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 <sstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <cfloat>
using namespace std;

int solve(vector<string>& tile)
{
    int h = tile.size();
    int w = tile[0].size();
    int sy, sx;
    for(int i=0; i<h; ++i){
        for(int j=0; j<w; ++j){
            if(tile[i][j] == '@'){
                sy = i;
                sx = j;
            }
        }
    }

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

    vector<vector<bool> > check(h, vector<bool>(w, false));
    check[sy][sx] = true;
    queue<pair<int, int> > q;
    q.push(make_pair(sy, sx));

    int ret = 0;
    while(!q.empty()){
        int y = q.front().first;
        int x = q.front().second;
        q.pop();
        ++ ret;
        for(int i=0; i<4; ++i){
            int y0 = y + dy[i];
            int x0 = x + dx[i];
            if(tile[y0][x0] == '.' && !check[y0][x0]){
                q.push(make_pair(y0, x0));
                check[y0][x0] = true;
            }
        }
    }
    return ret;
}

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

        vector<string> tile(h+2);
        tile[0] = tile[h+1] = string(w+2, '#');
        for(int i=0; i<h; ++i){
            cin >> tile[i+1];
            tile[i+1] = '#' + tile[i+1] + '#';
        }

        cout << solve(tile) << endl;
    }
}

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

#define rep(i, n) for (int i = 0; i < (int)n; ++i)
#define REP(i, a, b) for (int i = (int)a; i < (int)b; ++i)
#define rer(i, a, b) for (int i = (int)a; i <= (int)b; ++i)
#define each(i,c) for(__typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define all(v) v.begin(), v.end()
#define mset(a, n) memset(a, n, sizeof(a))

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<vector<int> > vvi;
typedef vector<pair<int, int> > vpii;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;

const int inf = 1000000000;
const int mod = 1000000007;
const double eps = 1e-9;
const int dx[] = { -1, 0, 1, 0};
const int dy[] = { 0, -1, 0, 1};

int W, H;
int ans;
string field[100];

void dfs(int y, int x) {
	if (x < 0 || y < 0 || x == W || y == H) return;
	if (field[y][x] == '#') return;
	ans++;
	field[y][x] = '#';
	rep(i, 4) dfs(y + dy[i], x + dx[i]);
}

int main() {
	while (cin >> W >> H, W) {
		ans = 0;
		rep(i, H) cin >> field[i];
		rep(i, H) rep(j, W) {
			if (field[i][j] == '@') dfs(i, j);
		}
	}
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define FOR(i,k,n) for(int i = (int)(k); i < (int)(n); i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(a) a.begin(), a.end()
#define MS(m,v) memset(m,v,sizeof(m))
#define D10 fixed<<setprecision(10)
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<string> vs;
typedef pair<int, int> pii;
const int MOD = 1000000007;
const int INF = MOD + 1;
const ld EPS = 1e-12;
template<class T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template<class T> T &chmax(T &a, const T &b) { return a = max(a, b); }

/*--------------------template--------------------*/
const int dx[] = { -1, 0, 0, 1 }, dy[] = { 0, -1, 1, 0 };
//[const int dx[] = { -1, -1, -1, 0, 0, 1, 1, 1 }, dy[] = { 0, -1, 1, -1, 1, 0, -1, 1 };
bool valid(int x, int y, int h, int w) { return (x >= 0 && y >= 0 && x < h&&y < w); }
int place(int x, int y, int w) { return w*x + y; }

struct UnionFind
{
	vector<int> par;
	UnionFind(int n) :par(n, -1) {}
	int find(int x) { return par[x] < 0 ? x : par[x] = find(par[x]); }
	void unite(int x, int y)
	{
		x = find(x); y = find(y);
		if (x == y) return;
		if (par[y] < par[x]) swap(x, y);
		if (par[x] == par[y]) par[x]--;
		par[y] = x;
		if (find(x) != find(y)) par[find(y)] = x;
	}
	int count()
	{
		int cnt = 0;
		REP(i, par.size()) if (par[i] < 0) cnt++;
		return cnt;
	}
	bool same(int x, int y)
	{
		return find(x) == find(y);
	}
};


int main()
{
	cin.sync_with_stdio(false);
	int h, w;
	while (cin >> w >> h, h)
	{
		vs fld(h);
		REP(i, h) cin >> fld[i];
		int st;
		REP(i, h)REP(j, w)
		{
			if (fld[i][j] == '@')
			{
				st = place(i, j, w);
				fld[i][j] = '.';
			}
		}
		UnionFind uf(h*w);
		REP(i, h)REP(j, w)
		{
			if (fld[i][j] == '#') continue;
			REP(k, 4)
			{
				int nx = i + dx[k], ny = j + dy[k];
				if (!valid(nx, ny, h, w) || fld[nx][ny] == '#') continue;
				uf.unite(place(i, j, w), place(nx, ny, w));
			}
		}
		int cnt = 0;
		REP(i, h*w)
		{
			if (uf.same(i, st)) cnt++;
		}
		cout << cnt << endl;
	}
	return 0;
}