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 <list>
#include <set>
#include <string>
#include <cmath>
#include <algorithm>
#include <map>

using namespace std;
using ll=long long;
using vi=vector<int>;
using vll=vector<ll>;
using pii=pair<int,int>;
using vs=vector<string>;
using vpii=vector<pii>;
using si=set<int>;

#define REP(i,a) for(int i=0;i<a;i++)
#define FOR(i,a,b) for(int i=a;i<b;i++)
#define rREP(i,a) for(int i=(a)-1;i>=0;i--)
#define rFOR(i,a,b) for(int i=(a)-1;i>=(b);i--)
#define out(S) cout<<(S)<<endl;
#define beginend(v) (v).begin(),(v).end()
#define IfOut(condition,text) if(condition){out(text);return 0;}
#define IfOutElse(condition,if_text,else_text) if(condition){out(if_text);return 0;}else{out(else_text);return 0;}
#define mod(s) ((s)%(ll)(1e9+7))
ll modpow(ll i, ll j) { ll tmp = 1; while (j) { if (j % 2)tmp = mod(tmp*i); i = mod(i*i); j /= 2; }return tmp; }
#define divmod(a,b) (mod(a*modpow((ll)b,(ll)(1e9+5))))
#define pb(i) push_back(i)
#define fst first
#define snd second
#define Foreach(item,collection) for(auto item:collection)
#define INF (int)1e10
#define removeAll(vec,deleg) auto itr=remove_if(beginend(vec),deleg);vec.erase(itr,vec.end());
void removeAt(string& s, int index) { s.erase(index, 1); }
template<typename T> void removeAt(vector<T>& v, int index) { v.erase(v.begin() + index); }
ll manhattanDistance(ll x1, ll y1, ll x2, ll y2) { return (abs(x2 - x1) + abs(y2 - y1)); }
vector<ll> sieveOfEratosthenes(int max) {ll prime;double sqrtmax = sqrt(max);vector<ll> primeVec, searchVec;FOR(i, 2, max + 1) searchVec.push_back(i);do{prime = searchVec[0];primeVec.push_back(prime);auto itr = remove_if(beginend(searchVec), [=](ll x)->bool {return x%prime == 0; });searchVec.erase(itr, searchVec.end());} while (prime < sqrtmax);primeVec.reserve(primeVec.size() + searchVec.size());primeVec.insert(primeVec.end(), beginend(searchVec));return primeVec;}
ll sum(vi v) {ll sum=0;Foreach(i,v){sum+=i;}return sum;}
#define ShowAll(v) Foreach(i,v) out(i);


int x,y;
vs box;
int dx[4]{0,1,0,-1};
int dy[4]{1,0,-1,0};
ll s=0;
void dfs(int px,int py){
    REP(i,4){
        int nx=px+dx[i];
        int ny=py+dy[i];
        if(nx>=0&&nx<x&&ny>=0&&ny<y&&box[ny][nx]=='.'){
            box[ny][nx]='d';
            s++;
            dfs(nx,ny);
        }
    }
    return;
}

int main(){
    vi ans;
    while (cin>>x>>y,(x||y))
    {
        box.resize(y);
        int sx=-1,sy;
        REP(i,y){cin>>box[i];if(sx==-1&&(sx=box[i].find('@'))!=-1){sy=i;}}
        s=1;
        dfs(sx,sy);
        ans.pb(s);
    }
    
    ShowAll(ans);
    return 0;
}

C++: #include <iostream>
#include <queue>
using namespace std;
typedef pair<int, int> P;

int dx[] = {1, 0, -1, 0};
int dy[] = {0, -1, 0, 1};
int h, w;
int sy, sx;
char field[22][22];

int main(){
 
  while(cin >> w >> h && (h || w)){
    fill(&field[0][0], &field[21][21] + 1, '#');
    for(int i = 1; i <= h; i++){
      for(int j = 1 ; j <= w; j++){
	cin >> field[i][j];
	if(field[i][j] == '@'){
	  sy = i;
	  sx = j;
	  field[i][j] = '#';
	}
      }
    }
    queue<P> que;
    que.push(P(sy, sx));
    int res = 1;
    while(!que.empty()){
      P p = que.front();
      que.pop();
      int y = p.first;
      int x = p.second;
      for(int i = 0; i < 4; i++){
	int y2 = y + dy[i];
	int x2 = x + dx[i];
	if(field[y2][x2] == '.'){
	  res++;
	  field[y2][x2] = '#';
	  que.push(P(y2, x2));
	}
      }
    }
    cout << res << endl;
  }
  return 0;
}