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<stdio.h>
char a[21][21];
int c;
void dfs(int y,int x){
  c++;
  a[y][x]=0;
  if(y!=0)
    if(a[y-1][x]=='.')
      dfs(y-1,x);
  if(y!=20)
    if(a[y+1][x]=='.')
      dfs(y+1,x);
  if(x!=0)
    if(a[y][x-1]=='.')
      dfs(y,x-1);
  if(x!=20)
    if(a[y][x+1]=='.')
      dfs(y,x+1);
}
int main(){
  int w,h,i,j;
  while(1){
    scanf("%d%d",&w,&h);
    if(w==0&&h==0)break;
    for(i=0;i<21;i++){
      for(j=0;j<21;j++){
	a[i][j]=0;
      }
    }
    for(i=0;i<h;i++){
      for(j=0;j<=w;j++){
	scanf("%c",&a[i][j]);
      }
    }
    c=0;
    for(i=0;i<h;i++){
      for(j=0;j<w;j++){
	if(a[i][j]=='@')dfs(i,j);
      }
    }
    printf("%d\n",c);

   
  }
  return 0;
}

C++: #include<stdio.h>
char a[21][21];
int c;
void dfs(int y,int x){
  c++;
  a[y][x]=0;
  if(y!=0)
    if(a[y-1][x]=='.')
      dfs(y-1,x);
  if(y!=20)
    if(a[y+1][x]=='.')
      dfs(y+1,x);
  if(x!=0)
    if(a[y][x-1]=='.')
      dfs(y,x-1);
  if(x!=20)
    if(a[y][x+1]=='.')
      dfs(y,x+1);
}
int main(){
  int w,h,i,j;
  char b[21];
  while(1){
    scanf("%d%d",&w,&h);
    if(w==0&&h==0)break;
    for(i=0;i<21;i++){
      for(j=0;j<21;j++){
	a[i][j]=0;
      }
    }
    for(i=0;i<h;i++){
	scanf("%s",b);
	for(j=0;b[j]!='\0';j++){
	  a[i][j]=b[j];
	}
    }
    c=0;
    for(i=0;i<h;i++){
      for(j=0;j<w;j++){
	if(a[i][j]=='@')dfs(i,j);
      }
    }
    printf("%d\n",c);

   
  }
  return 0;
}

C++: #include <bits/stdc++.h>

using namespace std;

int W, H;
char room[20][20];

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

int sx, sy;

int dfs(int x, int y)
{
  room[y][x] = '#';
  int cnt = 1;
  for(int i = 0; i < 4; i++){
    int nx = x + dx[i], ny = y + dy[i];
    if(0 <= nx && nx < W && 0 <= ny && ny < H && room[ny][nx] != '#'){
      cnt += dfs(nx, ny);
    }
  }
  return cnt;
}

int main()
{
  while(cin >> W >> H, W || H){
    for(int i = 0; i < H; i++){
      for(int j = 0; j < W; j++){
	cin >> room[i][j];
	if(room[i][j] == '@') sx = j, sy = i;
      }
    }
    cout << dfs(sx, sy) << endl;
  }
  return 0;
}

C++: #include <iostream>

using namespace std;

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

int fill(int x, int y) {
    int count = 1;
    map_[x][y] = '#';
    for (int i=0; i<4; i++) {
        int x_new = x+dx[i];
        int y_new = y+dy[i];
        if (0 <= x_new && x_new < w && 0 <= y_new && y_new < h) {
            if (map_[x_new][y_new] == '.') {
                count += fill(x_new, y_new);
            }
        }
    }
    return count;
}

int main() {
    int x_start, y_start;
    while (true) {
        cin >> w >> h;
        if (w == 0) {
            break;
        }
        for (int i=0; i<h; i++) {
            for (int j=0; j<w; j++) {
                cin >> map_[j][i];
                if (map_[j][i] == '@') {
                    x_start = j;
                    y_start = i;
                }
            }
            cin.ignore(1);
        }
        cout << fill(x_start, y_start) << endl;
    }
    return 0;
}

C++: #include<iostream>
#include<queue>
using namespace std;
int main() {
	int w, h;
	while (cin >> w >> h) {
		if (!w && !h)return 0;
		queue<pair<int, int> > q;
		char map[114][514];
		bool b[114][514] = {};
		for (int i = 1; i <= h; i++) {
			for (int j = 1; j <= w; j++) {
				cin >> map[i][j];
				if (map[i][j] == '@')q.push(make_pair(i, j));
			}
		}
		int ans = 0;
		while (q.size()) {
			ans++;
			int x = q.front().first;
			int y = q.front().second;
			q.pop();
			map[x][y] = '#';
			b[x][y] = true;
			if (map[x - 1][y] == '.' && !b[x - 1][y]) {
				q.push(make_pair(x - 1, y));
				b[x - 1][y] = true;
			}
			if (map[x + 1][y] == '.' && !b[x + 1][y]) {
				q.push(make_pair(x + 1, y));
				b[x + 1][y] = true;
			}
			if (map[x][y - 1] == '.' && !b[x][y - 1]) {
				q.push(make_pair(x, y - 1));
				b[x][y - 1] = true;
			}
			if (map[x][y + 1] == '.' && !b[x][y + 1]) {
				b[x][y + 1] = true;
				q.push(make_pair(x, y + 1));
			}
		}
		cout << ans << endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
char maze[21][21];
#define r(z,x) for(int z=0;z<x;z++)
int ans,a,b;
void dfs(int x,int y){
	int dx[4]={0,-1,0,1},dy[4]={1,0,-1,0};
	maze[x][y]='#',ans++;
	r(i,4)if(maze[x+dx[i]][y+dy[i]]=='.')dfs(x+dx[i],y+dy[i]);
}
int main(){
	while(cin>>a>>b,a||b){
		ans=0;
		fill(maze[0],maze[20],'#');
		r(i,b)r(j,a)cin>>maze[i][j];
		r(i,b)r(j,a)if(maze[i][j]=='@')dfs(i,j),cout<<ans<<endl;
	}
}