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>

using namespace std;

const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1};
int H,W,count=0;
char **map;
bool **isCount;

bool valid(int x,int y)
{
    return x<=(W-1)&&x>=0&&y<=(H-1)&&y>=0;
}

void bfs(int x,int y)
{
    if(valid(x,y)&&map[y][x]!='#' && !isCount[y][x])
    {
        count++;
        isCount[y][x]=true;
        for(int i=0;i<4;i++)
            bfs(x+dx[i],y+dy[i]);
    }
  /*int D[100];
  for(int i=0;i<100;i++)
    D[i]=-1;
  queue<int>Q;
  Q.push(src);
  D[src]=0;
  while(!Q.empty())
    {
      int cur=Q.front();
      Q.pop();
       for(int dst=0;dst<n;dst++)
	{
	  if(M[cur][dst]==1&&D[dst]==-1)
	    {
	      D[dst]=D[cur]+1;
	      Q.push(dst);
	    }
    }
    }
    for(int i=0;i<n;i++)
        cout<<i+1<<' '<<D[i]<<endl;
        */
}



int main()
{
    int x,y;
    cin>>W>>H;
    while(W!=0&&H!=0)
    {
        isCount=new bool*[H];
        map=new char*[H];
        for(int i=0;i<H;i++)
        {
            isCount[i]=new bool[W];
            map[i]=new char[W];
        }
        for(int i=0;i<H;i++)
        {
            for(int j=0;j<W;j++)
                isCount[i][j]=false;
        }
        for(int i=0;i<H;i++)
        {
            for(int j=0;j<W;j++)
            {
                cin>>map[i][j];
                if(map[i][j]=='@')
                {
                    x=j;
                    y=i;
                }
            }
        }
        bfs(x,y);
        cout << count << endl;
        delete map;
        delete isCount;
        cin>>W>>H;
        count=0;
    }
    return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>

using namespace std;

#define rep(i, n) for(int i=0; i<(n); ++i)
const int dx[] = {-1, 0, 1, 0}, dy[] = {0, -1, 0, 1};

int W, H;

inline bool inrect(int x, int y){
    return 0 <= x && x < H && 0 <= y && y < W;
}

int dfs(vector<string>& room, int x, int y){
    room[x][y] = '#';
    int res = 1;
    rep(i, 4){
        int nx = x + dx[i], ny = y + dy[i];
        if(!inrect(nx, ny) || room[nx][ny] == '#')continue;
        res += dfs(room, nx, ny);
    }
    return res;
}

int main(){
    while(cin >> W >> H, W|H){
        vector<string> room(H);
        int sx = -1, sy = -1;
        rep(i, H){
            cin >> room[i];
            rep(j, W)if(room[i][j] == '@')sx = i, sy = j;
        }
        cout << dfs(room, sx, sy) << '\n';
    }
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;

int w,h;
string g[100];
const int dy[] = {-1,0,1,0}, dx[] = {0,1,0,-1};

int dfs(int y, int x){
  g[y][x] = '#';

  int res = 1;
  for(int i=0;i<4;i++){
    int ny = y+dy[i], nx = x+dx[i];
    if(ny<0 || nx<0 || ny>=h || nx>=w)continue;
    if(g[ny][nx] == '.')res += dfs(ny,nx);
  }
  return res;
}

int main(){
  while(cin >> w >> h, w){
    for(int i=0;i<h;i++)cin >> g[i];

    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	if(g[i][j] == '@')cout << dfs(i,j) << endl;
      }
    }
  }
}

C++: #include <iostream>
using namespace std;
int a=0;
int box[20][20];
int  kazoe(int p,int P,int H,int W){
  int i,I;
  a++;
  box[p][P]=0;
  if(box[p][P+1]==1) kazoe(p,P+1,H,W);
  if(box[p][P-1]==1) kazoe(p,P-1,H,W);
  if(box[p-1][P]==1) kazoe(p-1,P,H,W);
  if(box[p+1][P]==1) kazoe(p+1,P,H,W);
  else {
  return a;
  }
}
int main(){
  int i,I,H,W,p,P;
  
  char v;
  while(1){
  cin>>W>>H;
  if(W==0&&H==0) break;
  for(i=0;i<H;i++){
    for(I=0;I<W;I++){
      cin>>v;
      if(v=='.') box[i][I]=1;
      if(v=='#') box[i][I]=2;
      if(v=='@') {
	box[i][I]=0;
	p=i;
	P=I;
      }
    }
  }
  cout<<kazoe(p,P,H,W)<<endl;
  a=0;
  for(i=0;i<H;i++){
    for(I=0;I<W;I++){
      box[i][I]=0;
    }
  }
  
 
  }
  return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <string>

using namespace std;

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

int paint(vector<string> &field, int x, int y) {
    int cnt = 0;

    for(int i = 0; i < 4; ++i) {
        int xx = x + dx[i];
        int yy = y + dy[i];

        if(xx < 0 || field[i].size() <= xx || yy < 0 || field.size() <= yy) continue;

        if(field[yy][xx] == '.') {
            field[yy][xx] = '#';
            cnt += paint(field, xx, yy)+1;
        }
    }
    return cnt;
}

int main() {
    while(true) {
        int W, H;
        cin >> W >> H;

        if(!W && !H) break;

        int sx, sy;
        vector<string> field(H);
        for(int i = 0; i < H; ++i) {
            cin >> field[i];
            for(int x = 0; x < W; ++x) {
                if(field[i][x] == '@') {
                    sx = x;
                    sy = i;
                }
            }
        }

        cout << paint(field, sx, sy)+1 << endl;
    }
    return 0;
}