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 <algorithm>
#include <cstdio>
#include <iostream>
#include <string>
#include <vector>
#include <map>
#define rep(i, n) for (int i=0; i<n; ++i)
#define FOR(i, a, b) for (int i=a; i<b; ++i)
#define pb(x) push_back(x)
#define ll long long
#define ul unsigned long long
#define dbg(x) cout  << #x << " : " << (x) << endl
#define out(x) cout << (x)
#define outl(x) cout << (x) << endl
#define int(n); int n; cin >> n;
#define long(n); long long n; cin >> n;
#define dbl(f); double f; cin >> f;
#define s(s); string s; cin >> s;
#define into(a, n) for (int i = 0; i < n; ++i) cin >> a[i]
using namespace std;

const int inf =1500000000;
char a[20][20];
int cnt; 
int w,h; 
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1}; 
void dfs(int x, int y) {
  a[y][x] = '#';
  cnt++;
  rep(i, 4) {
    int nx=x+dx[i];
    int ny=y+dy[i];
    if(nx<w&&ny<h&&nx>=0&&ny>=0&&a[ny][nx]=='.') dfs(nx, ny);
  }
}

int main () {
  while(true){
    cnt = 0;
    cin>>w>>h;
    if (w==0&& h==0) break;
    int x0,y0;
    rep(i, h) {
      rep(j, w) {
        cin>>a[i][j];
        if(a[i][j]=='@' ) {
          x0=j; y0=i;
          a[i][j]='.';
        }
      } 
    } 
    dfs(x0, y0);
    outl(cnt);
  }
  return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <cmath>
#include <iostream>
#include <sstream>
#include <vector>
#include <string>
#include <queue>
#include <set>
#include <map>
#include <utility>
#include <algorithm>
#include <functional>

using namespace std;

#define rep(i,n) for((i)=0;(i)<(int)(n);(i)++)
#define each(c,itr) for(__typeof((c).begin()) itr=(c).begin();itr!=(c).end();itr++)

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

int w,h,res;
char field[30][30];

void dfs(int y, int x){
  int i;

  rep(i,4){
    int my = y + dy[i], mx = x + dx[i];
    if(0 <= my && 0 <= mx && my < h && mx < w && field[my][mx] != '#'){
      field[my][mx] = '#';
      res++;
      dfs(my,mx);
    }
  }
}

int main(){
  int i,j;

  while(true){
    scanf("%d %d",&w,&h);
    if(w == 0 && h == 0) break;
    res = 0;
    rep(i,h) scanf("%s",field[i]);
    rep(i,h) rep(j,w) if(field[i][j] == '@') dfs(i,j);
    printf("%d\n",res);
  }

  return 0;
}

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

using namespace std;

// int count(int a, int b, int ans, int sikaku){
//   for(int i=-1;i<1;i++){
//     for(int j=-1;j<1;j++){
//       if(sikaku[a+i][b+j] == 1){
// 	sikaku[a+i][b+j] = 10;
// 	sikaku = count(a+i,b+j,ans,sikaku);
//       }
//     }
//   }

//   return sikaku;
// }

int main(){

  int w,h;
  int ans;
  char aaa;
  int a,b;

  // while(cin >> w >> h){
  //   if(w==0&&h==0)
  //     break;
  //   ans = 1;

  //   int space[w][h];
  //   for(int i=0;i<w;i++)
  //     for(int j=0;j<h;j++){
  // 	cin >> aaa;
  // 	if(aaa == '.')
  // 	  space[i][j] = 1;
  // 	else if(aaa == '#')
  // 	  space[i][j] = 0;
  // 	else{
  // 	 a = i;
  // 	 b = j;
  // 	}
  //     }

  //   ans = count(a,b,ans,space);
  // }
  ans = 0;
  cout << ans << endl;

  return 0;
}

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

template <class T>
void readVector(int n, vector<T> &v){
	for(int i=0; i<n; ++i){
		T t;
		cin >> t;
		v.push_back(t);
	}
}
typedef pair<long long, long long> P;
int dx[4] = {-1, 1, 0, 0};
int dy[4] = {0, 0, -1, 1};

void solve(int w, int h){
	vector<string> v;
	readVector(h, v);
	int x = 0, y = 0, ans = 0;
	for(int i=0; i<h; ++i)
		for(int j=0; j<w; ++j)
			if(v[i][j] == '@')
				x = i, y = j;
	queue<P> q;
	q.push(P(x, y));
	v[x][y] = ' ';
	++ans;
	while(!q.empty()){
		P p = q.front();
		q.pop();
		int nx = p.first;
		int ny = p.second;
		for(int i=0; i<4; ++i){
			int a = nx + dx[i];
			int b = ny + dy[i];
			if(0<=a && 0<=b && a<h && b<w && v[a][b] == '.'){
				q.push(P(a, b));
				v[a][b] = ' ';
				++ans;
			}
		}
	}
	cout << ans << "\n";
}

int main(){
//	cin.tie(0);
//	ios::sync_with_stdio(false);
	while(1){
		int w, h;
		cin >> w >> h;
		if(w == 0 && h == 0) return 0;
		solve(w, h);
	}
}