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>

using namespace std;
using P = pair<int, int>;

int map[22][22];

P dirs[] = {P(1, 0), P(-1, 0), P(0, 1), P(0, -1)};

int dfs(int h, int w) {
    if (!map[h][w]) return 0;
    map[h][w] = 0;
    int ans = 0;
    for (P dir : dirs) {
        ans += dfs(h + dir.first, w + dir.second);
    }
    return ans + 1;
}

int main() {
    while (1) {
        int w, h;
        cin >> w >> h;
        if (w == 0) break;

        for (int i = 0; i < 22; ++i)
            for (int j = 0; j < 22; ++j)
                map[i][j] = 0;
        
        int pw, ph;
        for (int i = 1; i <= h; ++i) {
            for (int j = 1; j <= w; ++j) {
                char c;
                cin >> c;
                if (c == '.') map[i][j] = 1;
                if (c == '@') { 
                    map[i][j] = 1;
                    ph = i;
                    pw = j;
                }
            }
        }
        cout << dfs(ph, pw) << endl;
    }
}

C++: #include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for (int i=0; i<(n); ++i)
#define RREP(i, n) for (int i=(int)(n)-1; i>=0; --i)
#define FOR(i, a, n) for (int i=(a); i<(n); ++i)
#define RFOR(i, a, n) for (int i=(int)(n)-1; i>=(a); --i)

#define SZ(x) ((int)(x).size())
#define all(x) begin(x),end(x)

#define dump(x) cerr<<#x<<" = "<<(x)<<endl
#define debug(x) cerr<<#x<<" = "<<(x)<<" (L"<<__LINE__<<")"<<endl;

template<class T>
ostream &operator<<(ostream &os, const vector <T> &v) {
    os << "[";
    REP(i, SZ(v)) {
        if (i) os << ", ";
        os << v[i];
    }
    return os << "]";
}

template<class T, class U>
ostream &operator<<(ostream &os, const pair <T, U> &p) {
    return os << "(" << p.first << " " << p.second << ")";
}

template<class T>
bool chmax(T &a, const T &b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
bool chmin(T &a, const T &b) {
    if (b < a) {
        a = b;
        return true;
    }
    return false;
}

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using P = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vi>;
using vvll = vector<vll>;

const ll MOD = 1e9 + 7;
const int INF = INT_MAX / 2;
const ll LINF = LLONG_MAX / 2;
const ld EPS = 1e-9;


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);

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

    for (;;) {
        int w, h; cin >> w >> h;
        if (w == 0) break;

        vector<string> s(h);
        int si, sj;
        REP(i, h) {
            cin >> s[i];
            REP(j, w) {
                if (s[i][j] == '@') {
                    si = i, sj = j;
                    s[i][j] = '.';
                }
            }
        }

        int ans = 0;
        vvi vis(h, vi(w));
        function<void(int,int)> dfs = [&](int i, int j) {
            vis[i][j] = 1;
            ++ans;
            REP(k, 4) {
                int y = i + dy[k], x = j + dx[k];
                if (y < 0 or x < 0 or y >= h or x >= w) continue;
                if (s[y][x] == '#') continue;
                if (vis[y][x]) continue;
                dfs(y, x);
            }
        };

        dfs(si, sj);

        cout << ans << endl;
    }

    return 0;
}

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

int calc(std::vector<std::vector<char> >&v, int sx, int sy){

	v[sx][sy] = '#';

	int res = 1;
	if (sx - 1 >= 0 && v[sx - 1][sy] == '.')res += calc(v, sx - 1, sy);
	if (sx + 1 < v.size() && v[sx + 1][sy] == '.')res += calc(v, sx + 1, sy);
	if (sy - 1 >= 0 && v[sx][sy - 1] == '.')res += calc(v, sx, sy - 1);
	if (sy + 1 < v[sx].size() && v[sx][sy + 1] == '.')res += calc(v, sx, sy + 1);

	return res;

}

int main(){

	int w, h;
	while (std::cin >> w >> h){
		if (w == 0 && h == 0)break;

		std::vector<std::vector<char> >v(h, std::vector<char>(w));

		int sx, sy;
		for (int i = 0; i < h; i++){
			for (int j = 0; j < w; j++){
				std::cin >> v[i][j];
				if (v[i][j] == '@')sx = i, sy = j;
			}
		}

		std::cout << calc(v, sx, sy) << std::endl;
	}
	return 0;
}