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>
#include <vector>
#include <algorithm>
#include <string>
#include <cmath>
#include <numeric>
#include <queue>
using namespace std;

int main(){
    bool end = false ;
    int w,h ;
    cin >> w >> h ;
    if (w == 0 && h == 0)
    {
        end = true ;
    }
    
    //外側が全て壁になるように、元の迷路より2マス大きく取る（エラー回避）
    
    while (end == 0)
    {
    vector<vector<int> > a(h+2,vector<int>(w+2,-2)) ;
    pair<int,int> start ;
        for (size_t i = 1; i < h+1; i++)
        {
            for (size_t j = 1; j < w+1; j++)
            {
                char status ;
                cin >> status ;
                if (status == '.')
                {
                    a.at(i).at(j) = -1 ;
                }else if (status == '@')
                {
                    a.at(i).at(j) = 0 ;
                    start.first = i ;
                    start.second = j ;
                }else if (status == '#')
                {
                    a.at(i).at(j) = -2 ;
                }
                
                
                
            }
            
        }
        
        queue<pair<int,int> > wait ;
        int del[4][2] = {
            {1,0},
            {0,1},
            {-1,0},
            {0,-1}
        };
        wait.push(start) ;
        pair<int,int> now ;
        int ans = 0 ;
        while (wait.empty() == 0)
        {
            now = wait.front() ;
            wait.pop() ;
            ans ++;
            for (size_t i = 0; i < 4; i++)
            {
                int look1 = now.first + del[i][0] ;
                int look2 = now.second + del[i][1] ;
                pair <int,int> look = make_pair(look1,look2) ;
                if (a.at(look.first).at(look.second) == -1)
                {
                    a.at(look.first).at(look.second) = 0 ;
                    wait.push(look) ;

                }
                
            }
            
        }
        
        cout << ans << endl ;

        cin >> w >> h ;
        if (w == 0 && h == 0)
        {
            end = true ;
        }
        
    }
    
}

C++: #include <iostream>
using namespace std;
int w,h;
int tx[4] = {0,-1,0,1};
int ty[4] = {-1,0,1,0};
char map[21][21];

int slv(int x,int y){
    map[y][x]='#';
    int ans=0;
    for(int i=0;i<4;++i)
        if( x+tx[i]!=-1 && y+ty[i]!=-1 && x+tx[i]!=w && y+ty[i]!=h)
            if( map[y+ty[i]][x+tx[i]] == '.' )
                ans+=slv( x+tx[i], y+ty[i] ) + 1 ;
    return ans;
}

int main()
{
    int ix,iy;
    while(1){
        cin >> w >> h;
        if(w==0&&h==0)break;
        for(int i=0;i<h;++i)
            for(int j=0;j<w;++j){
                cin >> map[i][j];
                if(map[i][j]=='@'){ix=j;iy=i;}
            }
        cout << slv( ix , iy ) + 1 << endl;
    }
    return 0;
}

C++: #include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
int check(int j){
	if (j==2 || j==4 ||j==3)
	return j;
	else
	return 3;
}
int main(){
	int board[22][22];
	int w,h;
	char t;
	while(1){
		cin >>w >>h;
		if(w==0 && h==0) break;
		for(int i=0;i<=21;i++){
			for(int j = 0;j<=21;j++){
				board[j][i]=4;
			}
		}//initialization
		for(int i=1;i<=h;i++){
			for(int j=1;j<=w;j++){
				cin >> t ;
				if(t == '.')board[j][i] = 1;
				else if(t=='#')board[j][i]=2;
				else board[j][i] = 3;
			}
		}// input end
		int cmem = 2000;
		int c  = 0;
		while(cmem-c !=0){
				cmem = c;
			for(int i=1;i<=h;i++){
				for (int j= 1;j<=w;j++){
					if(board[j][i] ==3){
					board[j-1][i] =check(board[j-1][i]);
					board[j+1][i] =check(board[j+1][i]);
					board[j][i-1] =check(board[j][i-1]);
					board[j][i+1] =check(board[j][i+1]);
					}
				}
			}
			c = 0;
			for(int i=1;i<=h;i++){
				for (int j= 1;j<=w;j++){
					if (board[j][i] ==3) c+=1;
				}
			}
	
		}	
	
		cout <<c << endl;	
	}//while loop end
	return 0;
}