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 <vector>
#include <algorithm>
#include <set>
#include <queue>
#include <stack>
#include <string>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>

#define rep(i, n) for(int i = 0; i < n; i ++)
#define ALL(T) T.begin(), T.end()
#define mp make_pair
#define pb push_back
#define X first
#define Y second
#define MEMSET(v, h) memset((v), h, sizeof(v))

using namespace std;

typedef pair<int, int> pii;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;

const int INF = 1 << 24;
const int dx[4] = {1, -1, 0, 0};
const int dy[4] = {0, 0, 1, -1};

int cnt, w, h;
char m[100][100];

void dfs(pii);

int main(void){
    while(cin >> w >> h, w){
        MEMSET(m, '#');

        cnt = 0;
        pii p;

        rep(y, h){
            cin >> m[y];
            rep(x, w) if(m[y][x] == '@') m[y][x] = '.', p = mp(x, y);
        }

        dfs(p);
        cout << cnt << endl;
    }

    return 0;
}

void dfs(pii p){
    if((p.X < 0 || w <= p.X) ||
       (p.Y < 0 || h <= p.Y)) return;

    if(m[p.Y][p.X] == '.') cnt ++;
    else return;
    m[p.Y][p.X] = '#';

    rep(i, 4){
        pii next = mp(p.X + dx[i], p.Y + dy[i]);
        dfs(next);
    }
}

C++: /*
 * http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=1130&lang=jp
 */


#include <iostream>
#include <limits>

constexpr size_t MAX_HEIGHT = 20;
const char START_TILE = '@';
const char PASSABLE_TILE = '.';
const char PROHIBITED_TILE = '#';
const char PASSED_TILE = 'P';

bool is_wall(std::string world[MAX_HEIGHT], size_t x, size_t y) {
	char tile = world[y][x];
	return tile == PROHIBITED_TILE || tile == PASSED_TILE;
}

bool valid(std::string world[MAX_HEIGHT], size_t width, size_t height, size_t x, size_t y) {
	return x < width && y < height // x and y is unsigned
		   && !is_wall(world, x, y);
}

int walk(std::string world[MAX_HEIGHT], size_t width, size_t height, size_t x, size_t y) {
	if (!valid(world, width, height, x, y)) {
		return 0; // ????????????????????\?????????????£?or??¢?´¢??????
	}
	int total = 1; // ????????????????????\?????????1?????¨??????????????????
	world[y][x] = PASSED_TILE; // ??????????????¨????????????????????¨?????????????????????????????????
	// ?????????????????§??????????????????valid???????????????walk????????§
	total += walk(world, width, height, x - 1, y);
	total += walk(world, width, height, x + 1, y);
	total += walk(world, width, height, x, y - 1);
	total += walk(world, width, height, x, y + 1);
	return total;
}

void process(size_t width, size_t height) {
	std::string world[MAX_HEIGHT];
	size_t x = std::numeric_limits<size_t>::max(), y = std::numeric_limits<size_t>::max();
	for (size_t i = 0; i < height; ++i) {
		std::cin >> world[i];
		size_t start_index = world[i].find(START_TILE);
		if (start_index != std::string::npos) {
			x = start_index;
			y = i;
		}
	}

	std::cout << walk(world, width, height, x, y);
}

int main() {
	size_t width, height;
	while (true) {
		std::cin >> width >> height;
		if (width == 0 || height == 0) {
			break;
		}
		else {
			process(width, height);
		}
	}
}

C++: #include<bits/stdc++.h>
using namespace std;
#define inf 1e9
#define ll long long
#define ull unsigned long long
#define M 1000000007
#define P pair<int,int>
#define PLL pair<ll,ll>
#define FOR(i,m,n) for(int i=m;i<n;i++)
#define RFOR(i,m,n) for(int i=m;i>=n;i--)
#define rep(i,n) FOR(i,0,n)
#define rrep(i,n) RFOR(i,n,0)
#define all(a) a.begin(),a.end()
const int vx[4] = {0,1,0,-1};
const int vy[4] = {1,0,-1,0};
#define PI 3.14159265


void f(int w,int h){
  string s[100];
  int sx,sy;

  rep(i,h){
    cin>>s[i];
    rep(j,w){
      if(s[i][j]=='@'){
        sx=j;sy=i;
      }
    }
  }

  bool a[100][100]={};
  queue<P> p;
  p.push(P(sx,sy));
  int ans=1;
  while(p.size()){
    P q = p.front(); p.pop();
    rep(i,4){
      int x = q.first+vx[i];
      int y = q.second+vy[i];
      if(0<=x&&x<w&&0<=y&&y<h){
        if(s[y][x]=='.'&&!a[y][x]){
          ans++;
          a[y][x]=true;
          p.push(P(x,y));
        }
      }
    }
  }

  cout<<ans<<endl;
}



int main(){
  
  while(1){
    int n,m;
    cin>>n>>m;
    if(n==0) break;
    f(n,m);
  }

  return 0;
}