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<string>
#include<cmath>
#include<sstream>
#include<cstdio>
#include<ctime>
#include<utility>
#include<cstdlib>
#include<iterator>
#include<queue>
#include<stack>
using namespace std;
typedef long long llong;
typedef unsigned long long ullong;
typedef unsigned int uint;
#define int long long
const int INF = 9223372036854775807;


namespace G {
}
//vector<vector<char>> map;
//vector<vector<bool>> flag;
int w, h;
int ans = 0;
int X, Y; //@?????????
queue<pair<int, int>> que;

/*
void check() {
	auto xy = que.front();
	que.pop();
	int x = xy.first;
	int y = xy.second;
	if (!flag[x][y]) {
		ans += 1;
		flag[x][y] = true;
	}
	else return;
	
	if ((map[x + 1][y] == '.') && (!flag[x + 1][y])) que.push(pair<int, int>(x + 1, y));
	if ((map[x - 1][y] == '.') && (!flag[x - 1][y])) que.push(pair<int, int>(x - 1, y));
	if ((map[x][y + 1] == '.') && (!flag[x][y + 1])) que.push(pair<int, int>(x, y + 1));
	if ((map[x][y - 1] == '.') && (!flag[x][y - 1])) que.push(pair<int, int>(x, y - 1));
}
*/

void check(vector<vector<char>> &map, vector<vector<bool>> &flag) {
	auto xy = que.front();
	que.pop();
	int x = xy.first;
	int y = xy.second;
	if (!flag[x][y]) {
		ans += 1;
		flag[x][y] = true;
	}
	else return;

	if ((map[x + 1][y] == '.') && (!flag[x + 1][y])) que.push(pair<int, int>(x + 1, y));
	if ((map[x - 1][y] == '.') && (!flag[x - 1][y])) que.push(pair<int, int>(x - 1, y));
	if ((map[x][y + 1] == '.') && (!flag[x][y + 1])) que.push(pair<int, int>(x, y + 1));
	if ((map[x][y - 1] == '.') && (!flag[x][y - 1])) que.push(pair<int, int>(x, y - 1));
}



signed main() {
	cin >> w >> h;
	while ((w != 0) && (h != 0)) {
		vector<vector<char>> map;
		vector<vector<bool>> flag;
		//cout << "w=" << w << " h=" << h << endl << endl;
		map = vector<vector<char>>(h + 2, vector<char>(w + 2,'#'));
		flag = vector<vector<bool>>(h + 2, vector<bool>(w + 2,true));
		for (int i = 1; i < h + 1; ++i) {
			for (int j = 1 ; j < w + 1; ++j) {
				cin >> map[i][j];
				flag[i][j] = false;
				if (map[i][j] == '@') {
					X = j;
					Y = i;
				}
			}
		}

		//cout << "map=" << map.size() << " map1=" << map[0].size() << endl;
		//cout << "X=" << X << " Y=" << Y << endl;

		ans = 0;
		while (!que.empty()) que.pop();
		que.push(pair<int,int>(Y,X));
		while (!que.empty()) {
			check(map,flag);
		}


		/*
		for (auto ngo : map) {
			for (auto ngoo : ngo) {
				cout << ngoo;
			}cout << endl;
		}
		cin >> w >> h;
		*/
		cout << ans << endl;
		cin >> w >> h;
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <utility>

#define REP(i,a,b) for(int i=int(a);i<int(b);i++)

using namespace std;

typedef long long int lli;
typedef pair<int,int> pii;

int main () {
    int W, H;
    while (cin >> H >> W, W + H) {
        vector<vector<char> > map(W + 2, vector<char>(H + 2, '#'));
        int sx, sy;
        REP (i, 1, W + 1) {
            REP (j, 1, H + 1) {
                cin >> map[i][j];
                if (map[i][j] == '@') {
                    sx = j;
                    sy = i;
                }
            }
        }
        queue<pii> q;
        q.push(make_pair(sx, sy));
        int dx[4] = {0, 1, 0, -1};
        int dy[4] = {1, 0, -1, 0};
        vector<vector<bool> > check(W + 2, vector<bool>(H + 2, false));
        while (q.size()) {
            pii qq = q.front();
            q.pop();
            if (check[qq.second][qq.first]) continue;
            check[qq.second][qq.first] = true;
            REP (k, 0, 4) {
                int xx = qq.first + dx[k];
                int yy = qq.second + dy[k];
                if (map[yy][xx] == '#') continue;
                q.push(make_pair(xx, yy));
            }
        }
        int ans = 0;
        REP (i, 1, W + 1) REP (j, 1, H + 1) ans += check[i][j];
        cout << ans << endl;
    }

    return 0;
}