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++: /* 1130: Red and Black */
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <iostream>
#include <sstream>
#include <vector>
#include <string>
#include <queue>
#include <set>
#include <map>
#include <utility>
#include <algorithm>
#include <functional>

using namespace std;

#define rep(i,n) for((i)=0;(i)<(int)(n);(i)++)
#define foreach(c,itr) for(__typeof((c).begin()) itr=(c).begin();itr!=(c).end();itr++)

#define MAX_W 20
#define MAX_H 20

using namespace std;

/* 隣接タイルへの移動用配列 */
int move[4][2] = {{1,0},{0,-1},{-1,0},{0,1}};
char tiles[MAX_H][MAX_W]; /* タイルの並びを入れる配列 */
int w,h; /* 床の幅、高さ */
int sum; /* 移動可能なタイル数 */

int i,j,k,l,m,n;

int dfs(int y, int x) {

//  rep(i,h){ rep(j,w) cout << tiles[i][j]; cout << endl; } cout << endl;
	for(int i=0; i<4; i++) {
		int nx = x+move[i][0], ny = y+move[i][1];
		
		if(
			/* 床の範囲内で */
			0<=nx&&nx<w &&
			0<=ny&&ny<h &&
			/* 黒いタイルだったら */
			tiles[ny][nx] == '.'
		) {
			/* 二重カウントしないよう予め書き換え */
			tiles[ny][nx] = '#';
			sum++;
			dfs(ny,nx);
		}
	}
	return 0;
}

int main(void) {
	
	int yy,xx;
	
	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 >> tiles[i][j];
			}
		}

//    rep(i,h){ rep(j,w) cout << tiles[i][j]; cout << endl; } cout << endl;
//    cout << "before xx " << xx << endl;
//    cout << "before yy " << yy << endl;
		
		/* @の位置を探す */
		for(yy=0; yy<h; yy++) {
//      cout << 'a' << endl;
			for(xx=0; xx<w; xx++) {
//        cout << 'b' << endl;
				if(tiles[yy][xx] == '@') break;
			}
//      cout << "yy " << yy << "xx " << xx << "tile " << tiles[yy][xx] << endl;
			if(0 <= yy && yy < h && 0 <= xx && xx < w && tiles[yy][xx] == '@') break;
		}

//    cout << "xx " << xx << endl;
//    cout << "yy " << yy << endl;
		
		sum = 1;
		dfs(yy,xx);
		
		cout << sum << endl;
	}
	
	return 0;
}

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

int w,h;
char m[22][32];

int dy[]={1,0,-1,0};
int dx[]={0,1,0,-1};
int dfs(int y,int x){
	if(m[y][x]=='#')return 0;
	int res=1;
	m[y][x]='#';
	for(int i=0;i<4;i++){
		res+=dfs(y+dy[i],x+dx[i]);
	}
	return res;
}
int main(){
	while(cin>>w>>h,w|h){
		memset(m,'#',sizeof(m));
		for(int i=1;i<=h;i++)for(int j=1;j<=w;j++)cin>>m[i][j];
		for(int i=1;i<=h;i++){
			for(int j=1;j<=w;j++){
				if(m[i][j]!='@')continue;
				cout<<dfs(i,j)<<endl;
				goto BREAK;
			}
		}
		BREAK:;
	}
	return 0;
}

C++: #include <cstdio>

char map[20][21];
int w, h;
int ans;
int x, y;
int xx, yy;
const int dx[4] = {0, 1, 0, -1}, dy[4] = {-1, 0, 1, 0};

void Move(int X, int Y) {
	ans++;
	map[Y][X] = '#';
	for(int i = 0; i < 4; i++) {
		xx = X + dx[i];
		yy = Y + dy[i];
		if(0 <= xx && xx < w && 0 <= yy && yy < h && map[yy][xx] == '.') {
			Move(xx, yy);
		}
	}
}

int main(void) {
	while(1) {
		ans = 0;
		scanf("%d %d", &w, &h);
		if(!w && !h) break;
		for(int i = 0; i < h; i++) {
			scanf("%s", map[i]);
			for(int j = 0; j < w; j++) {
				if(map[i][j] == '@') {
					x = j;
					y = i;
				}
			}
		}
		Move(x, y);
		printf("%d\n", ans);
	}
	return 0;
}

C++: #include<cstdio>
#include<stack>

using namespace std;

struct Coord {
  int row;
  int col;
};

int solve (int W, int H);
  
int main () {
  int W, H;

  while(W!=0 && H!=0)
    {
      scanf("%d%d", &W, &H);
      if(W!=0 && H!=0)
	{
          printf("%d\n", solve(W,H));
	}
    }
}

int solve (int W, int H) {
  int n=0;
  char M[H+2][W+2];
  Coord start;
  for (int i=0; i<=W+1; i++) { M[0][i] = '#'; M[H+1][i] = '#'; }
  for (int i=1; i<=H; i++) { M[i][0] = '#'; M[i][W+1] = '#'; } 
  for (int i=1; i<=H; i++) {
    for (int j=1; j<=W; j++) {
      scanf(" %c", &M[i][j]);
      if (M[i][j]=='@') {
	start.row = i;
	start.col = j;
      }
    }
  }

  
  stack<Coord> S;
  S.push(start);
  while (!S.empty()) {
    Coord current = S.top(); S.pop();
    n++;
    int D1[]={1,0,-1,0}; int D2[]={0,-1,0,1};
    for (int d=0; d<4; d++) {
      Coord nc = current;
      nc.row += D1[d];
      nc.col += D2[d];
      if(M[nc.row][nc.col]!='#' && M[nc.row][nc.col]!='*' && M[nc.row][nc.col]!='@')
	{
	  M[nc.row][nc.col] = '*';
	  S.push(nc);
	}
    } 
    
  }
  return n;
}