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 vin=vector<int>;
using vll=vector<long long>;
using vvin=vector<vector<int>>;
using vvll=vector<vector<long long>>;
using vstr=vector<string>;
using vvstr=vector<vector<string>>;
using vch=vector<char>;
using vvch=vector<vector<char>>;
using vbo=vector<bool>;
using vvbo=vector<vector<bool>>;
using vpii=vector<pair<int,int>>;
using pqsin=priority_queue<int,vector<int>,greater<int>>;
#define mp make_pair
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define rep2(i,s,n) for(int i=(s);i<(int)(n);i++)
#define all(v) v.begin(),v.end()
#define decp(n) cout<<fixed<<setprecision((int)n)
const int inf=1e9+7;
const ll INF=1e18;

vin dx={-1,0,1,0};
vin dy={0,1,0,-1};
int w,h;

int dfs(vvch &c,int x,int y,int &res){
    c[y][x]='?';
    int nx,ny;
    rep(i,4){
        nx=x+dx[i];
        ny=y+dy[i];
        if(0<=nx&&nx<w&&0<=ny&&ny<h&&c[ny][nx]=='.'){
            res++;
            dfs(c,nx,ny,res);
        }
    }
    return res;
}

int main(){
    queue<int> ans;int res;
    while(1){
        cin>>w>>h;
        if(w==0)break;
        vvch c(h,vch(w));int sx,sy;
        rep(i,h)rep(j,w){
            cin>>c[i][j];
            if(c[i][j]=='@'){
                sx=j;
                sy=i;
            }
        }
        res=1;
        ans.push(dfs(c,sx,sy,res));
    }
    while(ans.size()){
        cout<<ans.front()<<endl;
        ans.pop();
    }
}

C++: #include<stdio.h>
#include<math.h>
#include<algorithm>
#include<queue>
#include<deque>
#include<stack>
#include<string>
#include<string.h>
#include<vector>
#include<set>
#include<map>
#include<bitset>
#include<stdlib.h>
#include<cassert>
#include<time.h>
#include<bitset>

int main(){
  while(1){
    int h,w;
    cin>>w>>h;
    if(h==0&&w==0) break;
    char a[h][w];
    queue<pair<int,int>> q;
    bool used[h][w];
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
        cin>>a[i][j];
        used[i][j]=0;
        if(a[i][j]=='@') q.push(make_pair(i,j));
      }
    }
    int ans=0;
    while(!q.empty()){
      pair<int,int> p=q.front();
      q.pop();
      int x=p.first, y=p.second;
      if(used[x][y]) continue;
      used[x][y]=1;
      ans++;
      if(x>0){
        if(!used[x-1][y]&&a[x-1][y]=='.'){
          q.push(make_pair(x-1,y));
        }
      }
      if(x<h-1){
        if(!used[x+1][y]&&a[x+1][y]=='.'){
          q.push(make_pair(x+1,y));
        }
      }
      if(y>0){
        if(!used[x][y-1]&&a[x][y-1]=='.'){
          q.push(make_pair(x,y-1));
        }
      }
      if(y<w-1){
        if(!used[x][y+1]&&a[x][y+1]=='.'){
          q.push(make_pair(x,y+1));
        }
      }
    }
    cout<<ans<<endl;
  }
  return 0;
}

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

char map[20][20];
int W, H;
int sh, sw;
int dw[4] = {-1, 0, 1, 0};
int dh[4] = {0, 1, 0, -1};
int cnt;

void print_map(){
  for(int i = 0; i < H; i++){
    for(int j = 0; j < W; j++){
      cout << map[i][j];
    }
    cout << endl;
  }
}

void solve(int w, int h){
  map[h][w] = '#';
  cnt++;
  for(int i = 0; i < 4; i++){
    int nh = h + dh[i], nw = w + dw[i];
    if(0 <= nh && nh < H && 0 <= nw && nw < W && map[nh][nw] == '.') solve(nw, nh);
  }
  return;
}

int main(void){
  while(true){
    cin >> W >> H;
    if(W == 0 && H == 0) break;
    cnt = 0;
    for(int i = 0; i < H; i++){
      for(int j = 0; j < W; j++){
        cin >> map[i][j];
        if(map[i][j] == '@'){
          sh = i;
          sw = j;
        }
      }
    }
    solve(sw, sh);
    cout << cnt << endl;
  }
  return 0;
}

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

char a[20][20];
int w,h,cx,cy,count=1;

void check(int x,int y){
  if (a[y][x]=='.'){
    count ++;
    a[y][x]='*';
    if (x>0) check(x-1,y);
    if (x<w-1) check(x+1,y);
    if (y>0) check(x,y-1);
    if (y<h-1) check(x,y+1);
  }
}

int main(){
  cin >> w >> h;
  while (w!=0||h!=0){
    for (int i=0;i<h;i++){
      for (int j=0;j<w;j++){
	cin >> a[i][j];
	if (a[i][j]=='@'){
	  cx=j;
	  cy=i;
	}
      }
    }

    if (cx>0) check(cx-1,cy);
    if (cx<w-1) check(cx+1,cy);
    if (cy>0) check(cx,cy-1);
    if (cy<h-1) check(cx,cy+1);
    
    cout << count << endl;
    count =1;
    cin >> w >> h;
  }
  return 0;
}