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 <bits/stdc++.h>
using namespace std;

const int dx[] = {0, 0, 1, -1};
const int dy[] = {-1, 1, 0, 0};
int W, H;
char board[30][30];

int dfs(int y, int x) {
   board[y][x] = '#';
   int ret = 1;
   for (int i = 0; i < 4; i++) {
      int ny = y + dy[i], nx = x + dx[i];
      if (ny >= 0 && ny < H && nx >= 0 && nx < W && board[ny][nx] == '.') {
         ret += dfs(ny, nx);
      }
   }
   return ret;
}

int main() {
   while (cin >> W >> H, W) {
      for (int i = 0 ; i < H; i++) {
         cin >> board[i];
      }

      for (int i = 0 ; i < H; i++) {
         for (int j = 0 ; j < W; j++) {
            if (board[i][j] == '@') {
               cout << dfs(i, j) << endl;
            }
         }
      }
   }

   return 0;
}

C++: 

// http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=1130&lang=jp
#include <iostream>
#include <algorithm>
#include <queue>
#include <utility> // pair<int,int> xxxx;  make_pair(first,second);
using namespace std;

int W,H;
int X,Y; // 自分の位置
char s[20];
char c;
bool tile[20][20]; // タイルの状態。trueは黒、falseは赤(または壁)。
bool visited[20][20];
int answer;

// 右、上、左、下
const int dx[4] = { 1, 0, -1, 0 }; 
const int dy[4] = { 0, 1, 0, -1 };

bool valid (int x, int y) {
  return 0 <= x && x < H && 0 <= y && y < W && tile[x][y] ;
}

void init() { // 初期化
  fill(&tile[0][0], &tile[0][0]+20*20, 0);
  fill(&visited[0][0], &visited[0][0]+20*20, 0);
  answer = 0;
}

void make_tile() { // タイル読み込み
  for (int i=0; i<H; i++) {
    scanf("%s", s);
    for (int j=0; j<W; j++) {
      c = s[j];
      if (c == '.') {
	tile[i][j] = true;
      } else if (c == '#') {
	tile[i][j] = false;
      } else if (c == '@') {
	tile[i][j] = true;
	X = i;
	Y = j;
      }
    }
  }
}

void count_tile() {
  queue<pair<int,int>> Q; // 幅優先探索を使う。特に理由はないが、周辺から徐々に探査する感じになるのでイメージしやすい。
  pair<int,int> cur = make_pair(X,Y);
  Q.push(cur);
  while (!Q.empty()) {
    cur = Q.front();
    Q.pop();
    if (visited[cur.first][cur.second]) {
      continue;
    }
    visited[cur.first][cur.second] = true;
    answer++;
    for (int i=0; i<4; i++) {
      int a = cur.first+dx[i];
      int b = cur.second+dy[i];
      if (valid(a, b) && !visited[a][b]) {
	Q.push(make_pair(a, b));
      }
    }
  }
  printf("%d\n", answer);
}

int main() {
  while (scanf("%d%d", &W, &H) && W && H) {
    init();
    make_tile();
    count_tile();
  }
}

C++: //#define _GRIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
#define int long long

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

signed main()
{
    int h, w;
    while(cin>>w>>h, w) {
        char field[h+1][w+1];
        int sx, sy;
        for(int i=0; i<h; ++i) {
            for(int j=0; j<w; ++j) {
                char c;
                cin >> c;
                if(c=='@') {
                    sx = i;
                    sy = j;
                }
                field[i][j] = c;
            }
        }

        queue<pair<int, int> > q;
        q.push(make_pair(sx, sy));
        int ans = 0;
        while(!q.empty()) {
            ans++;
            pair<int, int> p = q.front();
            q.pop();
            int x = p.first;
            int y = p.second;
            for(int i=0; i<4; ++i) {
                int nx = x + dx[i];
                int ny = y + dy[i];
                if(nx>=0&&nx<h&&ny>=0&&ny<w&&field[nx][ny]=='.') {
                    q.push(make_pair(nx, ny));
                    field[nx][ny] = '#';
                }
            }
        }
        cout << ans << endl;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

int W, H;
char board[30][30];
bool is[30][30];

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

void dfs(int x, int y) {
   for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= H || ny < 0 || ny >= W) continue;
      if (board[nx][ny] == '#') continue;
      if (is[nx][ny]) continue;
      is[nx][ny] = true;
      dfs(nx, ny); // 次の一から再び4近傍を探索
   }
}

int main() {
   while (cin >> W >> H, W && H) {
      memset(is, false, sizeof(is));
      int x, y;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            cin >> board[i][j];
            if (board[i][j] == '@') {
               x = i, y = j;
            }
         }
      }

      is[x][y] = true;
      dfs(x, y);
      int cnt = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (is[i][j]) cnt++;
         }
      }
      cout << cnt << endl;
   }

   return 0;
}