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>
using namespace std;

#define repl(i, a, b) for(int i = ((int) a); i < ((int) b); i++)
#define rep(i, n) repl(i, 0, n)

int w, h;
char tile[20][20];
bool vis[20][20];
int dd[] = {-1, 0, 1, 0, -1};

bool range(int y, int x) {
  return 0 <= y && y < h && 0 <= x && x < w;
}

int dfs(int y, int x) {
  if (tile[y][x] == '#') return 0;
  // cout << y << " " << x << endl;
  vis[y][x] = true;
  int cnt = 1;
  rep(i, 4) {
    int ni = y + dd[i], nj = x + dd[i + 1];
    if(range(ni, nj) && tile[ni][nj] == '.' && !vis[ni][nj]) {
      cnt += dfs(ni, nj);
    }
  }
  return cnt;
}


int main(int argc, char const *argv[])
{
  while(cin >> w >> h, w | h) {
    int x, y;
    memset(vis, 0, sizeof(vis));
    rep(i, h)rep(j, w) {
      cin >> tile[i][j];
      if(tile[i][j] == '@') x = j, y = i;
    }

    cout << dfs(y, x) <<endl;

  }
  return 0;
}

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

char mas[20][20];
int dx[4]={0,1,0,-1};
int dy[4]={1,0,-1,0};
int sx,sy;
int w,h,cnt;

void dfs(int x,int y);

int main(){
  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++){
	cin>>mas[i][j];
	if(mas[i][j]=='@'){
	  sx=j;
	  sy=i;
	}
      }
    }
    cnt=0;
    dfs(sx,sy);
    cout<<cnt<<endl;
  }
}

void dfs(int x,int y){
  mas[y][x]='#';
  cnt++;
  for(int i=0;i<4;i++){
    int nextx=x+dx[i];
    int nexty=y+dy[i];
    if(0<=nextx && nextx<w && 0<=nexty && nexty<h && mas[nexty][nextx]=='.'){
      dfs(nextx,nexty);
    }
  }
  return;
}

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

void checkboard(void);
void startmove(void);
int checked(int i,int j);

char board[20][20];
int H,W,now[2];

int main(void){
	while(1){
		cin >> H >> W ;
		if (!H && !W) break;
		// count=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				cin >> board[i][j];
				if(board[i][j]=='@'){
					now[0]=i;
					now[1]=j;
				}
			}
		}
		startmove();
		checkboard();
	}

}


void startmove(void){
	int n;
	do{
		n=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				if(board[i][j]=='@')
					n+=checked(i,j);
			}
		}

	}while(n);
}

int checked(int i,int j){
	int a=0;
	if(i>=0 && board[i-1][j]=='.'){
		board[i-1][j]='@';
		a=1;
	}
	if(i<W && board[i+1][j]=='.'){
		board[i+1][j]='@';
		a=1;
	}
	if(j>=0 && board[i][j-1]=='.'){
		board[i][j-1]='@';
		a=1;
	}
	if(j<H && board[i][j+1]=='.'){
		board[i][j+1]='@';
		a=1;
	}
	// checkboard();
	return a;
}
void checkboard(void){
		int count=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				if(board[i][j]=='@') count++; 
				// cout << board[i][j];
			}
		}
			cout << count << endl;

}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;

int cnt;
int w, h;
int dx[] = {0, 1, 0, -1};
int dy[] = {1, 0, -1, 0};
char tile[20][20];

void dfs(int x, int y);

int main()
{
	int x, y;

	while (true){
		scanf("%d %d", &w, &h);

		if (w == 0){
			break;
		}

		for (int i = 0; i < h; i++){
			scanf("%s", tile[i]);

			for (int j = 0; j < w; j++){
				if (tile[i][j] == '@'){
					y = i;
					x = j;
				}
			}
		}

		cnt = 0;
		dfs(x, y);

		printf("%d\n", cnt);
	}

	return (0);
}

void dfs(int x, int y)
{
	tile[y][x] = '#';
	cnt++;

	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 && tile[ny][nx] == '.'){
			dfs(nx, ny);
		}
	}

}

C++: #include <iostream>
#include <cstdio>
#include <string>
#define rep(i, n) for (int i = 0; i < n; ++i)
using namespace std;
typedef long long ll;

const ll MAX_W = 20, MAX_H = 20;
int tiles[MAX_H][MAX_W];
int d[4][2] = {{-1, 0}, {0, -1}, {0, 1}, {1, 0}};

void dfs(ll x, ll y, ll w, ll h) {
  tiles[y][x] = 0;

  rep(i, 4) {
      int dx = d[i][0];
      int dy = d[i][1];
      if (x + dx < 0 || x + dx >= w) continue;
      if (y + dy < 0 || y + dy >= h) continue;

      if (tiles[y + dy][x + dx] == -1) {
        dfs(x + dx, y + dy, w, h);
      }
  }
}


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

    string line;
    int startX;
    int startY;
    rep(i, h) {
      cin >> line;
      rep(j, w) {
        if (line[j] == '.') {
          tiles[i][j] = -1;
        } else if (line[j] == '#') {
          tiles[i][j] = 1;
        } else {
          tiles[i][j] = -1;
          startX = j;
          startY = i;
        }
      }
    }

    dfs(startX, startY, w, h);
    ll cnt = 0;
    rep(i, h) {
      rep(j, w) {
        if (tiles[i][j] == 0) ++cnt;
      }
    }
    cout << cnt << endl;
  }


  return 0;
}