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 <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (int)n; ++i)
#define REP(i, a, b) for (int i = (int)a; i < (int)b; ++i)
#define rer(i, a, b) for (int i = (int)a; i <= (int)b; ++i)
#define each(i,c) for(__typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define all(v) v.begin(), v.end()
#define mset(a, n) memset(a, n, sizeof(a))

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<vector<int> > vvi;
typedef vector<pair<int, int> > vpii;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;

const int inf = 1000000000;
const int mod = 1000000007;
const double eps = 1e-9;
const int dx[] = { -1, 0, 1, 0};
const int dy[] = { 0, -1, 0, 1};

int W, H;
int ans;
string field[100];

void dfs(int y, int x) {
	if (x < 0 || y < 0 || x == W || y == H) return;
	if (field[y][x] == '#') return;
	ans++;
	field[y][x] = '#';
	rep(i, 4) dfs(y + dy[i], x + dx[i]);
}

int main() {
	while (cin >> W >> H, W) {
		ans = 0;
		rep(i, H) cin >> field[i];
		rep(i, H) rep(j, W) {
			if (field[i][j] == '@') dfs(i, j);
		}
		cout << ans << endl;
	}
	return 0;
}

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

int W, H;
char tile[25][25];
int cnt;

void dfs(int y, int x){
	cnt++;
	tile[y][x] = '#';
	int dy[4] = { 0, 1, 0, -1 };
	int dx[4] = { 1, 0, -1, 0 };
	for (int i = 0; i < 4; i++){
		int Y = y + dy[i];
		int X = x + dx[i];
		if (0 <= Y&&Y < H && 0 <= X&&X < W&&tile[Y][X] != '#')dfs(Y, X);
	}
}

int main(){
	
	
	while (cin >> W >> H,W != 0 && H != 0){
		cnt = 0;

		int y, x;
		for (int i = 0; i < H; i++){
			for (int j = 0; j < W; j++){
				cin >> tile[i][j];
				if (tile[i][j] == '@'){
					y = i;
					x = j;
				}
			}
		}
		dfs(y, x);

		cout << cnt << endl;

	}
return 0;
}

C++: #include <iostream>
#include <queue>
using namespace std;
class pos
{
	public:
	int x;
	int y;
	pos(int,int);
};

pos::pos(int a,int b)
{
	x=a;y=b;
}

int main()
{
	int w,h;
	int g[21][21];
/*	for(int i=0;i<21;i++)
		for(int j=0;j<21;j++)
			g[i][j]=-1;
*/	while(1)
	{
		cin>>w>>h;
		if(w==0&&h==0) break;
		for(int i=0;i<h;i++)
			for(int j=0;j<w;j++)
				g[i][j]=-1;
		int psx,psy;
		bool visited[21][21];
		for(int i=0;i<h;i++)
			for(int j=0;j<w;j++)
			{
				char s;
				cin>>s;
				switch(s)
				{
					case '.':
						g[i][j]=0;break;
					case '#':
						g[i][j]=1;break;
					case '@':
						g[i][j]=2;psx=i;psy=j;break;
				}
				visited[i][j]=false;
			}

/*		for(int i=0;i<h;i++)
		{
			for(int j=0;j<w;j++)
				cout<<g[i][j];
			cout<<endl;
		}
*/
		int num=0;
		queue<pos> Q;
		Q.push(pos(psx,psy));
		num++;
		visited[psx][psy]=true;
		while(!Q.empty())
		{
			pos now=Q.front();
			Q.pop();
	
			for(int i=-1;i<=1;i+=2)
			{
				int newx=now.x+i;
				int newy=now.y;
				if(newx>=0&&newx<h&&newy>=0&&newy<w&&g[newx][newy]==0&&visited[newx][newy]==false)
				{
					Q.push(pos(newx,newy));		
					num++;
					visited[newx][newy]=true;
				}
			}
			for(int i=-1;i<=1;i+=2)
			{                
				int newx=now.x;
				int newy=now.y+i;
				if(newx>=0&&newx<h&&newy>=0&&newy<w&&g[newx][newy]==0&&visited[newx][newy]==false)
				{
					Q.push(pos(newx,newy));
					num++;
					visited[newx][newy]=true;
				}
			}

		}
		cout<<num<<endl;
								


	}
}

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

int W,H;
int nx[4],ny[4];
string M[128];

const int dx[4] = { 1, 0, -1, 0, } , dy[4] = { 0, 1, 0, -1, };

bool good_cell(int x,int y) {
  return min(x,y) >= 0 && x<W && y<H;
}

bool safe_cell(int x, int y) {
  return good_cell(x,y) && M[y][x] == '.';
}

void neighbor4(int x, int y) {
  for (int d=0; d<4; ++d) {
    nx[d] = x + dx[d], ny[d] = y + dy[d];
  }
}

void walk1() {
  for (int y=0; y<H; ++y) {
    for(int x=0; x<W; ++x) {
      if (M[y][x] == '@') {
	neighbor4(x, y);
	for (int i=0; i<4; ++i) {
	  if (good_cell(nx[i],ny[i]) == 1)
	  if (M[ny[i]][nx[i]] == '.') M[ny[i]][nx[i]] = '@';
	}
      }
    }
  }
}

void print_map() {
  for (int y=0; y<H; ++y) cout << M[y] << endl;
}

int main() {
  while (cin >> W >> H && W > 0) {
    int c=0;
    for (int i=0; i<H; ++i)
      cin >> M[i];
  


for (int y=0; y<H; ++y) {
  for (int x=0; x<W; ++x) {
    if (M[y][x] == '@') {
    }
  }
 }
 for (int i=0; i<W*H; ++i) walk1();
 //print_map();
 for (int y=0; y<H; ++y) {
   for(int x=0; x<W; ++x) {
     if(M[y][x] =='@') {
       c++;
     }
   }
 }
 cout << c << endl;
  }
}