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<map>
#include<vector>
#include<algorithm>
#include<cmath>
#include<climits>
#include<ctime>
#include<cstring>
#include<stack>
#include<queue>
#include<sstream>
#include<string>
#include<set>
#include<array>

#define ALL(v) (v).begin(),(v).end()
#define REP(i,p,n) for(int i=p;i<(int)(n);++i)
#define rep(i,n) REP(i,0,n)
#define DUMP(list) cout << "{ "; for(auto nth : list){ cout << nth << " "; } cout << "}" << endl
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i);
#define UNIQUE(a) a.erase(unique(ALL(a)), a.end())

using namespace std;

int dx[4] = {1, 0, -1, 0};//{1,0,-1,0,1,1,-1,-1};//8??????
int dy[4] = {0, 1, 0, -1};//{0,1,0,-1,-1,1,1,-1};//8??????

int px;
int py;
int cnt;
int xr,yr;
vector<string> field;

int dfs(int x, int y) {
  //(x, y) ????????????????????????????????????
    
    if(yr<=y || xr<=x || 0>x || 0>y) return 0;
    if(field[y][x]=='#') return 0;
    if(field[y][x]=='.' || field[y][x]=='@') {field[y][x]='#';cnt++;}
    // ??¨???4??????????????????
    for(int r = 0; r < 4; r++) {
      int nx = x + dx[r];
      int ny = y + dy[r];
      dfs(nx, ny);
    }
  return 0;
}

int main(){

 while(1){
    int x,y;
    cin >> x >> y;
    if(x==0 && y==0) break;
    xr=x; yr=y;
    field.clear(); 
    cnt=0;
    rep(i,y){
      string str;
      cin >> str;
      rep(j,str.size()){
        if(str[j]=='@'){
          px=j;
          py=i;
        }
      }
      field.push_back(str);
    }
   dfs(px,py);
   
   cout << cnt << endl;
  }
}

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

int w, h, black;
char tile[24][24];
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

void dfs(int x, int y){
    black++;
    tile[x][y] = '#';
    for(int i = 0; i < 4; i++){
        if( x+dx[i] >= 0 && x+dx[i] < h &&
            y+dy[i] >= 0 && y+dy[i] < w &&
            tile[ x+dx[i] ][ y+dy[i] ] == '.' ){
            dfs( x+dx[i], y+dy[i] );
        }
    }
}

int main(){
    int i, j;
    while( cin >> w >> h, w||h ){
        for(i = 0; i < h; i++){
            cin >> tile[i];
        }
        black = 0;
        for(i = 0; i < h && !black; i++){
            for(j = 0; j < w && !black; j++){
                if( tile[i][j] == '@' ){
                    dfs(i, j);
                }
            }
        }
        cout << black << endl;
    }
    return 0;
}

C++: #include<iostream>
#include<queue>

using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};

int main(){

   while(1){
      int W, H;
      cin>> W>> H;
      if(!(H||W)) break;
      char fld[H][W];
      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;
            }
         }
      }

      bool visited[H][W];
      for(int i=0; i<H; i++)for(int j=0; j<W; j++)visited[i][j]=0;
      queue<pii> Q;
      Q.push(pii(sy, sx));
      while(!Q.empty()){
         pii now=Q.front(); Q.pop();
         visited[now.first][now.second]=1;
         for(pii dd: dxdy){
            int ni=now.first+dd.first;
            int nj=now.second+dd.second;
            if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
            if(fld[ni][nj]=='#') continue;
            if(visited[ni][nj]) continue;
            Q.push(pii(ni, nj));
         }
      }
      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=visited[i][j];
      }

      cout<< ans<< endl;
   }

   return 0;
}

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

int w , h , count ;
char map[21][21] = {} ;
int dx[] = { -1,0,1,0 } ;
int dy[] = { 0,1,0,-1 } ;

void move( int ny , int nx ){
	map[ny][nx] = '0' ;
	
	for( int i=0 ; i<4 ; i++ ){
		if( ny+dy[i] >= 0 && ny+dy[i] < h &&
			nx+dx[i] >= 0 && nx+dx[i] < w &&
			map[ny+dy[i]][nx+dx[i]] == '.' ){
				count++;
				move( ny+dy[i] , nx+dx[i] ) ;
		}
	}
}

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