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<queue>
#include<string>
#define N 20
using namespace std;

int main(){
	int w, h;
	while(cin >>w >>h){
		if(w == 0 && h == 0) break;
		int cnt = 1, dxh[4] = {-1, 0, 1, 0}, dxw[4] = {0, 1, 0, -1};
		string home[N];
		queue<int> quh, quw;
		for(int i = 0; i < h; i++){
			cin >>home[i];
			for(int j = 0; j < w; j++){
				if(home[i][j] == '@'){
					 quh.push(i);
					 quw.push(j);
				}
			}
		}
		while(quh.empty() != 1){
			int nowh = quh.front(), noww = quw.front();
			quh.pop();
			quw.pop();
			for(int i = 0; i < 4; i++){
				if(nowh + dxh[i] >= h || noww + dxw[i] >= w || nowh + dxh[i] < 0 || noww + dxw[i] < 0){
					continue;
				}
				if(home[nowh + dxh[i]][noww + dxw[i]] == '.'){
					quh.push(nowh + dxh[i]);
					quw.push(noww + dxw[i]);
					cnt++;
					home[nowh + dxh[i]][noww + dxw[i]] = '#';
				}
			}
		}
		cout <<cnt <<endl;
	}
	return 0;
}

C++: #include <bits/stdc++.h>
#define int long long
#define endl '\n'
#define FOR(i, a, n) for (int i = (a); i < (n); ++i)
#define REP(i, n) FOR(i, 0, n)
using namespace std;

const int dy[] = {-1, 0, 0, 1};
const int dx[] = {0, -1, 1, 0};
int w, h;
string s[20];
int sy, sx;

void dfs(int y, int x) {
    s[y][x] = '@';
    REP (i, 4) {
        int ny = y + dy[i];
        int nx = x + dx[i];
        if (ny >= 0 && ny < h && nx >= 0 && nx < w && s[ny][nx] == '.') {
            dfs(ny, nx);
        }
    }
}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    while (cin >> w >> h, w) {
        REP (i, h) {
            cin >> s[i];
            REP (j, w) {
                if (s[i][j] == '@') sy = i, sx = j;
            }
        }
        dfs(sy, sx);
        int ans = 0;
        REP (i, h) REP (j, w) ans += (s[i][j] == '@');
        cout << ans << endl;
    }
}

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

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;
            }
         }
      }

      board[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;
}

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

int main() {
	int w, h, sx, sy;
	while (cin >> w >> h , w != 0 || h != 0) {
		vector<string> field = vector<string>(h);

		for (int i = 0; i < h; i++) {
			cin >> field[i];
		}
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (field[i][j] == '@') {
					sx = j;
					sy = i;
				}
			}
		}

		int ans = 0;
		auto visited = vector<vector<bool>>(h, vector<bool>(w, false));
		queue<int> q = queue<int>();
		q.push(sy * 100 + sx);
		while (!q.empty()) {
			int x = q.front() % 100, y = q.front() / 100;
			q.pop();
			if (visited[y][x]) continue;
			ans++;
			visited[y][x] = true;
			int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
			for (int i = 0; i < 4; i++) {
				int nx = x + dx[i], ny = y + dy[i];
				if (nx < 0 || w <= nx || ny < 0 || h <= ny) continue;
				if (field[ny][nx] == '.' && !visited[ny][nx])
					q.push(ny * 100 + nx);
			}
		}

		cout << ans << endl;
	}
}

C++: #include <bits/stdc++.h>
#define rep(i,a,b) for(int i=a ; i<b ; i++)
#define dump(x) cout << #x << " = " << x << endl

using namespace std;

int h, w;
char maze[20][20];
int reach[20][20];
pair<int, int> st;

void disp(){
	rep(i,0,h){
		rep(j,0,w) cout << maze[i][j];
		cout << endl;
	}
	cout << endl;

	rep(i,0,h){
		rep(j,0,w) cout << reach[i][j];
		cout << endl;
	}
	cout << endl;
}

int dr[4] = {0, -1, 0, 1};
int dc[4] = {-1, 0, 1, 0};

void dfs(int r, int c){
	if(r<0 or r>h-1 or c<0 or c>w-1) return ;
	if(maze[r][c] == '#' or reach[r][c] == 1) return ;

	reach[r][c] = 1;
	rep(i,0,4){
		dfs(r+dr[i], c+dc[i]);
	}
	return;
}

void solve(){
	int ans=0;

	rep(i,0,h) rep(j,0,w){
		if(reach[i][j] == 1) ans++;
	}
	cout << ans << endl;
	return;
}

int main(){

	while(1){
		cin >> w >> h;
		if(h == 0) break;

		rep(i,0,h) rep(j,0,w){
			cin >> maze[i][j];
			reach[i][j] = 0;
			if(maze[i][j] == '@'){
				st.first = i;
				st.second = j;
			}
		}
		dfs(st.first, st.second);
		solve();
	}


	return 0;
}