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 <stdio.h>
char c[1000][1000];
bool went[1000][1000];
int ans,n,m;
void go(int x,int y){
    if(x>=n||y>=m||x<0||y<0)return ;
    if(c[x][y]=='#')return;
    if(went[x][y])return;
    went[x][y]=true;
    go(x+1,y);
    go(x,y+1);
    go(x-1,y);
    go(x,y-1);
    ans++;
    return ;
}
int main(){
    bool f;
    while(1){
        scanf("%d%d",&m,&n);
        if(n==0&&m==0)return 0;
        f=true;
        ans=0;
        for(int i=0;i<n;i++)scanf("%s",c[i]);
        for(int i=0;i<n;i++)for(int j=0;j<m;j++)went[i][j]=false;
        for(int i=0;i<n&&f;i++)for(int j=0;j<m&&f;j++)if(c[i][j]=='@'){
            go(i,j);
            f=false;
        }
        printf("%d\n",ans);
    }
}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;

int cnt;
int w, h;
int dx[] = {0, 1, 0, -1};
int dy[] = {1, 0, -1, 0};
char tile[20][20];

void dfs(int x, int y);

int main()
{
	int x, y;

	while (true){
		scanf("%d %d", &w, &h);

		if (w == 0){
			break;
		}

		for (int i = 0; i < h; i++){
			scanf("%s", tile[i]);

			for (int j = 0; j < w; j++){
				if (tile[i][j] == '@'){
					y = i;
					x = j;
				}
			}
		}

		cnt = 0;
		dfs(x, y);

		printf("%d\n", cnt);
	}

	return (0);
}

void dfs(int x, int y)
{
	tile[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 && tile[ny][nx] == '.'){
			dfs(nx, ny);
		}
	}
}

C++: #include <iostream>
#include <algorithm>
#include <cstdlib>
#include <cstring>
#include <cstdio>
#include <vector>
#include <queue>
using namespace std;
int dx[]={-1,0,1,0},dy[]={0,-1,0,1};
int H,W;
char fld[20][20];
int main(){
	queue<pair<int,int> > p;
	while(cin>>W>>H,H,W){
		for(int i=0;i<H;i++){
			for(int j=0;j<W;j++){
				cin>>fld[j][i];
				if(fld[j][i]=='@'){
					fld[j][i]='#';
					p.push(make_pair(j,i));
				}
			}
		}
		int cnt=1;
		while(!p.empty()){
			int x=p.front().first,y=p.front().second;
			p.pop();
			for(int i=0;i<4;i++){
				int nx=x+dx[i],ny=y+dy[i];
				if(nx>=0 && nx<W && ny>=0 && ny<H && fld[nx][ny]=='.'){
					fld[nx][ny]='#';
					p.push(make_pair(nx,ny));
					cnt++;
				}
			}
		}
		cout<<cnt<<endl;
	}
}

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

using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
char fld[20][20];
int d[20][20];
const int inf=1e8;

int main(){

   while(1){
      int W, H;
      cin>> W>> H;
      if(!(H||W)) break;
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
            }
         }
      }

      for(int i=0; i<H; i++)for(int j=0; j<W; j++)d[i][j]=inf;
      queue<pii> Q;
      Q.push(pii(sy, sx));
      d[sy][sx]=0;
      while(!Q.empty()){
         pii now=Q.front(); Q.pop();
         for(pii dd: dxdy){
            int ni=now.first+dd.first;
            int nj=now.second+dd.second;
            if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
            if(fld[ni][nj]=='#') continue;
            if(d[ni][nj]<=d[now.first][now.second]+1) continue;
            d[ni][nj]=d[now.first][now.second]+1;
            Q.push(pii(ni, nj));
         }
      }

      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=(d[i][j]!=inf);
      }

      cout<< ans<< endl;
   }

   return 0;
}

C++: #include <iostream>
#include <vector>
using namespace std;
int dx[4]={-1,0,1,0};
int dy[4]={0,1,0,-1};
int dfs(int x, int y, int acc, vector<vector<bool> > &map)
{
    acc=0;
    if(map[x][y]==false)return 0;
map[x][y]=false;
for(int i=0;i<4;i++){
    int nx=x+dx[i];
    int ny=y+dy[i];

    acc+=dfs(nx,ny,acc,map);
}
acc++;
return acc;
}
/*void dfs(int x,int y,vector<vector<bool> > &map,vector<int> &ans){
    if(map[x][y]){
        ans.push_back(x);
    map[x][y]=false;
for(int i=0;i<4;i++){
    int nx=x+dx[i];
    int ny=y+dy[i];
    dfs(nx,ny,map,ans);
}
    }
}*/
int main(void)
{
    int h, w;
    while (1)
    {
        cin >> h >> w;
        if (h == 0)
            break;
        vector<vector<bool> > map(w + 2, vector<bool>(h + 2, false));
        char temp;
        int x, y;
        for (int i = 1; i <= w; i++)
        {
            for (int j = 1; j <= h; j++)
            {
                cin>>temp;
                if(temp!='#')
                map[i][j]=true;
                if(temp=='@'){
                    x=i;
                    y=j;
                 }
            }
        }
        cout<<dfs(x,y,0,map)<<endl;
   /*     vector<int> ans;
dfs(x,y,map,ans);
cout<<ans.size()<<endl;*/

    }
    return 0;
}