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 <iostream>
#include <algorithm>
#include <vector>
#include <iomanip>
using namespace std;

// #define ll long long
#define vvi vector< vector<int> >
#define All(X) X.begin(),X.end()
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define REP(i,n) for(int i=0;i<(int)(n);i++)
#define pb push_back

int w,h,fx,fy;
char ban[30][30];
int dx[4] = {-1,0,1,0};
int dy[4] = {0,-1,0,1};

bool inrange(int px,int py){
  if(px<0||py<0||px>=h||py>=w) return false;
  else return true;
}
int bsearch(int px,int py){
  int cnt = 0;
  REP(i,4){
    int nx = px+ dx[i]; int ny = py+dy[i];
    if(!inrange(nx,ny)) continue;
    if(ban[nx][ny]=='#') continue;
    //cnt++;
    //cout << nx << " " <<ny<<endl;
    if(ban[nx][ny]=='.'){
      ban[nx][ny]='!';
      cnt++;
      cnt += bsearch(nx,ny);
    }
  }
  return cnt;
}

int main(){
  while(1){
    cin >> w >> h;
    if((w==0)&&(h==0)) break;
    REP(i,h){
      char buf[21];
      cin >> buf;
      REP(j,w) {
        ban[i][j] = buf[j];
        if(ban[i][j] == '@'){
          fx = i; fy = j;
        }
      }
    } 
    cout << bsearch(fx,fy)+1 << endl;
  }
  return 0;
}

C++: //??¨??¢?´¢:??±???????????¢?´¢??????????????¢?´¢ 

//??????????????¢?????¬?????????
 
//?????°??¢??°???????????????(?????????)????????\???(?????????)
#include <stdio.h>
#include <queue>
#include <string.h>
using namespace std;	
int W,H;
char c[101][101];
bool visited[100][100];
 
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
 	
void compute();//????????????????????£?¨? 
	
int main(){
	
	while(scanf("%d %d",&W,&H),W |H){
	    int i;
		
		for(i=0;i<H;i++){
			scanf(" %s",&c[i][0]);
		}
	}
	return 0;
}

void compute(){
	
	memset(visited,false,sizeof visited);  //{????????????????????????1bit??????????????????1bit????????§?????????} 
	
	queue<int> que;
	int i,j;
	for(j=0;j<W;j++){ 
	 for(i=0;i<H;i++){
	  if(c[i][j]=='@'){
	  	que.push(j+i*H);
	  	visited[i][j]==true;
	  	 break;
	  }
	 }
	}
  int answer=1;
  while(!que.empty()){
	int cur=que.front();
	        que.pop();
	        int x=cur%W;
	        int y=cur/W;
    

  for(i=0;i<4;i++){
	int nx=x+dx[i],ny=y+dy[j];
	
  
  
  if(!(0<=nx&&nx<W&&0<=ny))continue;
  if(c[ny][nx]=='#'||visited[ny][nx])continue;

  que.push(nx+ny*W);
  visited[ny][nx]=true;

    }
   }
 
 }

C++: #include<bits/stdc++.h>
using namespace std;
int W,H;
char fld[20][21];
const int dx[]={-1,0,1,0};
const int dy[]={0,-1,0,1};
bool used[20][20];
int rec(int y,int x){
    if(fld[y][x]!='.'||used[y][x])return 0;
    used[y][x]=true;
    int ret=0;
    for(int i=0;i<4;i++){
        int ty=y+dy[i],tx=x+dx[i];
        if(ty<0||ty>=H||tx<0||tx>=W)continue;
        ret+=rec(ty,tx);
    }
    return ret+1;
}
int main(){
    while(cin>>W>>H,W||H){
        fill_n(*used,400,false);
        for(int i=0;i<H;i++)cin>>fld[i];

        int sy,sx;
        for(int i=0;i<H;i++){
            for(int j=0;j<W;j++){
                if(fld[i][j]!='@')continue;
                sy=i;
                sx=j;
                fld[i][j]='.';
            }
        }

        cout<<rec(sy,sx)<<endl;
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <queue>
#include <utility>
using namespace std;
using P = std::pair<int, int>;


int main() {
    int W, H;
    while(cin >> W >> H, W) {
        vector<vector<char>> tile(W, vector<char>(H));
        vector<vector<int>> ret(W, vector<int>(H));
        queue<P> que;
        for(int i=0; i<H; ++i) {
            for(int j=0; j<W; ++j) {
                cin >> tile[j][i];
                if(tile[j][i] == '@') {
                    que.push(make_pair(j, i));
                    ret[j][i] = 1;
                }
            }
        }
        int dx[4] = {0, 1, 0, -1};
        int dy[4] = {1, 0, -1, 0};
        while(!que.empty()) {
            P p = que.front();
            que.pop();
            for(int i=0; i<4; ++i) {
                int nx = p.first + dx[i];
                int ny = p.second + dy[i];
                if(nx >= 0 && nx < W && ny >= 0 && ny < H && tile[nx][ny] != '#' && ret[nx][ny] == 0) {
                    ret[nx][ny] = ret[p.first][p.second] + 1;
                    que.push(make_pair(nx, ny));
                }
            }
        }
        int ans = 0;
        for(int i=0; i<H; ++i) {
            for(int j=0; j<W; ++j) {
                if(ret[j][i] != 0) {
                    ans += 1;
                }
            }
        }
        cout << ans << endl;
    }
}