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<iostream>
#include<string>
#include<map>
#include<vector>
#include<algorithm>
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)

using namespace std;
int ans = 0;

void bfs(char **tile, int w, int h){
  ans++;
  tile[w][h] = '#';

  if(tile[w][h-1] == '.')
  bfs(tile, w, h-1);

  if(tile[w][h+1] == '.')
  bfs(tile, w, h+1);

  if(tile[w-1][h] == '.')
  bfs(tile, w-1, h);

  if(tile[w+1][h] == '.')
  bfs(tile, w+1, h);

  return;
}


int main(){
  int W,H,index_w,index_h;

  while(true){
    cin >> W >> H;
    if(W == 0 && H == 0) break;

    char **tile;
    tile = new char* [H+2];
    REP(i,H+2) tile[i] = new char[W+2];

    REP(i,H+2){
      REP(j,W+2){
        if(i == 0 || i == H+1 || j == 0 || j == W+1) tile[i][j] = '#'; //sentry
        else scanf(" %c",&tile[i][j]);

        if(tile[i][j] == '@') {
          index_h = j;
          index_w = i;
        }

      }
    }
    ans = 0;
    bfs(tile,index_w,index_h);
    
    REP(i,H) delete[] tile[i];
    delete[] tile;
    
    cout << ans << endl;
  }
  return 0;
}

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<queue>
#include<string>
using namespace std;

int dx[] = {0, 0, 1, -1}, dy[] = {1, -1, 0, 0};
int W, H;
string mem[20];
bool res[20][20];

int dfs(int sx, int sy)
{
  for(int i = 0; i < 20; i++)
    for(int j = 0; j < 20; j++)
      res[i][j] = 0;
  queue<int> q;
  q.push(sx);
  q.push(sy);
  while(!q.empty())
  {
    int x = q.front(); q.pop();
    int y = q.front(); q.pop();
    res[y][x] = true;
    for(int i = 0; i < 4; i++)
    {
      
      int tx = x + dx[i];
      int ty = y + dy[i];
      if(tx >= 0 && ty >= 0 && tx < W && ty < H && mem[ty][tx] == '.' && !res[ty][tx])
      {
        q.push(tx);
        q.push(ty);
      }
    }
  }
  int ans = 0;
  for(int y = 0; y < H; y++)
    for(int x = 0; x < W; x++)
      ans += res[y][x];
  return ans;
}

int main()
{
loop:
  for(int i = 0; i < 20; i++)
    mem[i] = "";
  int sx = -1, sy;
  cin >> W >> H;
  if(!(W || H))
    return 0;
  for(int i = 0; i < H; i++)
  {
    cin >> mem[i];
    if(sx != -1)
      continue;
    for(int j = 0; j < W; j++)
    {
      if(mem[i][j] == '@')
      {
        sx = j;
        sy = i;
        break;
      }
    }
  }
  cout << dfs(sx, sy) << endl;
  goto loop;
}