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 <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <fstream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <cstring>
#include <assert.h>

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n)  FOR(i,0,n)
#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 ALL(s) (s).begin(),(s).end()
#define ITE(c) typeof((c).begin())

#define clr(a) memset((a),0,sizeof(a))
#define nclr(a) memset((a),-1,sizeof(a))
#define pb push_back
#define INRANGE(x,s,e) ((s)<=(x) && (x)<(e))
#define MP(a,b) make_pair((a),(b))

#define MAX 20

using namespace std;

int Map[MAX][MAX];

int w, h;
int sx, sy;

int cou;
int d_x[4]={-1,0,1,0};
int d_y[4]={0,-1,0,1};
void dfs(int x,int y){
	Map[x][y]=0;
	for(int i=0;i<4;i++){
		int nx=x+d_x[i];
		int ny=y+d_y[i];
		if(nx>=0&&nx<w&&ny>=0&&ny<h&&Map[nx][ny]){
			dfs(nx,ny);
			cou++;
		}
	}
}		

void setMap(int i,int j,char s){
	switch(s){
		case '.':Map[i][j]=1;break;
		case '#':Map[i][j]=0;break;
		case '@':sx=i;sy=j;Map[i][j]=0;break;
	}
}


char getMap(int i,int j){
	if(Map[i][j]){
		return '.';
	}else{
		return '#';
	}
}


int main(){
	char s;
	while(cin>>w>>h&&w||h){
		rep(i,h){
			rep(j,w){
				cin>>s;
				setMap(j,i,s);
			}
		}
		cou=1;
		dfs(sx,sy);
		cout<<cou<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <stack>
#include <vector>
#include <map>

using namespace std;

void input_field(vector< vector< char > > &field, int w, int h);
int reach_tile_num(vector< vector< char > > &field);
pair< int, int> search_start(vector< vector< char > > &field);

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

int main(){
  int w, h;

  while(cin >> w >> h, w&&h){
    vector< vector< char > >field(w+2, vector< char >(h+2, 0));
    
    input_field(field, w+2, h+2);
    cout << reach_tile_num(field) << endl;
  }
}


void input_field(vector< vector< char > > &field, int w, int h){
  for(int y = 0; y < h; y++){
    for(int x = 0; x < w; x++){
      if(x == 0 || y == 0
         || x == w-1 || y == h-1){
	field[y][x] = '#';
      }else{
	cin >> field[y][x];
      }
    }
  }
}

int reach_tile_num(vector< vector< char > > &field){
  int sum = 0;
  pair< int, int > start, cur;
  stack< pair< int, int > > s;

  start = search_start(field);
  s.push(start);

  while(!(s.empty())){
    cur = s.top();
    s.pop();
    if(field[cur.second][cur.first] == '.'
       || field[cur.second][cur.first] == '@'){
      sum++;
      for(int i = 0; i < 4; i++){
        s.push(make_pair(cur.first+dx[i], cur.second+dy[i]));
	field[cur.second][cur.first] = '#';
      }
    }
  }
  return sum;
}

pair< int, int> search_start(vector< vector< char > > &field){
  for(int y = 0; y < field.size(); y++){
    for(int x = 0; x < field[y].size(); x++){
      if(field[y][x] == '@') return make_pair(x,y);
    }
  }
}

C++: #include "bits/stdc++.h"

using namespace std;

typedef long long ll;
typedef pair<int, int> P;
typedef vector<int> VI;
typedef vector<VI> VVI;
const double PI = 3.14159265358979323846;
const double EPS = 1e-12;
const int INF = numeric_limits<int>::max() / 2;
const int NEG_INF = numeric_limits<int>::min() / 2;
const int MOD = 1e9 + 7;

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

    int w,h;
    int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
    while(cin>>w>>h,w||h){
        vector< vector<int> > field(h,vector<int>(w,1));
        int x,y;
        for(int i=0;i<h;i++){
            string s;cin>>s;
            for(int j=0;j<w;j++){
                if(s[j]=='.') field[i][j]=0;
                else if(s[j]=='@'){x=i;y=j;}
            }
        }
        queue<P> q;
        q.push(P(x,y));
        int res=0;
        while(!q.empty()){
            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(nx<0||h<=nx||ny<0||w<=ny) continue;
                if(field[nx][ny]==1) continue;
                res++;
                field[nx][ny]=1;
                q.push(P(nx,ny));
            }
        }
        cout<<res+1<<endl;
    }
}

C++: #include<stdio.h>
#include<string.h>
char m[22][22],*p;
int F(int x,int y)
{
	if(m[y][x]=='#')return 0;
	m[y][x]='#';
	return 1+F(x+1,y)+F(x-1,y)+F(x,y+1)+F(x,y-1);
}
int main()
{
	int w,h,i,x,y;
	while(scanf("%d%d",&w,&h),w)
	{
		for(i=1;i<=h;++i)
		{
			scanf("%s",m[i]+1);
			if(p=strchr(m[i]+1,'@'))x=p-m[y=i];
		}
		for(i=0;i<h+2;++i)m[i][0]=m[i][w+1]='#';
		for(i=0;i<w+2;++i)m[0][i]=m[h+1][i]='#';
		printf("%d\n",F(x,y));
	}
	return 0;
}