);

	cout<<ans<<endl;
  }
}

C++: #include <bits/stdc++.h>
using namespace std;
vector<string>v;
vector<int>ansl;
string s;
int ary[100005] = {0};
char mp[22][22];
int cnt = 0;
int d_x[] = {0,1,0,-1};
int d_y[] = {1,0,-1,0};
int W,H;

void dfs(int x,int y){
  mp[y][x] = '#';
  cnt++;
  for(int d = 0;d<4;d++){
    int nx = x+d_x[d],ny = y+d_y[d];
    if((nx>=0)&&(ny>=0)&&(ny<H)&&(nx<W)&&(mp[ny][nx] == '.')){
      dfs(nx,ny);
    }
  }
}
int main(){
  cin >>W>>H;

  while((W!=0)&&(H!=0)){
    cnt = 0;
    for(int i = 0;i<22;i++){
      for(int j = 0;j<22;j++){
        mp[i][j] = '#';
      }
    }
    for(int i = 0;i<H;i++){
      for(int j = 0;j<W;j++){
        cin >>mp[i][j];
      }
    }
    for (int y = 0; y < H; y++){
      for (int x = 0; x < W; x++) {
        if (mp[y][x] == '@') {
          dfs(x, y);
        }
      }
    }
    cout <<cnt<<endl;
    cin >>W>>H;
  }
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int main(){
  int mp[21][21];
  int w,h;
  int x[4]={1,-1,0,0};
  int y[4]={0,0,1,-1};
  int m;
  bool b;
  char s;
  int ans;
  while(1){
    cin>>w>>h;
    if(w==0 && h==0) break;
    ans=0;
    for(int i=0;i<21;i++){
      for(int j=0;j<21;j++){
	mp[i][j]=0;
      }
    }
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin>>s;
	if(s=='#') mp[i][j]=1;
	else if(s=='@') mp[i][j]=2;
      }
    }
    b=true;
    while(b){
      b=false;
      for(int i=0;i<h;i++){
	for(int j=0;j<w;j++){
	  if(mp[i][j]==2){
	    /*for(int k=0;k<4;k++){*/
	      if(mp[i+1][j]==0){
		mp[i+1][j]=2;
		b=true;
	      }
	      if(mp[i-1][j]==0){
		mp[i-1][j]=2;
		b=true;
	      }
	      if(mp[i][j+1]==0){
		mp[i][j+1]=2;
		b=true;
	      }
	      if(mp[i][j-1]==0){
		mp[i][j-1]=2;
		b=true;
	      }
	  }
	}
      }
    }
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	/*cout<<mp[i][j]<<" ";*/
	if(mp[i][j]==2) ans++;
      }
      /* cout<<endl;*/
    }
    cout<<ans<<endl;
  }
  return 0;
}

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

const int dx[] = {1,0,-1,0};
const int dy[] = {0,-1,0,1};
queue<pair<int,int> > M;
int W,H;
int q[20+5][20+5] = {};	
char p[20+5][20+5];
int counter;
int x,y;
int main() {
	while(cin >> W >> H && W != 0){
		counter = 0;
		for (int i = 1; i <= H; i++){
			for (int j = 1; j <= W; j++){
				cin >> p[i][j];
				if (p[i][j] == '@' ){
					M.push(make_pair(i,j));
					counter = 1;
					q[i][j] = 1;
				}
			}
		}
		while(!M.empty()){
			x = M.front().first;
			y = M.front().second;
			M.pop();
			for (int k = 0; k <= 3; k++)
			{		
				if (p[x+dx[k]][y+dy[k]] == '.' && q[x+dx[k]][y+dy[k]] == 0)
				{
					M.push(make_pair(x+dx[k], y+dy[k]));
					q[x+dx[k]][y+dy[k]] = 1;
					counter += 1;
				}
			}
		}
		cout << counter << endl;
	}
}