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<cstdio>
#include<vector>
#include<algorithm>
#include<queue>
using namespace std;
typedef long long ll;
//Global
int x, y;
const int MAX_N = 20;
char maze[MAX_N][MAX_N+1];
const int INF = 1000000;
int X, Y;//"@"???????????????
int memo[MAX_N][MAX_N];
int dx[4] = { 1,0,-1,0 };
int dy[4] = { 0,1,0,-1 };
int counter;

void bsd(int a, int b) {
	maze[b][a] = '#';
	memo[b][a] = INF;
	counter++;
	for (int i = 0; i < 4; i++) {
		int nx = a + dx[i];
		int ny = b + dy[i];
		if (memo[ny][nx] == 0 && maze[ny][nx] == '.' && 0 <= nx && nx < x && 0 <= ny && ny < y) {
			bsd(nx, ny);
		}
	}

}



//solve
//??±???????????¢?´¢??§"@"??¨???????????£?????????"."????????°????±???????
void solve()
{
	//memo????????????
	memset(memo.0, sizeof(memo));
	//for (int i = 0; i < y; i++) {
		//for (int j = 0; j < x; j++) {
			//memo[i][j] = 0;
	//	}
	//}

	//"@"??¨??£??\????????????  ??????  INF??§??????  ??????  ????????????????????£?????????????????????"."??§??????
	counter = 0;//"@"??????
	bsd(X, Y);


	cout << counter << endl;

}


//main
int main()
{
	while (true) {
		cin >> x >> y;
		if (x == 0 && y == 0)break;
		//"W"??¨"."??¨"@"?????\???
		for (int i = 0; i < y; i++) {
			for (int j = 0; j < x; j++) {
				cin >> maze[i][j];
				if (maze[i][j] == '@') {
					X = j; Y = i;//@???????????????????¨????
				}
			}
		}
		solve();
	}
    return 0;
}

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

int moved = 0;
int w, h;
char myMap[20][20];

bool safe(int x, int y){
    return x < h && y < w;
}

void move(int x, int y){
    myMap[x][y] = '#';
    moved++;
    if (myMap[x+1][y] == '.' && safe(x+1, y)) move(x+1, y);
    if (myMap[x-1][y] == '.' && safe(x-1, y)) move(x-1, y);
    if (myMap[x][y+1] == '.' && safe(x, y+1)) move(x, y+1);
    if (myMap[x][y-1] == '.' && safe(x, y-1)) move(x, y-1);
    return;
}

int main(){
    while (1){
	cin >> w >> h;
	if (w == 0 && h == 0) break;
	moved = 0;
	int initX, initY;
	
	for (int i = 0; i < h; i++){
	    for (int j = 0; j < w; j++){
		cin >> myMap[i][j];
		if (myMap[i][j] == '@'){
		    initX = i; initY = j;
		}
	    }
	}
	
	move(initX, initY);
	
	cout << moved << endl;
    }
}

C++: #include <iostream>
using namespace std;
char mas[25][25];
int dx[4] = {1,0,-1,0};
int dy[4] = {0,1,0,-1};
int w,h,sx,sy,cnt;
void dfs(int,int);

int main(){
    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 >> mas[i][j];
                if(mas[i][j] == '@'){
                    sx = j;
                    sy = i;
                }
            }
        }
        cnt = 0;
        dfs(sx,sy);
        cout << cnt << endl;
    }
}

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

C++: #include <iostream>
#include <string>
#include <utility>
#include <stack>
#include <vector>
#include <queue>
#include <algorithm>
#include <map>
#include <climits>
#include <set>
#include <numeric>
#include <iomanip>
#include <cmath>
#include <cstring>
using namespace std;
string grid[20];
bool used[20][20];
int move_h[4] = {-1, 1, 0, 0};
int move_w[4] = {0, 0, -1, 1};

void dfs(int cur_h, int cur_w, int h, int w){
  used[cur_h][cur_w] = true;
  for(int i = 0; i < 4; i++){
    int next_h = cur_h + move_h[i];
    int next_w = cur_w + move_w[i];
    if(0 <= next_h && next_h < h && 0 <= next_w && next_w < w
        && !used[next_h][next_w] && grid[next_h][next_w] != '#'){
      dfs(next_h, next_w, h, w);
    }
  }
}

int main(){
  while(true){
    int W;
    int H;
    cin >> W >> H;
    if(W == 0 && H == 0){
      break;
    }
    for(int i = 0; i < H; i ++){
      cin >> grid[i];  
    }

    int start_h = 0;
    int start_w = 0;
    for(int i = 0; i < H; i++){
      for(int j = 0; j < W; j++){
        used[i][j] = false;
        if(grid[i][j] == '@'){
          start_h = i;
          start_w = j;
        }
      }
    }

    dfs(start_h, start_w, H, W);

    int ans = 0;
    for(int i = 0; i < H; i++){
      for(int j = 0; j < W; j++){
        if(used[i][j]){
          ans ++;
        }
      }
    }
    cout << ans << endl;
  }
  return 0;
}