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;
int main(){
  int w,h,a[20][20],str1,str2,count=0,m;
  char in;
  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>>in;
	if(in=='#') a[i][j]=0;
	else if(in=='@') {
	  a[i][j]=2;
	}
	else a[i][j]=1;
      }
    }
    count=0;
    while(1){
      m=0;
      for(int i=0;i<h;i++){
	for(int j=0;j<w;j++){
	  if(a[i][j]==2){
	    m=1;
	    count++;
	    a[i][j]=3;
	    if(i==0){
	      if(j==0){
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	      }
	      else {
		if(a[i][j-1]==1) a[i][j-1]=2;
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	      }
	    }
	    else if(j==0){
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	    }
	    else if(i==h-1){
	      if(j==w-1){
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
	      }
	      else {
		if(a[i][j+1]==1) a[i][j+1]=2;
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
	      }
	    }
	    else if(j==w-1){
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
	    }
	    else {
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	    }
	  }
	}
      }
      if(m==0) break;
    }
    cout<<count<<endl;
  }
    
    
  return 0;
}

C++: #include <string>
#include <iostream>
using namespace std;
  
int w,h,ans,dx[]={0,-1,0,1}, dy[]={-1,0,1,0};
char c[22][22];
 
void solve(int x,int y){
    ans++;
    c[x][y]='#';
    for(int i=0;i<4;i++){
        int nx=x+dx[i], ny=y+dy[i];
        if(0<=nx && nx<w && 0<=ny && ny<h && c[nx][ny]=='.')solve(nx,ny);
    }
}
 
int main(){
    while(cin>>w>>h, w){
        ans=0;
        int x,y;
        for(int i=0;i<h;i++){
            for(int j=0; j<w; j++){
                cin>>c[j][i];
                if(c[j][i]=='@')x=j,y=i;
            }
        }
        
        solve(x,y);
        cout<<ans<<endl;
    }
}

C++: #include<iostream>
#include<queue>

using namespace std;

typedef pair<int,int> P;
int W,H;
int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
char ma[21][21];
int sx,sy;

int bfs(){
  int count=1;
  int d[21][21];
  queue<P> que;
   for(int i=0;i<W;i++)
   for(int j=0;j<H;j++) d[i][j]=1000000;
  que.push(P(sx,sy));
  d[sx][sy]=0;

  while(!que.empty()){
    P p=que.front(); que.pop();
    for(int i=0;i<4;i++){
	int nx=p.first+dx[i],ny=p.second+dy[i];
	if(0<=nx && nx<W && 0<=ny && ny<H && ma[nx][ny]!='#' &&
	   d[nx][ny] == 1000000){
	  que.push(P(nx,ny));
	  d[nx][ny]=0;
	  // cout<<nx<<"  "<<ny<<endl;
	  count++;
      }
    }
  }
    return count;
  }


int main(){
  while(1){
  cin>>H>>W;
  if(H+W==0) break;
  for(int i=0;i<W;i++){
    for(int j=0;j<H;j++){
      cin>>ma[i][j];
      if(ma[i][j]=='@') {
	sx=i;
	sy=j;
      }
    }
  }
  
  int ans =bfs();
  cout<<ans<<endl;
  }
  return 0;
}

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

using namespace std;

#define pb push_back
#define fi first
#define se second
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define REP(i, n) FOR(i, 0, n)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define RREP(i, n) FOR(i, n, 0)
#define MFOR(i, m) for(auto i=(m).begin();i!=(m).end();i++)
#define ALL(a) (a).begin(), (a).end()
#define SZ(x) ((int)(x).size())

typedef long long int ll;
typedef pair<int, int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;

const double eps = 1e-10;
const int MOD = 1000000007;

template<typename T>
void printv(vector<T> const& s) {
  REP(i, s.size()) {
    cout << s[i] << " ";
  }
  cout << endl;
}

int main () {
  cin.tie(0);
  cout << setprecision(10);
  vi ans;

  while(1) {
    int w, h;
    cin >> w >> h;
    if(w == 0 && h == 0) break;
    vector<vector<char>> tiles(h);
    P st;
    REP(i, h) {
      tiles[i].resize(w);
      REP(j, w) {
        cin >> tiles[i][j];
        if(tiles[i][j] == '@') st = make_pair(i, j);
      }
    }
    queue<P> q;
    vector<vector<bool>> sel(h);
    REP(i, h) {
      sel[i].resize(w, false);
    }
    q.push(st);
    sel[st.fi][st.se] = true;
    vi dx = {-1, 1, 0, 0}, dy = {0, 0, -1, 1};
    int res = 0;
    
    while(!q.empty()) {
      P now = q.front(); q.pop();
      res++;
      REP(i, 4) {
        int nextx = now.fi + dx[i], nexty = now.se + dy[i];
          
        if(0 <= nextx && nextx <= h-1 &&
           0 <= nexty && nexty <= w-1 &&
           !sel[nextx][nexty] && tiles[nextx][nexty] == '.') {
          q.push(make_pair(nextx, nexty));
          sel[nextx][nexty] = true;
        }
      }
    }

    ans.pb(res);
  }

  REP(i, ans.size()) {
    cout << ans[i] << endl;
  }
}