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
#define endl "\n"
#ifdef LOCAL
#define lout cerr
#else
ostream devnull(0);
#define lout devnull
#endif
#define AMULET \
  cin.tie(0);  \
  ios::sync_with_stdio(false);
// https://github.com/kurokoji/.cpp-Template/blob/master/template/template2.cpp
#define GET_MACRO(_1, _2, _3, _4, NAME, ...) NAME
#define rep(...)                                 \
  GET_MACRO(__VA_ARGS__, rep4, rep3, rep2, rep1) \
  (__VA_ARGS__)
#define rep1(n) rep2(_, n)
#define rep2(i, n) rep3(i, 0, n)
#define rep3(i, a, n) rep4(i, a, n, 1)
#define rep4(i, a, n, d) rep5(i, a, n, d, <)
#define rep5(i, a, n, d, o) for (auto i = decltype(n)(a), _n = (n); i o _n; i += (d))

#define rrep(...)                                    \
  GET_MACRO(__VA_ARGS__, rrep4, rrep3, rrep2, rrep1) \
  (__VA_ARGS__)
#define rrep1(a) rrep2(_, a)
#define rrep2(i, a) rrep3(i, a, 0)
#define rrep3(i, a, n) rrep4(i, a, n, -1)
#define rrep4(i, a, n, d) rep5(i, (a)-1, n, d, >=)

#define dump(x) lout << #x << " = " << x << endl;
// alias
typedef long long ll;
typedef long long lint;
typedef pair<int, int> pii;
typedef pair<lint, lint> pll;
// func
template <class C>
inline void vcin(C &c);
template <class C, class D>
inline void vcin(C &c, D &d);
template <class C>
inline void vcout(const C &c, const string separate = "\n");
template <class T = int>
inline T in();

int w, h;
int cnt;
char c[20][20];

void dfs(int x, int y);

void solve()
{
  while (true)
  {
    cin >> w >> h;
    if (w == 0 && h == 0)
    {
      break;
    }

    int sx, sy;

    rep(j, h) rep(i, w)
    {
      cin >> c[i][j];
      lout << i << "," << j << " ";
      dump(c[i][j]);
      if (c[i][j] == '@')
      {
        sx = i,
        sy = j;
      }
    }
    dump(sx);
    dump(sy);

    cnt = 0;

    dfs(sx, sy);

    cout << cnt << endl;
  }
}

void dfs(int x, int y)
{
  if (x < 0 || y < 0 || x >= w || y >= h || c[x][y] == '#')
    return;

  cnt++;
  dump(x);
  dump(y);

  c[x][y] = '#';

  dfs(x - 1, y);
  dfs(x + 1, y);
  dfs(x, y - 1);
  dfs(x, y + 1);

  return;
}

signed main()
{
  AMULET
  solve();
  return 0;
}

template <class C>
inline void vcin(C &c)
{
  for (auto &a : c)
  {
    cin >> a;
  }
}
template <class C, class D>
inline void vcin(C &c, D &d)
{
  for (auto it1 = c.begin(), it2 = d.begin(); it1 != c.end() && it2 != d.end(); it1++, it2++)
  {
    cin >> *it1 >> *it2;
  }
}
template <class C>
inline void vcout(const C &c, const string separate)
{
  for (auto it = c.begin(); it != c.end(); it++)
  {
    if (it != c.begin())
      cout << separate;
    cout << *it;
  }
}
template <class T = int>
inline T in()
{
  T x;
  cin >> x;
  return (x);
}

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

typedef long long ll;
typedef pair<int,int> P;

int h,w;
queue<P>q;
char c[25][25];
int si,sj,ans=0;
int dx[4]={1,-1,0,0};
int dy[4]={0,0,1,-1};
bool seen[25][25];

void f(){
        for(int i=0;i<h;i++){
                for(int j=0;j<w;j++){
                        seen[i][j]=false;
                }
        }
}

void bfs(int x,int y){
        q.push(P(x,y));
        ans++;
        seen[x][y]=true;
        while(q.size()){
                P p=q.front();q.pop();
                for(int i=0;i<4;i++){
                        int nx=p.first+dx[i],ny=p.second+dy[i];
                        if(0<=nx&&nx<h&&0<=ny&&ny<w&&c[nx][ny]!='#'){
                                if(!seen[nx][ny]){
                                        seen[nx][ny]=true;
                                        q.push(P(nx,ny));
                                        ans++;
                                }
                        }
                }
        }
        return;
}

int main(){
        while(cin>>w>>h||w,h){
                ans=0;
                f();
                for(int i=0;i<h;i++){
                        for(int j=0;j<w;j++){
                                cin>>c[i][j];
                                if(c[i][j]=='@')si=i,sj=j;
                        }
                }
                bfs(si,sj);
                cout<<ans<<endl;
        }
        return 0;
}