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 <bits/stdc++.h>
using namespace std;
/*{{{*/  //template
#define rep(i,n) for(int i=0;i<n;i++)
constexpr int INF = numeric_limits<int>::max()/2;
constexpr long long LINF = numeric_limits<long long>::max()/3;
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define sz(x) (int)(x).size()
#define debug(x) cerr<<#x<<":"<<x<<endl
#define debug2(x,y) cerr<<#x<<","<<#y":"<<x<<","<<y<<endl
//struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double{ double d; explicit Double(double x) : d(x){} };
ostream& operator<<(ostream& os,const Double x){ os << fixed << setprecision(20) << x.d; return os; }
template<typename T> ostream& operator<<(ostream& os,const vector<T>& vec){ os << "["; for(const auto& v : vec){ os << v << ","; } os << "]"; return os; }
template<typename T,typename U> ostream& operator<<(ostream& os,const pair<T,U>& p){ os << "(" << p.first << ","<< p.second <<")"; return os; }
template<typename T> ostream& operator<<(ostream& os,const set<T>& st){ os<<"{"; for(T v:st) os<<v<<","; os <<"}"; return os; }
template<typename T,typename U> inline void chmax(T &x,U y){ if(y>x) x = y; }
template<typename T,typename U> inline void chmin(T &x,U y){ if(y<x) x = y; }
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
ll gcd(ll a,ll b){ if(b==0) return a; else return gcd(b,a%b); }
//constexpr double eps = 1e-14; 
constexpr double eps = 1e-10; 
constexpr ll mod = 1e9+7;
const int dx[]={1,0,-1,0} ,dy[] = {0,1,0,-1};
/*}}}*/

int w,h;
int sx,sy;

void solve(){
    vector<string> t(h);
    rep(i,h) cin >> t[i];

    rep(i,h) rep(j,w) if(t[i][j]=='@'){
        sx = i;
        sy = j;
        break;
    }

    bool visited[32][32];
    rep(i,32) rep(j,32) visited[i][j]=false;
    visited[sx][sy]=true;
    queue<pii> que;
    que.push(pii(sx,sy));
    while(!que.empty()){
        auto p = que.front();
        que.pop();

        int x = p.fi;
        int y = p.se;

        rep(i,4){
            int nx = x + dx[i];
            int ny = y + dy[i];
            if(nx<0 or ny<0 or nx>=h or ny>=w) continue;
            if(t[nx][ny]=='#') continue;
            if(visited[nx][ny]) continue;

            visited[nx][ny] = true;
            que.push(pii(nx,ny));
        }
    }

    int ans = 0;
    rep(i,h) rep(j,w) if(visited[i][j]) ans++;
    cout << ans << endl;
}

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

C++: #include <bits/stdc++.h>
using namespace std;

#define INF 1.1e9
#define LINF 1.1e18
#define FOR(i,a,b) for (int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define BIT(x,n) bitset<n>(x)
#define PI 3.14159265358979323846

typedef long long ll;
typedef pair<int,int> P;
typedef pair<ll,P> PP;

//-----------------------------------------------------------------------------

int sx,sy,h,w;
string fld[20];
int dx[]={1,-1,0,0},dy[]={0,0,1,-1};

int bfs() {
	bool used[h][w];
	REP(i,h) REP(j,w) used[i][j]=false;
	int cnt=1;
	queue<P> q;
	used[sy][sx]=true;
	q.push(P(sx,sy));
	while(!q.empty()) {
		P p=q.front();q.pop();
		REP(i,4) {
			int nx=p.fi+dx[i],ny=p.se+dy[i];
			if(nx<0||nx>=w||ny<0||ny>=h||used[ny][nx]||fld[ny][nx]=='#') continue;
			used[ny][nx]=true;
			cnt++;
			q.push(P(nx,ny));
		}
	}
	return cnt;
}

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

	while(true) {
		cin>>w>>h;
		if(w==0) break;
		REP(i,h) {
			cin>>fld[i];
			REP(j,w) if(fld[i][j]=='@') sx=j,sy=i;
		}
		cout<<bfs()<<endl;
	}

	return 0;
}

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

#define REP(i, n) for (int i = 0; i < n; i++)
#define debug(x) cerr << #x << " = " << (x) << endl;

using namespace std;

int map[21][21], w, h;

bool check(int j, int k) {
	return (j >= 0 && j < h && k >= 0 && k < w && map[j][k] >= 1);
}

int main() {
	while (1) {
		int sj, sk, j, k, diff[] = {-1, 0, 1, 0, -1}, ans;
		string input;
		queue<pair<int, int> > bfs;

		cin >> w >> h;
		if (w==0 && h==0) break;

		REP(j, h) { REP(k, w) map[j][k] = -1; }
		ans = 0;

		REP(j, h) {
			cin >> input;
			REP(k, w) {
				map[j][k] = (input[k] == '#') ? 0 : 1;
				if (input[k] == '@') {
					sj = j;
					sk = k;
				}
			}
		}

		bfs.push(make_pair(sj, sk));
		map[sj][sk] = -1;
		ans++;

		while (!bfs.empty()) {
			j = bfs.front().first;
			k = bfs.front().second;
			bfs.pop();
			REP(i, 4) {
				if (check(j + diff[i], k + diff[i + 1])) {
					/*debug(j);
					debug(k);
					debug(j+diff[i]);
					debug(k+diff[i+1]);*/
					bfs.push(make_pair(j + diff[i], k + diff[i + 1]));
					map[j + diff[i]][k + diff[i + 1]] = -1;
					ans++;
				}
			}
		}

		cout << ans << endl;
	}
	return 0;
}