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>
#include <iostream>
#include <string>
#include <cmath>
#include <vector>
#include <cstring>
#include <climits>
#include <queue>
#include <utility>

#define INF INT_MAX / 2
using namespace std;

typedef pair<int, int> P;
typedef long long ll;

int dx[] = {-1,0,1,0};
int dy[] = {0,-1,0,1};
bool used[30][30];

int bfs(int sy,int sx,const vector<vector<int> >&tiles){
	queue<pair<int,int> > q;
	q.push(make_pair(sy,sx));
	int X = tiles[0].size();
	int Y = tiles.size();
	memset(used,0,sizeof(used));
	used[sy][sx] = true;
	int ret= 0;
	while(!q.empty()){
		P p = q.front();q.pop();
		int y = p.first;int x = p.second;
		for(int i=0;i<4;i++){
			int ny = y + dy[i];
			int nx = x + dx[i];
			if( ny < 0 || nx < 0 || ny >= Y || nx >= X || used[ny][nx] || tiles[ny][nx]==0)continue;
			used[ny][nx] = true;
			q.push(make_pair(ny,nx));
			ret++;
		}
	}
	return ret+1;
}

int main(void) {
	int w,h;
	while(cin >> w >> h,w+h!=0){
	vector<vector<int> >tiles(h,vector<int>(w));
	int sx,sy;
	for(int i=0;i<h;i++){
		for(int j=0;j<w;j++){
			char c;cin >> c;
			if(c=='.')
				tiles[i][j] = 1;
			else if(c=='#')
				tiles[i][j] = 0;
			else{
				tiles[i][j] = 1;
				sx = j;sy = i;
			}
		}
	}
	cout << bfs(sy,sx,tiles) << endl;
}
}

// command ctrl 上下
// command D
// command caps G → '' ""
// vector<vector<int>> a(N,vector<int>(M,-1));

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <cstring>
#include <string>
#include <cstdlib>
#include <cstdio>
using namespace std;


int W, H;
char Map[25][25];
int d[4][2]={{0,1},{1,0},{0,-1},{-1,0}};

int dfs(int x, int y){
	if(Map[y][x] == '#') return 0;
	Map[y][x] = '#';
	int ret = 1;
	for(int i=0;i<4;i++){
		ret += dfs(x+d[i][1],y+d[i][0]);
	}
	return ret;
}
main(){
	int i,j;
	while(cin >> W >> H, W){
		int sx, sy;
		for(i=0;i<=H+1;i++)
			for(j=0;j<=W+1;j++)
				Map[i][j] = '#';
		for(i=1;i<=H;i++){
			for(j=1;j<=W;j++){
				cin >> Map[i][j];
				if(Map[i][j] == '@'){
					sx = j;sy = i;
				}
			}
		}
		cout << dfs(sx, sy) << endl;
	}
	return 0;
}

C++: #include <iostream>

using namespace std;

#define MAX 21
 
string str[MAX];
int n,m,cnt,dx[4] = {1,0,-1,0},dy[4] = {0,1,0,-1};
 
void dfs(int x,int y){
    str[x][y] = '#';
 
    for(int i = 0 ; i < 4 ; i++){
	int nx = x + dx[i] , ny = y + dy[i];
	if(0 <= nx && nx < m && 0 <= ny && ny < n){
	    if(str[nx][ny] == '.'){
		cnt++;
		dfs(nx,ny);
	    }
	}
    }
}
 
int main(){
    while(cin >> n >> m , n + m){
	for(int i = 0 ; i < m ; i++){
	    cin >> str[i];
	}
     
	int x,y;
	for(int i = 0 ; i < n ; i++){
	    for(int j = 0 ; j < m ; j++){
		if(str[j][i] == '@'){
		    x = j;
		    y = i;
		    break;
		}
	    }
	}
	cnt = 1;
	dfs(x,y);
	cout << cnt << endl;
    }
    return 0;
}

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

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

int W, H, ans;
string maze[20];

bool check(int x, int y){
	return x >= 0 && x < W && y >= 0 && y < H && maze[y][x] == '.';
}

void dfs(int x, int y){
	
	for(int i = 0; i < 4; ++i){
		int nx = x + dx[i], ny = y + dy[i];
		if(check(nx, ny)){
			maze[ny][nx] = '#';
			ans++;
			dfs(nx, ny);
		}
	}
}

int main() {
	while(cin >> W >> H && (W || H)){
		for(int i = 0; i < H; ++i)
				cin >> maze[i];
		
		ans = 0;
		for(int i = 0; i < H; ++i){
			for(int j = 0; j < W; ++j){
				if(maze[i][j] == '@'){
					dfs(j, i);
				}
			}
		}
		cout << ans + 1 << endl;
	}
	return 0;
}

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

using ll = long long;
#define debug(x) cerr << #x << ": " << x << endl;
#define line() cerr << "---------------" << endl;

int W, H;
char board[30][30];
bool visited[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 (visited[nx][ny]) continue;
      visited[nx][ny] = true;
      dfs(nx, ny);
   }
}

int main() {
   while (cin >> W >> H, W || H) {
      memset(visited, false, sizeof(visited));
      int sx = 0, sy = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            cin >> board[i][j];
            if (board[i][j] == '@') {
               sx = i, sy = j;
            }
         }
      }

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

   return 0;
}