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 <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <functional>
using namespace std;
#define MOD 1000000007
#define ADD(X,Y) ((X) = ((X) + (Y)%MOD) % MOD)
typedef long long i64; typedef vector<int> ivec; typedef vector<string> svec; typedef pair<int, int> pi;
int dd[4] = { 0, 1, 0, -1 };


int main()
{
	while (true)
	{
		int W, H;
		cin >> W >> H;
		if (W == 0 && H == 0) break;

		char C[30][30];
		int B[30][30];
		for (int i = 0; i < H; i++)
		{
			cin >> C[i];
		}
		int sx = -1; int sy = -1;
		for (int i = 0; i < H; i++)
		{
			for (int j = 0; j < W; j++)
			{
				B[i][j] = 0;
				if (C[i][j] == '@')
				{
					sy = i;
					sx = j;
				}
			}
		}

		queue<pi> Q;
		Q.push(pi(sy, sx));
		B[sy][sx] = 1;
		int ans = 1;
		while (!Q.empty())
		{
			int ty = Q.front().first; int tx = Q.front().second;
			Q.pop();

			for (int r = 0; r < 4; r++)
			{
				int y = ty + dd[r];
				int x = tx + dd[r + 1];
				if (y < 0 || y >= H || x < 0 || x >= W)continue;
				if (C[y][x] == '#')continue;
				if (B[y][x])continue;
				ans++;
				B[y][x] = 1;
				Q.push(pi(y, x));
			}
		}

		cout << ans << endl;
	}
	return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<vector>
#include<queue>
#include<algorithm>
#include<cmath>
#include<climits>
#include<string>
#include<set>
#include<numeric>
#include<map>
#include<iostream>
using namespace std;
#define rep(i,n) for(int i = 0;i<((int)(n));i++)
#define reg(i,a,b) for(int i = ((int)(a));i<=((int)(b));i++)
#define irep(i,n) for(int i = ((int)(n)-1);i>=0;i--)
#define ireg(i,a,b) for(int i = ((int)(b));i>=((int)(a));i--)
typedef long long ll;
typedef pair<ll, ll> mp;

//?????????WA????????????
//????????????????????£??????

int W,H,f[21][21],ans[1000],cnt=0,sx,sy;
string m[21];

void find(int x,int y){
	if(x>=W || x<0 || y>=H || y<0)return;
	string t;
	t = m[x][y];
	if(t=="#")return;
	if(f[x][y]!=0)return;
	f[x][y] = 1;
	ans[cnt]++;
	find(x+1,y);
	find(x-1,y);
	find(x,y+1);
	find(x,y-1);
}

int main(void){
	while(1){
		cin>>W>>H;
		if(W==0 && H==0)break;
		ans[cnt]=0;
		rep(i,H){
			string s;
			cin>>s;
			rep(j,W){
				f[j][i]=0;
				m[j][i]=s[j];
				string t;
				t=m[j][i];
				if(t=="@"){
					sx=j;
					sy=i;
				}
			}
		}
		find(sx,sy);
		cnt++;
	}
	rep(i,cnt)cout<<ans[i]<<endl;
	return 0;
}

C++: #include <iostream>
using namespace std;
void func(int x,int y);
int h,w;
char a[111][111];
int dx[4]={0,-1,0,1};
int dy[4]={-1,0,1,0};
int nx,ny;
int cnt=0;
int main(){
  int fx,fy;
  while(1){
    cin >> w >> h;
    if(h==0 && w==0)break;
    for(int i=1;i<=h;i++){
      for(int j=1;j<=w;j++){
	cin >> a[i][j];
	if(a[i][j]=='@'){
	  fx=i;
	  fy=j;
	}
      }
    }
    func(fx,fy);
    cout << cnt << endl;
    cnt=0;
  }
}


void func(int x,int y){
  if(x>h || x<=0 || y>w || y<=0 || a[x][y]=='#' || a[x][y]=='0')return;
  cnt++;
  a[x][y]='0';
  for(int i=0;i<4;i++){
    nx=dx[i]+x;
    ny=dy[i]+y;
    func(nx,ny);
  }
  
}

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

string line[30];
int w, h, x, y, sum;

void countup(int x, int y){
	if(line[y][x] != '#'){
		line[y][x] = '#';
		countup(x,y-1);
		countup(x,y+1);
		countup(x+1,y);
		countup(x-1,y);
		sum++;
	}
	return;
}

int main(){
	while(cin >> w >> h){
		if (!w && !h) break;
		sum = 0;
		for(int i = 1;i<=h;i++){
			cin >> line[i];
			line[i] = "#" + line[i] + "#";
			for(int j = 0;j<=w+1;j++){
				if(line[i][j] == '@'){
					x = j;
					y = i;
				}
			}
		}
		line[h + 1] = line[0] = "#####################";
		for (int i = 0; i <= h + 1;i++) {
			cout << line[i] << endl;
		}
		cout << x << " " << y << endl;
		countup(x, y);
		cout << sum << endl;
	}
return 0;
}

C++: #include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<cmath>

using namespace std;

int black_count( vector<vector<char>> &v,int h,int w){
	int ret = 0;
	v[h][w] = '#';
	if(v[h+1][w] == '.')ret += black_count(v,h+1,w)+1;
	if(v[h-1][w] == '.')ret += black_count(v,h-1,w)+1;
	if(v[h][w+1] == '.')ret += black_count(v,h,w+1)+1;
	if(v[h][w-1] == '.')ret += black_count(v,h,w-1)+1;
	
	return ret;
}
		
	
int main()
{
	int w,h,startw,starth;
	while(1){
		cin >> w >> h;
		
		
		if(w == 0 && h == 0)break;
		vector<vector<char>> v(h+2,vector<char>(w+2,'#'));
		
		for(int i=1;i<h+1;i++){
			for(int j=1;j<w+1;j++){
				cin >> v[i][j];
				if(v[i][j] == '@'){
					starth = i;
					startw = j;
				}
			}
		}
		
		cout << black_count(v,starth,startw)+1 << endl;
		
		
	}
	return 0;
}