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;
using LL = long long;
using P = pair<int, int>;
using Tapris = tuple<int, int, int>;

#define REP(i, n) for(LL i = 0; i < n; ++i)
#define FOR(i, a, n) for(LL i = a; i < n; ++i)
#define pb(a) push_back(a)
#define all(x) (x).begin(),(x).end()

const int INF = (int)1e9;
const LL INFL = (LL)1e15;
const int MOD = 1e9 + 7;

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

//#define int long long

/*************** using variables ***************/
int h, w;
char t[21][21];
/**********************************************/

void dfs(int x, int y){
    if(t[y][x] != '.' && t[y][x] != '@') return;
    t[y][x] = '*';
    if(x+1 < w)  dfs(x+1, y);
    if(x-1 >= 0) dfs(x-1, y);
    if(y+1 < h)  dfs(x, y+1);
    if(y-1 >= 0) dfs(x, y-1);
    return;
}

int solve(){
    int atx = -1, aty = -1;
    REP(i, h){
        if(atx != -1 && aty != -1) break;
        REP(j, w){
            if(t[i][j] == '@'){
                aty = i; atx = j;
                break;
            }
        }
    }
    dfs(atx, aty);
    int ans = 0;
    REP(i, h){
        REP(j, w){
            if(t[i][j] == '*') ans++;
        }
    }
    return ans;
}

signed main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    while(cin >> w >> h, h + w){
        REP(i, h){
            REP(j, w){
                cin >> t[i][j];
            }
        }
        cout << solve() << endl;
    }
}

C++: #include<bits/stdc++.h>
// 

/*
..#.#..
..#.#..
###.###
...@...
###.###
..#.#..
..#.#..
 */

using namespace std;

int W,H;
char c[100][101]; // c[H][W]

bool visited[100][100]; // <----

int dx[4] = {1,0,-1,0}; // ??? ??? ??? ???
int dy[4] = {0,1,0,-1};

void compute() {

  memset(visited,false,sizeof visited); //memset(???????????????????????????1bit???????????????????????????) ?????????1bit????????§?????????

  queue<int> que;
  int i,j;
  for(i=0;i<H;++i) {
    for(j=0;j<W;++j) {
      if( c[i][j] == '@' ) {
	que.push(j+i*W);
	visited[i][j] = true;
	//break;
	goto Skip;
      }
    }
  }
 Skip:;

  int answer = 1; // <---???????????????
  while( !que.empty() ) { // !0 => 1  !1 => 0
    int cur = que.front(); que.pop(); // que -> [1,2,3,4], front -> 1, pop -> [2,3,4]
    int x = cur % W, y = cur / W; //???????????§?¨?
    
    for(i=0;i<4;++i) {
      int nx = x + dx[i], ny = y + dy[i]; //?¬??????§?¨?????¨?????????§?¨?

      if( !( 0 <= nx && nx < W && 0 <= ny && ny < H ) ) continue; // 0 && 0 => 0, 1 && 0 => 0, 0 && 1 => 0, 1 && 1 => 1
      if( c[ny][nx] == '#' || visited[ny][nx] ) continue;         // 0 || 0 => 0, 1 || 0 => 1, 0 || 1 => 1, 1 || 1 => 1
      
      que.push(nx+ny*W);
      visited[ny][nx] = true;
      ++answer;// <---???????????????

    }
    
  }

  cout << answer << endl;// <---???????????????

}

int main() {
  while( scanf("%d %d",&W,&H), W | H ) {
    //while( scanf("%d %d",&W,&H), W == 0 && H == 0 ) {
    int i;
    for(i=0;i<H;++i) {
      scanf(" %s",&c[i][0]);  // c[y][x]
    }
    compute();
  }
  return 0;
}

C++: #include <iostream>
#include <cassert>
#define REP(i,l,n) for(int i=l;i<n;++i)
#define rep(i,n) REP(i,0,n)
#define MAX 21
int ans,baseX,baseY;
int w,h;
char tile[MAX][MAX];
void seek(int x,int y);
using namespace std;
int main() {
  while(1){
    cin >> w;
    cin >> h;
    if(w==0&&h==0) return 0;
    rep(i,h){
      rep(j,w){
        cin >> tile[i][j];
        if(tile[i][j] == '@'){
          baseX = i;
          baseY = j;
        }
      }
      tile[i][w] = '#';
    }
    rep(i,w) tile[h][i] = '#';
    ans = 1;
    seek(baseX,baseY);
    cout << ans << endl;
  }
  return 0;
}

void seek(int x,int y){
  if(tile[x+1][y] == '.'){
    tile[x+1][y] = '#';
    ans++;
    seek(x+1,y);
  }
  if(tile[x][y+1] == '.'){
    tile[x][y+1] = '#';
    ans++;
    seek(x,y+1);
  }
  if(tile[x-1][y] == '.'){
    tile[x-1][y] = '#';
    ans++;
    seek(x-1,y);
  }
  if(tile[x][y-1] == '.'){
    tile[x][y-1] = '#';
    ans++;
    seek(x,y-1);
  }
}

C++: #include<iostream>
using namespace std;
int w,h;
char tail[21][21];
int a[]={-1,0,1,0};
int b[]={0,1,0,-1};
int cnt=0;
void func(int x,int y);
main(){
  int sx,sy;
  while(cin>> w>> h){
    cnt=0;
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin>> tail[i][j];
	if(tail[i][j]=='@'){
	  sx=j;
	  sy=i;
	}
      }
    }
    func(sx,sy);
    cout<< cnt<< endl;
  }
}

void func(int x,int y){
  tail[y][x]='#';
  cnt++;
  for(int i=0;i<4;i++){
    int nx=x+a[i];
    int ny=y+b[i];
    if(nx>=0 && nx<w && ny>=0 && ny<h && tail[ny][nx]!='#'){
      func(nx,ny);
    }
  }
}