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 <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

template<class T> inline T sqr(T x) {return x*x;}

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;

#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

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

const double EPS = 1e-10;
const double PI  = acos(-1.0);

#define CLR(a) memset((a), 0 ,sizeof(a))

#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

int dx[8] = {-1, 0, 1, 0, -1, -1, 1, 1};
int dy[8] = {0, -1, 0, 1, -1, 1, -1, 1};

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

void dfs(int x, int y) {
  if(0 <= x && x < h && 0 <= y && y < w && field[x][y] == '.') { 
    cnt++;
    field[x][y] = '#';
    REP(i, 4) {
      dfs(x+dx[i], y+dy[i]);
    }
  }
}

int main() {
  while(cin >> w >> h, w+h) {
    int px, py;
    REP(i, h) {
      REP(j, w) {
	cin >> field[i][j];
	if(field[i][j] == '@') {
	  px = i;
	  py = j;
	  field[i][j] = '.';
	}
      }
    }

    cnt = 0;
    dfs(px, py);
    cout << cnt << endl;
  } 
}

C++: #include <bits/stdc++.h>
using namespace std;
using vi=vector<int>;
using vvi=vector<vi>;
using vs=vector<string>;
using msi=map<string,int>;
#define rep(i,n) for(int i=0;i<n;i++)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
vvi f;
int ans=0;
void search(int x,int y){
    vi v={1,0,-1,0,1};
    rep(i,4){
        if(f[y+v[i]][x+v[i+1]]==1){
            f[y+v[i]][x+v[i+1]]=0;
            ans++;
            search(x+v[i+1],y+v[i]);
        }
    }
}

int main(void){
    int w,h;
    while(cin>>w>>h,w){
        ans=0;
        f=vvi(h+2,vi(w+2,0));
        int x,y;
        rep(i,h)rep(j,w){
            char c; cin>>c;
            if(c=='.')f[i+1][j+1]=1;
            if(c=='@')y=i+1,x=j+1;
        }
        search(x,y);
        cout<<ans+1<<endl;
    }
}

C++: #include <stdio.h>
int main()
{
  int W, H, i, j, f, d, count;
  int dx[4] = {0, 1, 0, -1};
  int dy[4] = {1, 0, -1, 0};
  char map[32][32];
  char s[32];

  while (1) {
    gets(s);
    sscanf(s, "%d %d", &W, &H); if (W == 0 && H == 0) { break; }
    for (i = 0; i < 30; i++) {
      strcpy(map[i], "##############################");
    }
    
    for (i = 0; i < H; i++) {
      gets(&map[i+1][1]);
      map[i+1][W+1] = '#';
      map[i+1][W+2] = '\0';
    }

    count = 1;
    while (1) {
      f = 0;
      for (i = 1; i <= H; i++) {
        for (j = 1; j <= W; j++) {
          if (map[i][j] == '@') {
            for (d = 0; d < 4; d++) {
              if (map[i+dy[d]][j+dx[d]] == '.') {
                map[i+dy[d]][j+dx[d]] = '@';
                count++;
                f = 1;
              }
            }
          }
        }
      }
      if (!f) {
        break;
      }
    }
    printf("%d\n", count);
  }
  return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <algorithm>
#include <stdlib.h>
#include <vector>
#include <stack>

using namespace std;

char tile[20][20];
int cnt;
int add_y[4] = {0, 0, 1,-1};
int add_x[4] = {1,-1, 0, 0};

void sarch(int y, int x, int h, int w){
	if(y<0 || y==h || x<0 || x==w) return;

	if(tile[y][x] == '.'){
		cnt++;
		tile[y][x] = '#';

		/*
		for(int i=0 ; i<h ; i++){
			for(int j=0 ; j<w ; j++){
				cout << tile[i][j];
			}
			cout << endl;
		}
		*/

	}else if(tile[y][x] == '#'){
		return;
	}

	for(int i=0 ; i<4 ; i++){
		sarch(y+add_y[i], x+add_x[i], h, w);
	}
	return;
}

int main(){

	int w, h;
	int y, x;

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

		cnt = 0;

		for(int i=0 ; i<20 ; i++){
			for(int j=0 ; j<20 ; j++){
				tile[i][j] = '_';
			}
		}

		string temp[h];
		getline(cin , temp[0]);
		for(int i=0 ; i<h ; i++){
			getline(cin , temp[i]);
			for(int j=0 ; j<w ; j++){
				tile[i][j] = temp[i][j];
				if(tile[i][j] == '@'){
					y = i;
					x = j;
					tile[i][j] = '.';
				}
			}
		}
		sarch(y, x, h, w);
		cout << cnt << endl;
	}
	return 0;
}