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>

using namespace std;

int solve( int w, int h, vector< vector<char> > &v );
void dfs( int W, int H, int w_index, int h_index, vector< vector<char> > &V, int &ans );

int main(int argc, char const *argv[]) {
    vector< vector<char> > vec;

    int w, h;
    vector<int> sums;

    while (1) {
        cin >> w >> h;

        if( w==0 || h==0 ) break;

        vec.resize(w);
        for (int i = 0; i < w; i++) {
            vec[i].resize(h);
        }

        for (int i = 0; i < w; i++) {
            for (int j = 0; j < h; j++) {
                cin >> vec[i][j];
            }
        }

        sums.push_back( solve( w, h, vec ) );
    }

    for( auto it = sums.begin(); it != sums.end(); it ++ ){
        cout << *it << endl;
    }

    return 0;
}

int solve( int w, int h, vector< vector<char> > &v ){
    int w_index, h_index, total;
    int ans = 0;

    for (auto it = v.begin(); it != v.end(); it++ ) {
        auto iter = find(it->begin(), it->end(), '@');

        if(iter!=it->end()){
            w_index = distance( v.begin(), it );
            h_index = distance( it->begin(), iter );
        }
    }

    dfs( w, h, w_index, h_index, v, ans );

    return ans;
}

void dfs( int W, int H, int w_index, int h_index, vector< vector<char> > &V , int &ans ){
    int dx[4] = { 1, 0, -1, 0 };
    int dy[4] = { 0, 1, 0, -1 };
    V[w_index][h_index] = '#';

    for( int i = 0; i < 4; i++ ){
        int nx = w_index + dx[i];
        int ny = h_index + dy[i];

        if( 0 <= nx && nx < W && 0 <= ny && ny < H && V[nx][ny] == '.' ){
            ans++;
            dfs( W, H, nx, ny, V, ans );
        }
    }

    return;
}

C++: #include <iostream>
#include <algorithm>
#include <string>
#include <vector>

using namespace std;

int tile[22][22] = {};
int w, h;
int cnt;

void fill(int x, int y) {
	tile[x][y] = 0;
	cnt++;
	if (tile[x - 1][y] == 1) fill(x - 1, y);
	if (tile[x][y - 1] == 1) fill(x, y - 1);
	if (tile[x][y + 1] == 1) fill(x, y + 1);
	if (tile[x + 1][y] == 1) fill(x + 1, y);
}

int main() {
	int x = 0, y = 0;
	while (cin >> w >> h && (w || h)) {
		cnt = 0;
		for (int i = 1; i <= h; ++i){
			for (int j = 1; j <= w; ++j) {
				char tmp;
				cin >> tmp;
				if (tmp == '.') tile[i][j] = 1;
				else if (tmp == '#') tile[i][j] = 0;
				else x = i, y = j, tile[i][j] = 1;
			}
		}
		fill(x, y);
		cout << cnt << endl;
	}

	return 0;
}

C++: #include<algorithm>
#include<climits>
#include<cmath>
#include<cstring>
#include<deque>
#include<iomanip>
#include<iostream>
#include<map>
#include<queue>
#include<set>
#include<stack>
#include<unordered_map>
#include<unordered_set>
#include<utility>
#include<vector>

using namespace std;

using lint = long long;
using P = pair<int, int>;
using LLP = pair<long long, long long>;

#define REP(i, x, n) for(int i = (x), i##_len = int(n) ; i < i##_len ; ++i)
#define rep(i, n) for(int i = 0, i##_len = int(n) ; i < i##_len ; ++i)
#define reps(i, n) for(int i = 1, i##_len = int(n) ; i <= i##_len ; ++i)
#define rrep(i, n) for(int i = int(n) - 1 ; i >= 0 ; --i)
#define rreps(i, n) for(int i = int(n) ; i > 0 ; --i)
#define SORT(x) sort((x).begin(), (x).end())
#define SORT_INV(x) sort((x).rbegin(), (x).rend())
#define TWINS(x) cout << ((x) ? "Yay!" : ":(") << endl

const int IINF = (1 << 30) - 1;
const long long LLINF = 1LL << 61;
const int dx4[] = {1, 0, -1, 0}, dy4[] = {0, 1, 0, -1};
const int dx8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dy8[] = {0, -1, -1, -1, 0, 1, 1, 1};
const double EPS = 1e-8;

int dfs(int y, int x, vector< vector<char> > &v){
    v[y][x] = '#';

    int res = 1;
    rep(i, 4){
        if(v[y + dy4[i]][x + dx4[i]] == '.'){
            res += dfs(y + dy4[i], x + dx4[i], v);
        }
    }

    return res;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    int w, h;
    while(cin >> w >> h, w){
        vector< vector<char> > v(h + 2, vector<char>(w + 2, '#'));
        int sy, sx;
        reps(i, h){
            reps(j, w){
                cin >> v[i][j];
                if(v[i][j] == '@'){
                    sy = i;
                    sx = j;
                }
            }
        }

        int ans = dfs(sy, sx, v);
        cout << ans << endl;
    }

    return 0;
}