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;
int a=0;
void kazoeruyo(int p,int P,int box[20][20]){
  a++;
  if(box[p][P+1]==1) {
    box[p][P+1]=0;
    P=P+1;
    kazoeruyo(p,P,box);
  }
  if(box[p+1][P]==1){
    box[p+1][P]=0;
    p=p+1;
    kazoeruyo(p,P,box);
  }
  if(box[p][P-1]==1){
    box[p][P-1]=0;
    P=P-1;
    kazoeruyo(p,P,box);
  }
  if(box[p-1][P]==1){
    box[p-1][P]=0;
    p=p-1;
    kazoeruyo(p,P,box);
  }
}
int main(){
  int i,I,H,W,p,P;
  int  box[20][20];
  char v;
  while(1){
  cin>>W>>H;
  if(W==0&&H==0) break;
  for(i=0;i<H;i++){
    for(I=0;I<W;I++){
      cin>>v;
      if(v=='.') box[i][I]=1;
      if(v=='#') box[i][I]=2;
      if(v=='@') {
	box[i][I]=0;
	p=i;
	P=I;
      }
    }
  }
  kazoeruyo(p,P,box);
  for(i=0;i<H;i++){
    for(I=0;I<W;I++){
      cout<<box[i][I];
    }
    cout<<endl;
  }

  cout<<a<<endl;
 
  a=0;
  }
  return 0;
}

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

int w,h,count;
char room[20][20];

void rec(int y, int x){
  if(y<0||x<0||y>=h||x>=w){
      return;
    }
    if(room[y][x] == '#'){
      return;
    }
    count++;
    room[y][x] = '#';
    rec(y-1,x);
    rec(y,x+1);
    rec(y+1,x);
    rec(y,x-1);
    }
  
  int main(){
    
    int i,j,x,y;
    while(1){
      count=0;
      cin >> w >> h;
      if( w==0 && h==0){
	break;
      }
      for(j = 0;j < h;j++){
	for(i = 0;i < w;i++){ 
	  cin >> room[j][i] ;
	  if(room[j][i] =='@'){
	    x = i;
	    y = j;
	  }
	}
      }

      rec(y,x);
      cout << count << endl;
    }
  }

C++: //
//  RedAndBlack.cpp
//  
//
//  Created by ??°????????? on 2015/10/30.
//
//

#include <iostream>
using namespace std;

int w, h, num = 0, room[20][20];

void dfs(int x, int y) {
    if (room[y][x] == 0) return;
    
    num++;
    room[y][x] = 0;
    
    // ??????????????¢?´¢
    if (y-1 > -1 && room[y-1][x]) dfs(x, y-1);
    if (y+1 < h && room[y-1][x]) dfs(x, y+1);
    if (x-1 > -1 && room[y-1][x]) dfs(x-1, y);
    if (x+1 < w && room[y-1][x]) dfs(x+1, y);

    return;
}

int main() {
    while (cin >> w >> h) {
        if (w == 0 && h == 0) break;
        
        int manX, manY;
        
        for (int i = 0; i < h; i++) {
            string s;
            cin >> s;
            for (int j = 0; j < w; j++) {
                if (s[j] == '.') {
                    room[i][j] = 1;
                } else if (s[j] == '#') {
                    room[i][j] = 0;
                } else if (s[j] == '@') {
                    room[i][j] = 1;
                    manX = j;
                    manY = i;
                }
            }
        }
        
        dfs(manX, manY);
        
        cout << num << endl;
        
        num = 0;
    }
    
    return 0;
}

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

#define rep(i,n) for(int i=0; i<n; i++)

int main(){
	int w,h;
	char cfield[20][21];
	int ifield[20][20];
	int already[20][20];
	char point;

	while(std::cin >> w >> h && (w || h)){
		rep(i,20){
			memset(cfield[i],'#',20);
			rep(j,20){
				ifield[i][j] = 0;
				already[i][j] = 0;
			}
		}
		int x=-1,y=-1;
		int count = 1;
		int sum;
		rep(i,h){
			std::cin >> cfield[i];
		}
		rep(i,20){
			rep(j,20){
				if(cfield[i][j] == '@'){
					x=i,y=j;
					already[x][y] = 1;
					break;
				}
			}
			if (cfield[x][y] == '@') {
				break;
			}
		}
		while(1){
			rep(i,3) {
				point = cfield[x - 1 + i][y];
				if (point == '@' || point == '.' || point == '#') {
					if (point == '@') {
						already[x - 1 + i][y] = 1;
						ifield[x - 1 + i][y] = 0;
					}
					if (point == '.') {
						if (already[x - 1 + i][y] != 1) {
							ifield[x - 1 + i][y] = 1;
						}else{
							ifield[x - 1 + i][y] = 0;
						}
					}
					if(point == '#'){
						ifield[x - 1 + i][y] = 0;
					}
				}
			}
			rep(j,3) {
				point = cfield[x][y - 1 + j];
				if (point == '@' || point == '.' || point == '#') {
					if (point == '@') {
						already[x][y - 1 + j] = 1;
					}
					if (point == '.') {
						if (already[x][y - 1 + j] != 1) {
							ifield[x][y - 1 + j] = 1;
						}else{
							ifield[x][y - 1 + j] = 0;
						}
					}
					if (point == '#') {
						ifield[x][y - 1 + j] = 0;
					}
				}
			}

			sum = 0;
			rep(i,20){
				rep(j,20){
					sum += ifield[i][j];
					if(ifield[i][j] == 1){
						cfield[i][j] = '@';
						cfield[x][y] = '.';
						x=i,y=j;
					}
				}
			}
			if(sum == 0)break;
			count++;
		}
		std::cout << count << std::endl;
	}
	return 0;
}