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<string>
#include<vector>
#include<algorithm> 
#include<functional> 
#include<cassert>
#include<numeric> // std::accumulate(vec.begin(),vec.end(),0) sum of element
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<queue>
#include<map>
#include<set>


using namespace std;


typedef long long ll;
typedef std::pair<int, int> P;
typedef vector<int> vi;


#define rep(i,n) for(int i=0;i<n;i++)
#define rep1(i,n) for(int i=1;i<=(int)(n);i++)
#define rrep(i,n) for(int i=n-1;i>=0;i--)
#define all(c) c.begin(),c.end()
#define show(x) cout << #x << " = " << x << endl

#define fi first
#define se second
#define pb push_back



#define DEBUG


// void print_vector(const std::vector<int> &t);
// void unique_vector(std:: vector<int> &t);

template <class X>
void print_vector(const std::vector<X> &t);
template <class X>
void unique_vector(std:: vector<X> &t);


int w, h;

vector<int> xl={-1, 0, 1, 0};
vector<int> yl={0, 1, 0, -1};

int main(){

  vi ans;
  while(true){
    cin >> w >> h;
    if(w == 0 && h == 0) break;
    vector<vector<char>> f(h, vector<char>(w));
    queue<P> ser;
    for(int i = 0; i < h; i++){
      for(int j = 0; j < w; j++) {
	cin >> f[i][j];
	if(f[i][j] == '@')
	  ser.push(make_pair(i, j));
      }
    }
    int c = 0;
    while(!ser.empty()){
      P t = ser.front();
      ser.pop();
      int y = t.fi, x = t.se;
      // show(x); show(y);
      if(f[y][x] == '@') f[y][x] = '#';
      for(int i = 0; i < 4; i++) {
	int dx = x+xl[i], dy=y+yl[i];
	// cout << dx << ' ' << dy << endl;
	// show(dx); show(dy);
	if(dy < 0 || dy >= h || dx < 0 || dx >= w || f[dy][dx] == '#')
	  continue;
	
	if(f[dy][dx] == '.'){
	  c++;
	  ser.push(make_pair(dy, dx));
	  f[dy][dx] = '#';
	}
      }	
      // for(int i = 0; i < h; i++) {
      // 	for(int j = 0; j < w; j++) {
      // 	  cout << f[i][j] << ' ';
      // 	}
      // 	cout << endl;
      // }
      // cout << endl;
    }
    ans.pb(c+1);
  }
  for(auto &i : ans) cout << i << endl;
}

template <class X>
void unique_vector(std::vector<X> &t)
{
  std::sort(t.begin(), t.end());
  t.erase( std::unique(t.begin(), t.end()), t.end() );
}

template <class X>
void print_vector(const std::vector<X> &t){
  for(auto itr=t.begin(); itr != t.end(); itr++){
    std::cout << *itr;
    if(itr != t.end()-1) std::cout << ' ';
  }
  std::cout << std::endl;
}

C++: #if 0
08-162015?????????????????°???????????????????????????
pair???
#endif

#include <iostream>
#include <queue>
#include <string.h>
#include <utility>
using namespace std;

int w,h;
int tot=1; //?????£????????????(?????????????????????)
const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1}; //??????????§??????¨
char P[30][30]; //??§?¨?????????????????????¨???

bool valid(int x,int y){ //??????????????????
	return (x>=0)&&(x<w)&&(y>=0)&&(y<h)&&(P[x][y]!='#');
}

void bfs(int x,int y){ //???????????¢?´¢
	queue<pair<int,int> > q;
	pair<int,int> p;
	p = make_pair(x,y);
	while(!q.empty()){
		p=q.front();
		q.pop();
		for(int i=0;i<4;++i){
			if(valid(p.first+dx[i],p.second+dy[i]))
				tot+=1;
				P[p.first+dx[i]][p.second+dy[i]]='#'; //???????????£?????´???????°???????
				p.first+=dx[i]; p.second+=dy[i];
				q.push(p);
		}
	}
}

int main(){
	int x1,y1;
	while((cin>>w>>h)&&(w!=0&&h!=0)){
		for(int i=0;i<h;++i){
			for(int j=0;j<w;++j){
				cin>>P[j][i];
				if(P[j][i]=='@')
					x1=j; y1=i; 
			}
		}
		bfs(x1,y1); //??????????????°????????£??\????????¢?´¢????§?
		cout << tot << endl;
	}
}

C++: #include <iostream>

using namespace std;

int W, H;
int dx[4] = {0, 1, 0, -1},
    dy[4] = {-1, 0, 1, 0};
char field[22][22];     // 1-origin

int dfs(int y, int x)
{
  if (field[y][x] == '#') {
    return 0;
  }
  
  field[y][x] = '#';
  int res = 1;
  for (int k = 0; k < 4; k++) {
    res += dfs(y + dy[k], x + dx[k]);
  }

  return res;
}

int main()
{
  // pre exec
  for (int i = 0; i < 22; i++)
    for (int j = 0; j < 22; j++) 
      field[i][j] = '#';

  while (cin >> W >> H && (W && H)) {

    // read
    int sx = 1, sy = 1;
    for (int i = 1; i <= H; i++) {
      for (int j = 1; j <= W; j++) {
        cin >> field[i][j];
        if (field[i][j] == '@') {
          sx = j; sy = i;
          field[i][j] = '*';
        }
      }
    }

    // depth first search
    cout << dfs(sy, sx) << endl;
  }
}