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 <iostream>
using namespace std;

const int NUM = 7;

void grooping(int y, int x, int w, int h, char** room, unsigned long*** group, unsigned long* next)
{
	if (room[y][x] != '#') {
		for (int i = y - 1; i <= y + 1; i++) {
			for (int j = x - 1; j <= x + 1; j++) {
				if (i < 0 || j < 0 || i >= h || j >= w) {
					continue;
				}
				if ((i == y && j != x) || (i != y && j == x)) {
					for (int k = 0; k < NUM; k++) {
						group[y][x][k] |= group[i][j][k];
					}
				}
			}
		}
		for (int i = 0; i < NUM; i++) {
			if (group[y][x][i] == 0UL && next[i] != 0UL) {
				group[y][x][i] |= next[i];
				next[i] <<= 1;
				break;
			}
		}
	}
}

int main()
{
	int w, h;
	while (true) {
		cin >> w >> h;
		if (w == 0 && h == 0) {
			break;
		}

		unsigned long next[NUM] = { 1UL, 1UL, 1UL, 1UL, 1UL, 1UL, 1UL };
		char** room = new char*[h];
		unsigned long*** group = new unsigned long**[h];
		int x, y;
		for (int i = 0; i < h; i++) {
			room[i] = new char[w];
			group[i] = new unsigned long*[w];
			for (int j = 0; j < w; j++) {
				cin >> room[i][j];
				if (room[i][j] == '@') {
					x = j;
					y = i;
				}
				group[i][j] = new unsigned long[NUM];
				for (int k = 0; k < NUM; k++) {
					group[i][j][k] = 0UL;
				}
			}
		}

		for (int k = 0; k < 2; k++) {
            bool flag=true;
			for (int i = (flag ? 0 : h-1); i >= 0; (flag ? i++ : i--)) {
				for (int j = (flag ? 0 : w-1); j >=0 ; (flag ? j++ : j--)) {
					grooping(i, j, w, h, room, group, next);
                    if (i==h-1 && j==w-1) {
                        flag=!flag;
                    }
				}
			}
			for (int j = (flag ? w-1 : 0); j >= 0; flag ? j-- : j++) {
				for (int i = (flag ? h-1 : 0); i >= 0; flag ? i-- : i++) {
					grooping(i, j, w, h, room, group, next);
                    if (i==h-1 && j==w-1) {
                        flag=!flag;
                    }
				}
			}
		}

		int count = 0;
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				for (int k = 0; k < NUM; k++) {
					if ((group[i][j][k] & group[y][x][k]) != 0UL) {
						count++;
						break;
					}
				}
			}
		}

		cout << count << '\n';

		for (int i = 0; i < h; i++) {
			delete[] room[i];
			for (int j = 0; j < w; j++) {
				delete[] group[i][j];
			}
			delete[] group[i];
		}
		delete[] room;
		delete[] group;
	}

	return 0;
}

C++: #include<iostream>
using namespace std;

int h,w;
string grid[20];
bool visit[20][20];

int dy[4] = {-1,0,1,0}, dx[4] = {0,1,0,-1};

int rec(int y,int x){      //この場所から到達できるタイルの数を返す
  if(visit[y][x])return 0; //もう来たことがあるならカウントしない
  visit[y][x] = true;

  int res = 1;             //今いるタイルですでに1枚到達
  for(int i=0;i<4;i++){
    int ny = y+dy[i], nx = x+dx[i];

    if(ny<0 || nx<0 || ny>=h || nx>=w)continue;
    if(grid[ny][nx] == '#')continue;
    res += rec(ny,nx);     //次の場所から到達できる枚数を足す
  }
  return res;
}

int main(){
  while(cin >> w >> h,h){
    for(int i=0;i<h;i++){
      cin >> grid[i];
      for(int j=0;j<w;j++)visit[i][j] = false;
    }

    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	if(grid[i][j] == '@')cout << rec(i,j) << endl;
      }
    }
  }
}

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

using namespace std;

#define rep(a,b) for(int (a)=0;(a)<(b);(a)++)

int main(){
    int w,h;
    while(cin>>w>>h){
        if(w==0 && h==0)break;
        int sx,sy;
        int sum=0;
        int dy[]={-1,0,1,0};
        int dx[]={0,-1,0,1};
    
    
        string data[30]={};
    
        for(int i=0;i<h;i++){
            cin>>data[i];
        }
    
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                if(data[i][j]=='@'){
                    sx=j;sy=i;
                    break;
                }
            }
        }
    
        bool visited[30][30]={};
    
        queue<pair<int,int> > que;
    
        que.push( pair<int,int>(sy,sx) );
        visited[sy][sx]=true;
    
        while(que.size()){
            int qff=que.front().first,qfs=que.front().second;
            sum++;
            rep(i,4){
                if( qff+dy[i]<h && qff+dy[i]>=0 && qfs+dx[i]<w && qfs+dx[i]>=0 && visited[ qff+dy[i] ][ qfs+dx[i] ]==false && data[ qff+dy[i] ][ qfs+dx[i] ]!='#' ){
                    que.push( pair<int,int>(qff+dy[i] , qfs+dx[i]) );
                    visited[ qff+dy[i] ][ qfs+dx[i] ]=true;
                }
            }
            
            que.pop();
        }
        cout<<sum<<endl;
        
//        rep(i,h){rep(j,w)cout<<(visited[i][j]?'*':'_');cout<<endl;}
    }
    
    return 0;
}