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 <algorithm>
#include <iostream>
#include <vector>

using namespace std;

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

#define MOD 1000000007
#define REP(i, N) for (int i = 0; i < N; ++i)
#define REP1(i, N) for (int i = 1; i <= N; ++i)
#define RREP(i, N) for (int i = N - 1; i >= 0; --i)
#define ALL(a) a.begin(), a.end()

int w, h;

char c[20][20];
bool visit[20][20];

int dy[4] = {-1, 0, 1, 0};
int dx[4] = {0, 1, 0, -1};

void dfs(int y, int x) {
  visit[y][x] = true;
  REP(i, 4) {
    int ny = y + dy[i];
    int nx = x + dx[i];
    if (0 <= ny && ny < h && 0 <= nx && nx < w && c[ny][nx] == '.' &&
        !visit[ny][nx])
      dfs(ny, nx);
  }
}

int main() {
  while (cin >> w >> h) {
    if (w == 0 && h == 0) break;
    REP(i, h) REP(j, w) cin >> c[i][j];
    REP(i, h) REP(j, w) visit[i][j] = false;
    REP(i, h) REP(j, w) {
      if (c[i][j] == '@') dfs(i, j);
    }
    int ans = 0;
    REP(i, h) REP(j, w) {
      if (visit[i][j]) ans++;
    }
    cout << ans << endl;
  }
  return 0;
}

C++: #include <iostream>
#include <vector>
#include <queue>
#include <string>
#include <utility>
#include <unordered_set>

using namespace std;

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

int main(){
    int w, h;
    while(cin >> w >> h && w > 0 && h > 0){
        string tmp;
        pair<int, int> init;
        vector<vector <int> > grid(25, vector<int>(25, -1));
        vector<vector <int> > visited(25, vector<int>(25, 0));
        for(int i = 0; i < h; i++){
            cin >> tmp;
            for (int j = 0; j < w; j++){
                if(tmp[j] == '.'){
                    grid[i][j] = 0;
                }else if(tmp[j] == '#'){
                    grid[i][j] = -1;
                }else if(tmp[j] == '@'){
                    grid[i][j] = 0;
                    init = make_pair(i, j);
                }
            }
        }
        queue<pair<int, int> > Q;
        Q.push(init);
        visited[init.first][init.second] = 1; 
        while(!Q.empty()){
            pair<int, int> cur = Q.front(); 
            Q.pop();
            int cur_x = cur.first;
            int cur_y = cur.second;
            for(int i = 0; i < 4; i++){
                pair<int, int> dst = make_pair(cur_x + dx[i], cur_y + dy[i]);
                int x = dst.first;
                int y = dst.second;
                if(0 <= x && 0 <= y && grid[x][y] == 0 && visited[x][y] == 0){
                    Q.push(dst);
                    visited[x][y] = 1;
                }
            }
        }
        int ans = 0;
        for(int i = 0; i < h; i++){
            for (int j = 0; j < w; j++){
                ans += visited[i][j];
            }
        }
        cout << ans << endl << endl << endl;
    }
}

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

using namespace std;

// repetition
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

// container util
#define all(x) (x).begin(), (x).end()

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

// typedef
typedef long long lint;
typedef unsigned long long ull;
typedef complex<long double> Complex;
typedef pair<int, int> P;
typedef tuple<int, int, int> TP;
typedef vector<int> vec;
typedef vector<vec> mat;

// constant
const int MOD = (int)1e9 + 7;
const int INF = (int)1e18;
const int dx[] = {0, 1, 0, -1};
const int dy[] = {1, 0, -1, 0};
const int ddx[] = {0, 1, 1, 1, 0, -1, -1, -1};
const int ddy[] = {1, 1, 0, -1, -1, -1, 0, 1};

// conversion
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T>
inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}

//
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
//

int sx, sy;

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

  int w, h;

  while (1) {
    cin >> w >> h;
    if (w == 0 && h == 0) break;
    char tile[22][22];
    queue<pair<int, int>> que;
    rep(y, h) {
      rep(x, w) {
        cin >> tile[y][x];
        if (tile[y][x] == '@') {
          que.push(make_pair(x, y));
        }
      }
    }

    int ans = 0;

    while (!que.empty()) {
      pair<int, int> p = que.front();
      que.pop();

      rep(i, 4) {
        int nx = p.first + dx[i];
        int ny = p.second + dy[i];
        if (0 <= nx && nx < w && 0 <= ny && ny < h && tile[ny][nx] == '.') {
          ans++;
          tile[ny][nx] = '#';
          que.push(make_pair(nx, ny));
        }
      }
    }

    cout << ans + 1 << "\n";
  }

  return 0;
}