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>

using namespace std;

void dfs( int W, int H, int w_index, int h_index, vector< vector<char> > &V, int &ans );

int main(int argc, char const *argv[]) {
    vector< vector<char> > vec;

    int w, h, w_index, h_index;
    int ans = 1;
    vector<int> sums;

    while (1) {
        cin >> w >> h;

        if( w==0 || h==0 ) break;

        vec.resize(h);
        for (int i = 0; i < h; i++) {
            vec[i].resize(w);
        }

        for (int i = 0; i < h; i++) {
            for (int j = 0; j < w; j++) {
                cin >> vec[i][j];
            }
        }

        for (auto it = vec.begin(); it != vec.end(); it++ ) {
            auto iter = find(it->begin(), it->end(), '@');

            if(iter!=it->end()){
                h_index = distance( vec.begin(), it );
                w_index = distance( it->begin(), iter );
            }
        }

        dfs( w, h, w_index, h_index, vec, ans );

        std::cout << ans << std::endl;
    }

    for( auto it = sums.begin(); it != sums.end(); it ++ ){
        cout << *it << endl;
    }

    return 0;
}

void dfs( int W, int H, int w_index, int h_index, vector< vector<char> > &V , int &ans ){
    int dx[4] = { 1, 0, -1, 0 };
    int dy[4] = { 0, 1, 0, -1 };
    V[h_index][w_index] = '#';

    for( int i = 0; i < 4; i++ ){
        int nx = w_index + dx[i];
        int ny = h_index + dy[i];

        if( 0 <= nx && nx < W && 0 <= ny && ny < H && V[ny][nx] != '#' ){
            ans++;
            dfs( W, H, nx, ny, V, ans );
        }
    }

    return;
}

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

char s[32][32];
int cnt;
int dx[4] = {0,0,1,-1};
int dy[4] = {1,-1,0,0};

void dfs(int y, int x)
{
	cnt++;
	s[y][x]=0;
	for(int i=0; i<4; i++)
	{
		int ny=y+dy[i];
		int nx=x+dx[i];
		if(s[ny][nx]=='.')
			dfs(ny,nx);
	}
}

int main()
{
	int w,h;
	while(cin >> w >> h, w||h)
	{
		cnt = 0;
		memset(s,0,sizeof(s));
		for(int i=1; i<=h; i++)
		{
			cin >> s[i]+1;
		}

		for(int i=1; i<=h; i++)
		for(int j=1; j<=w; j++)
		if(s[i][j]=='@')
		{
			dfs(i,j);
			break;
		}
		cout << cnt << endl;
	}
}

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

int w, h, m;
string s[20];

void dfs(int i, int j) {
	s[i][j] = '#';
	m++;

	if (i > 0 && s[i - 1][j] == '.')
		dfs(i - 1, j);
	if (i < h - 1 && s[i + 1][j] == '.')
		dfs(i + 1, j);

	if (j > 0 && s[i][j - 1] == '.')
		dfs(i, j - 1);

	if (j < w - 1 && s[i][j + 1] == '.')
		dfs(i, j + 1);
}

int main()
{
	cin >> w >> h;

	while (w > 0 && h > 0) {
		m = 0;

		for (int i = 0; i < h; i++)
			cin >> s[i];

		for (int i = 0; i < h; i++)
			for(int j = 0; j < w; j++)
				if (s[i][j] == '@')
					dfs(i, j);

		cout << m << endl;
		cin >> w >> h;
	}
}

C++: #include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
int INF=1e9;

int main(){
	int dx[4]={0,1,0,-1};
    int dy[4]={1,0,-1,0};

	int w,h;
	while(cin>>w>>h and w>0){
		queue<pair<int,int>> q;
	    int sy,sx;
		char c[h][w]; int d[h][w];
		rep(i,h){
	        rep(j,w){
	            cin>>c[i][j]; d[i][j]=INF;
				if(c[i][j]=='@'){
					sy=i; sx=j;
				}
	        }
	    }
		d[sy][sx]=0;
	    q.push(make_pair(sy,sx));

		int ans=1;
		while(q.size()){
	        pair<int,int> p=q.front(); //キューの先頭を取り出す
	        q.pop(); //取り出したら削除
	        int prev=d[p.first][p.second]; //一手前の手数prev
	        rep(i,4){ //4方向探索
	            int cy=p.first+dy[i];
	            int cx=p.second+dx[i];
	            if(c[cy][cx]=='.' and d[cy][cx]==INF){ //通行可能かつ未到達
					if(0<=cy and cy<h){
						if(0<=cx and cx<w){
							d[cy][cx]=prev+1;
			                q.push(make_pair(cy,cx));
							ans++;
						}
					}
	            }
	        }
	    }
		cout<<ans<<endl;
	}
    return 0;
}

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

   return 0;
}