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>
#include<cstdio>
#include<vector>
#include<algorithm>
#include<queue>
using namespace std;
typedef long long ll;
//Global
int x, y;
const int MAX_N = 20;
char maze[MAX_N][MAX_N+1];
const int INF = 1000000;
int X, Y;//"@"???????????????
int memo[MAX_N][MAX_N];
int dx[4] = { 1,0,-1,0 };
int dy[4] = { 0,1,0,-1 };
int counter;

void bsd(int a, int b) {
	maze[b][a] = '#';
	memo[b][a] = INF;
	counter++;
	for (int i = 0; i < 4; i++) {
		int nx = a + dx[i];
		int ny = b + dy[i];
		if (memo[ny][nx] == 0 && maze[ny][nx] == '.' && 0 <= nx && nx < x && 0 <= ny && ny < y) {
			bsd(nx, ny);
		}
	}

}



//solve
//??±???????????¢?´¢??§"@"??¨???????????£?????????"."????????°????±???????
void solve()
{
	//memo????????????
	//memset(memo,0, sizeof(memo));
	for (int i = 0; i < y; i++) {
		for (int j = 0; j < x; j++) {
			memo[i][j] = 0;
		}
	}

	//"@"??¨??£??\????????????  ??????  INF??§??????  ??????  ????????????????????£?????????????????????"."??§??????
	counter = 0;//"@"??????
	bsd(X, Y);


	cout << counter << endl;

}


//main
int main()
{
	while (true) {
		cin >> x >> y;
		if (x == 0 && y == 0)break;
		//"W"??¨"."??¨"@"?????\???
		for (int i = 0; i < y; i++) {
			for (int j = 0; j < x; j++) {
				cin >> maze[i][j];
				if (maze[i][j] == '@') {
					X = j; Y = i;//@???????????????????¨????
				}
			}
		}
		solve();
	}
    return 0;
}

C++: #include <bits/stdc++.h>
#define FOR(v, a, b) for(int v = (int)(a); v < (int)(b); ++v)
#define FORE(v, a, b) for(int v = (int)(a); v <= (int)(b); ++v)
#define REP(v, n) FOR(v, 0, n)
#define REPE(v, n) FORE(v, 0, n)
#define RS resize
#define CLR clear
#define PB push_back
#define ALL(x) (x).begin(), (x).end()
#define C complex
#define CI complex< int >
#define CD complex< double >
using namespace std;

void split(string str, char chr, vector< string > *res){
	stringstream ss(str);
	string itm;
	while(getline(ss, itm, chr)){
		(*res).PB(itm);
	}
}


int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	int w, h, x, y, cnt;
	vector< vector< char > > field;
	stack< CI > st;
	
	while(cin >> w >> h , w){
		field.RS(h);
		REP(i, h){
			field[i].RS(w);
			REP(j, w){
				cin >> field[i][j];
				if(field[i][j] == '@'){
					x = j;
					y = i;
					field[i][j] = '.';
				}
			}
		}
		
		st.push(CI(x, y));
		cnt = 0;
		
		while(!st.empty()){
			complex< int > cur = st.top();
			st.pop();
			
			if(field[cur.imag()][cur.real()] != '.') continue;
			
			field[cur.imag()][cur.real()] = '#';
			++cnt;
			
			if(cur.imag() > 0){
				if(field[cur.imag() - 1][cur.real()] == '.'){
					st.push(cur + CI(0, -1));
				}
			}
			if(cur.imag() < h - 1){
				if(field[cur.imag() + 1][cur.real()] == '.'){
					st.push(cur + CI(0, 1));
				}
			}
			if(cur.real() > 0){
				if(field[cur.imag()][cur.real() - 1] == '.'){
					st.push(cur + CI(-1, 0));
				}
			}
			if(cur.real() < w - 1){
				if(field[cur.imag()][cur.real() + 1] == '.'){
					st.push(cur + CI(1, 0));
				}
			}			
		}
		
		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]='#';
			}
		}
	}
	if(room[20]=="00")return 0;
	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;
}

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

using namespace std;
using ll = long long;
using ld = long double;
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define reps(i,s,n) for(int i=(int)(s);i<(int)(n);i++)

int w, h;
char t[20][20];
void dfs(int i, int j) {
	t[i][j] = '*';
	for (int di = -1; di <= 1;di++) {
			for (int dj = -1;dj <= 1;dj++) {
				int ni = i + di;
				int nj = j + dj;
				if (di!=dj && di+dj!=0 && ni >= 0 && ni < h&&nj >= 0 && nj < w&&t[ni][nj] == '.') {
					dfs(ni, nj);
				}
			}
		}
	return;
}

int main() {
	cin.sync_with_stdio(false);
	vector<int>ans;
	while (1) {

		cin >> w >> h;
		if (w == 0)break;
		rep(i, h) {
			rep(j,w) {
				cin >> t[i][j];
			}
		}
		rep(i, h) {
			rep(j, w) {
				if (t[i][j] == '@') {
					dfs(i, j);
					break;
				}
			}
		}
		int count = 0;
		rep(i, h) {
			rep(j, w) {
				if (t[i][j] == '*') {
					count++;
				}
			}
		}
		ans.push_back(count);
	}
	rep(i, ans.size()) {
		cout << ans[i] << endl;
	}

	return 0;
}