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++: #define _CRT_SECURE_NO_WARNINGS

#include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <fstream>
#include <set>
#include <utility>
#include <algorithm>
#include <queue>
#include <stack>
#include <deque>
#include <numeric>
#include <sstream>
#include <list>
#include <map>
#include <functional>
#include <iterator>

#define rep(i,p,n) for(int i=(int)p;i<(int)n;i++)
#define repp(i,p,n) for(int i=(int)p;i>=(int)n;i--)
#define Sort(x) sort(x.begin(),x.end())
#define PSort(x) sort(begin(x), end(x), pairCompare)
#define ll long long
#define INF 2000000000
#define pb push_back
#define mp make_pair
#define PI 3.14159265358979
int dx[4] = { -1,0,1,0 }, dy[4] = { 0,1,0,-1 };
using namespace std;

int w, h;

vector<vector<char>> v;
int ans = 0;

void dfs(int y,int x) {
	if (v[y][x] == '.') {
		v[y][x] = '#';
		ans++;
	}

	rep(i, 0, 4) {
		int nx = x + dx[i], ny = y + dy[i];
		if (0 <= nx && 0 <= ny&&nx < w&&ny < h&&v[ny][nx] == '.')dfs(ny, nx);
	}
}

int main(){
	while (1) {
		int sx, sy;
		cin >> w >> h;
		if (w == 0 && h == 0)break;
		v.resize(h, vector<char>(w));
		rep(i, 0, h) {
			rep(j, 0, w) {
				cin >> v[i][j];
				if (v[i][j] == '@') {
					sx = j;
					sy = i;
					v[i][j] = '.';
				}
			}
		}

		dfs(sy, sx);
		cout << ans << endl;

		v.clear();
		ans = 0;
	}


	

	return 0;
}

C++: # -*- coding: utf-8 -*-

count = 0
def dfs(i, j, island):
    global count
    print i, j
    dxy = [(1, 0), (0, 1), (-1, 0), (0, -1)]
    
    if i < 0 or i >= len(island) or j < 0 or j >= len(island[0]):
        return

    if island[i][j] == '#' or island[i][j] == 'x':
        return

    island[i][j] = 'x'
    count += 1
    
    for d in dxy:
        dfs(i+d[0], j+d[1], island)

def solve(w, h, island):
    s_i = 0
    s_j = 0
    for i, row in enumerate(island):
        for j, s in enumerate(row):
            if s == '@':
                s_i = i
                s_j = j
                break

    dfs(s_i, s_j, island)
    print count

def func():
    '''

    '''
    while True:
        global count
        wh = map(int, raw_input().split())
        w = wh[0]
        h = wh[1]
        island = []
        count = 0
        for i in range(h):
            row = []
            line = raw_input()
            for j in line:
                row.append(j)
            island.append(row)
        
        for row in island:
            for j in row:
                print j
            print ''

        solve(w, h, island)
    return None


func()

C++: #include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < (n); i++)

using namespace std;

int d4x[] = {0, 1, 0, -1};
int d4y[] = {-1, 0, 1, 0};

int W, H;
int reachable(vector<string>& map, int x, int y){
  //cout << x << "," << y << endl;
  map[y][x] = '#';
  int res = 0;
  rep(i, 4){
    int sx = d4x[i] + x, sy = d4y[i] + y;
    if(0 <= sx && sx < W && 0 <= sy && sy < H){
      if(map[sy][sx] != '#'){
        res += reachable(map, sx, sy);
      }
    }
  }
  return res + 1;
}

int main(){
  while(cin >> W >> H, W != 0){
    vector<string> map(H);
    rep(i, H) cin >> map[i];
    int gy, gx;
    rep(y, H) {
      rep(x, W) {
        if (map[y][x] == '@'){
          gy = y; gx = x;
          break;
        }
      }
    }
    //cout << "OK" << endl;
    cout << reachable(map, gx, gy) << endl;
  }
}

C++: #include<iostream>
using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
char fld[20][20];
bool used[20][20];
int W, H;

void rec(int i, int j){
   used[i][j]=1;
   for(pii dd: dxdy){
      int ni=i+dd.first;
      int nj=j+dd.second;
      if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
      if(used[ni][nj]) continue;
      if(fld[ni][nj]!='.') continue;
      rec(ni, nj);
   }
}

int main(){

   while(1){
      cin>> W>> H;
      if(!(H||W)) break;
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
            }
         }
      }

      for(int i=0; i<20; i++)for(int j=0; j<20; j++) used[i][j]=0;
      used[sy][sx]=1;
      rec(sy, sx);
      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=used[i][j];
      }
      cout<< ans<< endl;

   }

   return 0;
}