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 < n; i++)
#define rrep(i,n) for(int i = n - 1; i >= 0; i--)
#define REP(i,k,n) for(int i = k; i < n; i++)

#define vi vector<int>
#define pb push_back
using namespace std;

int dfs(int x,int y);
int dx[] = {0,-1,0,1,0};

int w,h;
string g[21];

int main(){
    while(cin >> w >> h,w) {
        rep(i,h){
            cin >> g[i];
        }
        rep(y,h) {
            rep(x,w){
                if(g[y][x] == '@') {
                    int cnt = dfs(x,y);
                    cout << cnt << endl;
                }
            }
        }
    }
    return 0;

}

int dfs(int x,int y){
    g[y][x] = '#';
    int cnt = 1;
    rep(i,4){
        int nx = x + dx[i], ny = y + dx[i + 1];
        if(nx >= 0 && ny >= 0 && nx < w && ny < h && g[ny][nx] == '.') {
            cnt += dfs(nx,ny);
        }
    }
    return cnt;
}

C++: #include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <algorithm>
#include <cmath>
#include <string>

using namespace std;

typedef vector <int> VI;
typedef vector <VI> VVI;
typedef vector <string> VS;
typedef pair<int,int> PII;
typedef long long LL;
typedef unsigned long long ULL;
typedef pair <int, LL> PIL;
typedef vector <PIL> VPIL;


#define each(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); i++)
#define sort(c) sort((c).begin(),(c).end())

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

#define MAX_INT 2147483647

typedef struct Coor{
	int x;
	int y;
	Coor(int cx,int cy){
		x = cx;
		y = cy;
	}
} Coor;

Coor dxy[] = {Coor(1,0), Coor(0,1), Coor(-1,0), Coor(0,-1)};
string field[20];
int dfs(Coor here, Coor lb, Coor ub){
	int result = 0;
	if(field[here.y][here.x] != '#'){
		result++;
		field[here.y][here.x] = '#';
		rep(i,4){
			Coor nxt = Coor(here.x + dxy[i].x, here.y + dxy[i].y);
			if (lb.x <= nxt.x && nxt.x < ub.x && lb.y <= nxt.y && nxt.y < ub.y)
				result += dfs(nxt, lb, ub);
		}
	}
	return result;
}
int main(){
	int W, H;
	while (1){
		cin >> W >> H;
		if (!(W and H)) break; 
		rep(i,H) cin >> field[i];
		rep(i,H) 
			rep(j,W) 
			if (field[i][j] == '@') 
				cout << dfs(Coor(j,i),Coor(0,0),Coor(W,H)) << endl; 
	}
	return 0;
}

C++: #include <iostream>
#include <cassert>
#define REP(i,l,n) for(int i=l;i<n;++i)
#define rep(i,n) REP(i,0,n)
#define MAX 20
int ans,baseX,baseY;
int w,h;
char tile[MAX][MAX];
void seek(int x,int y);
using namespace std;
int main() {
  while(1){
    cin >> w;
    cin >> h;
    if(w==0&&h==0) return 0;
    rep(i,h){
      rep(j,w){
        cin >> tile[i][j];
        if(tile[i][j] == '@'){
          baseX = i;
          baseY = j;
        }
      }
      tile[i][w] = '#';
    }
    rep(i,w) tile[h][i] = '#';
    ans = 1;
    seek(baseX,baseY);
    cout << ans << endl;
  }
  return 0;
}

void seek(int x,int y){
  if(tile[x+1][y] == '.'){
    tile[x+1][y] = '#';
    ans++;
    seek(x+1,y);
  }
  if(tile[x][y+1] == '.'){
    tile[x][y+1] = '#';
    ans++;
    seek(x,y+1);
  }
  if(tile[x-1][y] == '.'){
    tile[x-1][y] = '#';
    ans++;
    seek(x-1,y);
  }
  if(tile[x][y-1] == '.'){
    tile[x][y-1] = '#';
    ans++;
    seek(x,y-1);
  }
}

C++: #include<iostream>
#include<vector>
#include<queue>
#include<map>
#include<string>
#define pb push_back
#define INF 1000000000

using namespace std;
typedef pair<int,int> P;
int w,h;
bool inf(int y,int x){
	if(y>=0&&y<h&&x>=0&&x<w)
	return true;
	return false;
}
int main(){
	
	int dx[]={0,0,1,-1},dy[]={1,-1,0,0};
	while(cin>>w>>h,w||h){
		vector<string> s(h);
		vector<vector<int> > d(h,vector<int>(w,INF));
		queue<P> que;
		for(int i=0;i<h;i++){
			cin>>s[i];
			for(int j=0;j<h;j++){
				for(int k=0;k<w;k++){
					if(s[j][k]=='@'){
						d[j][k]=0;
						que.push(P(j,k));
					}
				}
			}
		}
		while(!que.empty()){
			P q=que.front();
			que.pop();
			int x=q.second,y=q.first;
			for(int i=0;i<4;i++){
				int ny=dy[i]+y,nx=dx[i]+x;
				if(inf(ny,nx)&&s[ny][nx]!='#'&&d[ny][nx]>d[y][x]+1){
					d[ny][nx]=d[y][x]+1;
					que.push(P(ny,nx));
				}
			}
			
			
		}
		int cnt=0;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(d[i][j]!=INF)
					cnt++;
			}
		}
		cout<<cnt<<endl;
	}
	return 0;
}

C++: #include<iostream>
#include<utility>
#include<queue>
using namespace std;
char room[21][21];
int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};
typedef pair<int,int> p;


int main(){
	int x,y;
	queue<p> que;
	int w,h;
	cin>>w>>h;
	
	for(int a=0;a<h;a++){
		for(int b=0;b<w;b++){
			cin>>room[a][b];
			if(room[a][b]=='@'){
				x=b,y=a;
				que.push(p(b,a));
				room[a][b]='#';
			}
		}
		
	}
	int cnt=1;
	int nx,ny;
	while(que.size()){
		p ap=que.front();
		que.pop();
		for(int i=-1;i<=1;i++){
			nx=ap.first+dx[i];
			ny=ap.second+dy[i];
			if(0<=nx&&nx<=w&&0<=ny&&ny<=h&&room[ny][nx]!='#'){
				room[ny][nx]='#';
				que.push(p(nx,ny));
				cnt++;
			}
		}
	}
	cout<<cnt;
	return 0;
}