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 <queue>
#include <map>
#include <utility>
#include <cstring>

#define BLACK 1
#define RED 0

using namespace std;

int main() {
  while(true) {
    int h, w, ans = 0;
    cin >> w >> h;
    if(w == 0) break;
    map<pair<int, int>, bool> visits;
    map<pair<int, int>, int> table;
    queue<pair<int, int> > q;
    char t;
    pair<int, int> now;
    for(int i = 0; i < h; i++) {
      for(int j = 0; j < w; j++) {
	cin >> t;
	pair<int, int> index = make_pair(i, j);
	if(t == '.') {
	  visits[index] = false;
	  table[index] = BLACK;
	}
	else if(t == '#') {
	  visits[index] = false;
	  table[index] = RED;
	}
	else {
	  visits[index] = true;
	  table[index] = BLACK;
	  now = index;
	}
      }
    }
    q.push(now);
    ans++;
    while(!q.empty()){
      now = q.front();
      q.pop();
      pair<int, int> s = now;
      if(s.first > 0) {
	s.first--;
	if(table[s] == BLACK && visits[s] == false) {
	  ans++;
	  q.push(s);
	  visits[s] = true;
	}
	s = now;
      }
      if(s.first < h-1) {
	s.first++;
	if(table[s] == BLACK && visits[s] == false) {
	  ans++;
	  q.push(s);
	  visits[s] = true;
	}
	s = now;
      }
      if(s.second > 0) {
	s.second--;
	if(table[s] == BLACK && visits[s] == false) {
	  ans++;
	  q.push(s);
	  visits[s] = true;
	}
	s = now;
      }
      if(s.second < w-1) {
	s.second++;
	if(table[s] == BLACK && visits[s] == false) {
	  ans++;
	  q.push(s);
	  visits[s] = true;
	}
	s = now;
      }
    }
    cout << ans << endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
static const int MAX= 20;
char map[MAX][MAX];

int main() {
    int W, H, sx, sy, nx, ny, sum;
    int dx[4] = {0, 0, -1, 1};
    int dy[4] = {-1, 1, 0, 0};
    for (; ;) {
        std::cin >> W >> H;
        if (H == 0 && W == 0) break;
        for (int i = 0; i < H; i++) {
            for (int j = 0; j < W; j++) {
                std::cin >> map[i][j];
                if (map[i][j] == '@') sx = i, sy =j;
            }
        }
        std::queue<std::pair<int, int> > q;
        q.push({sx, sy});
        map[sx][sy] = '*';
        while ( !q.empty() ) {
            std::pair<int, int> p = q.front(); q.pop();
            for (int i = 0; i < 4; i++) {
                nx = p.first + dx[i], ny = p.second + dy[i];
                if (0 <= nx && nx < H && 0 <= ny && ny < W && map[nx][ny] == '.') {
                    map[nx][ny] = '*';
                    q.push({nx, ny});
                }
            }
        }
        sum = 0;
        for (int i = 0; i < H; i++) {
            for (int j = 0; j < W; j++) if (map[i][j] == '*') sum++;
        }
        std::cout << sum << std::endl;
    }


    return  0;
}

C++: #include<bits/stdc++.h>
#include<queue>
using namespace std;
int W,H;
char c[101][101];
bool visited [100][100];

int dx[4]={1,0,-1,0};
int dy [4]={0,1,0,-1} ;

void compute(){
  memset(visited,false,sizeof visited);


  queue<int> que;
int i,j;
 for(i=0;i<H;++i){
   for(j=0;j<W;++j){
     if(c[i][j]=='@'){
       que.push(j+i*W);
       visited[i][j]=true;
       break;}
       
	 }


   }
 int answer=1;
 
 while(!que.empty()){
   int cur = que.front();
   que.pop();
   int x = cur % W,y=cur/W;

   for(i=0;i<4;i++)
     {int nx = x + dx[i],ny= y+dy[i];
       if(!(0<=nx && nx< W && 0<=ny && ny < H))continue;
       if(c[ny][nx]=='#'||visited[ny][nx])continue;
       
       que.push(nx+ny*W);
       visited[ny][nx]=true;
       ++answer;
       
     }
 }
 cout<<answer<<endl;
 
}





int main(){
  while(cin>>W>>H,W|H) {
    for(int i=0;i<H;++i) {
      cin>>c[i];	    
    }
    compute();
  }
  return 0;
}

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

string v[20];
int dx[4]={0,1,0,-1};
int dy[4]={-1,0,1,0};
int W,H,x,y;
int cnt;

void search(int y, int x)
{
  int i;
  if(v[y][x]=='#') return;
  cnt++;
  v[y][x] = '#';
  for(i=0;i<4;i++){
    if(x+dx[i]<0||W<=x+dx[i]) continue;
    if(y+dy[i]<0||H<=y+dy[i]) continue;
    if(v[y+dy[i]][x+dx[i]] != '.') continue;
    search(y+dy[i],x+dx[i]);
  }
  return;
}

int main()
{
  int i;
  while(cin>>W>>H,W||H){
    for(i=0;i<H;i++){
      cin>>v[i];
      if(v[i].find("@")!=string::npos){
	x=i;
	y=v[i].find("@");
      }
    }
    cnt=0;
    search(x,y);
    cout<<cnt<<endl;
  }
}