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<iostream>
#include<algorithm>
using namespace std;
int solve(int x,int y);
char room[30][30];
int dx[4]={0,1,0,-1};
int dy[4]={-1,0,1,0};
int w,h;
int main(void){
  int ans;
  while(1){
    scanf("%d %d",&w,&h);
    if(w==0 && h==0) break;
    ans=0;
    for(int i=0;i<h;i++) scanf("%s",room[i]);
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	if(room[i][j]=='@') ans=solve(i,j);
      }
      if(ans)break;
    }
    printf("%d\n",ans);
  }
}

int solve(int x,int y){
  int res=1;
  room[x][y]='#';
  for(int i=0;i<4;i++){
    int nx=x+dx[i];
    int ny=y+dy[i];
    if(0<=nx && nx<h && 0<=ny && ny<w){
      if(room[nx][ny]=='.') res+=solve(nx,ny);
    }
  }
  return res;
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <cmath>
#include <utility>

#define ll (long long)
#define REP(i,n) for(int i = 0;i<(int)n;i++)
#define FOR(i,a,b) for(int i = (a);i < (b);i++)
#define RFOR(i,a,b) for(int i = (a);i>(b);i--)
#define COUNTER int i= 0,j = 0,k = 0,l = 0,m = 0, n= 0;
#define RETURN cout << endl;
/*vector<vector<int>>ArrayName = vector<vector<int>>(sizeofx, vector<int>(sizeofy, 0));*/
using namespace std;



int dfs (int x,int y,vector<vector<char>>&v);

int lim_x,lim_y;
int mx[] = {-1,1,0,0};
int my[] = {0,0,-1,1,};

int main (void){
    
    int x,y;
    int sh_x = 0,sh_y = 0;
    
    while(cin >> lim_x >> lim_y,lim_x+lim_y)
        {
            vector<vector<char>>v = vector<vector<char>>(lim_y, vector<char>(lim_x, 0));
            REP(i,lim_y)
            REP(j,lim_x)
            {
                cin >> v[i][j];
                if(v[i][j] == '@')
                    {
                        sh_y = i;
                        sh_x = j;
                    }
            }
            //cout << lim_x << " " << lim_y << endl;
            //cout << sh_x  <<" "<< sh_y << endl;
            cout << dfs(sh_x,sh_y,v) << endl;
            /*v is array name(pointer)*/
      
        }
    return 0;
}

int dfs(int x,int y, vector<vector<char>>&v)
{
    int counter = 1;
    for(int i = 0; i<4;i++){
            int tx = x+mx[i],ty = y+my[i];
            if((tx >= 0 && tx<lim_x) && (ty >= 0 && ty <lim_y) && (v[ty][tx] == '.'))
            {
                v[ty][tx] = '#';
                //cout << v[ty][tx] << endl;
                counter += dfs(tx, ty, v);
                //cout << "come on !" << endl;
            }
                
        }
            
    return counter;
}

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

using namespace std;

int mapToNum(char c)
{
	switch (c) {
	case '#':
			return -1;
	case '.':
			return 0;
	case '@':
			return 1;
	default:
			return -2;
	}
}

int main()
{
	while (1) {
		int w, h;
		
		cin >> w >> h;
		
		if (w == 0 && h == 0) break;
		
		stack< pair<int, int> > stk;
		vector< vector<int> > tiles(h + 2, vector<int>(w + 2));
		char c;
		
		for (int i = 0; i < w + 2; ++i) tiles.at(0).at(i) = tiles.at(h + 1).at(i) = -1;
		
		for (int i = 1; i < h + 1; ++i) {
			tiles.at(i).at(0) = tiles.at(i).at(w + 1) = -1;
			for (int j = 1; j < w + 1; ++j) {
				cin >> c;
				tiles.at(i).at(j) = mapToNum(c);
				if (c == '@') stk.push(make_pair(i, j));
			}
		}
		
		int num = 1;
		
		while (!stk.empty()) {
			pair<int, int> p = stk.top();
			stk.pop();
			int x = 1, y = 0, t;
			
			for (int i = 0; i < 4; ++i) {
				if (tiles.at(p.first + x).at(p.second + y) == 0) {
					tiles.at(p.first + x).at(p.second + y) = -1;
					stk.push(make_pair(p.first + x, p.second + y));
					++num;
				}
				
				t = -y;
				y = x;
				x = t;
			}
		}
		
		cout << num << endl;
	}
	
	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
char m[21][21];
int cnt=1,w,h;
void dfs(int x,int y){
  int i,j;
  m[y][x]='#';
  for(i=-1;i<=1;++i){
    if(y+i<0) continue;
    if(m[y+i][x]=='.'){
      cnt++;
      dfs(x,y+i);
    }
  }
  for(j=-1;j<=1;++j){
    if(x+j<0) continue;
    if(m[y][x+j]=='.'){
      cnt++;
      dfs(x+j,y);
    }
  }
}
int main(){
  int i,j,x,y;
  while(cin>>w>>h){
    if(w==0&&h==0) break;
    for(i=0;i<21;++i)
      for(j=0;j<21;++j)
	m[i][j]=0;
    for(i=0;i<h;++i){
      for(j=0;j<w;++j){
	cin>>m[i][j];
	if(m[i][j]=='@'){
	  x=j;
	  y=i;
	}
      }
    }
    cnt=1;
    dfs(x,y);
    cout<<cnt<<endl;
  }
  return 0;
}