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 <iomanip>
#include <cstdio>
#include <cmath>
#include <iostream>
#include <vector>
#include <string>
#include <stack>
#include <queue>
#include <map>
#include <set>

#define rep(i,n) for (int i = 0; i < (n); ++i)

using namespace std;

typedef long long ll;

int h, w;
char grid[30][30];
int ans;

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

void dfs(int x, int y) {
  grid[y][x] = '#';
  ans++;
  for (int i = 0; i < 4; i++) {
    int nx = x + dx[i];
    int ny = y + dy[i];
    if (nx < 0 || ny < 0 || nx >= w || ny >= h ) {
      continue;
    }
    if ( grid[ny][nx] == '#' ) {
      continue;
    }
    dfs(nx, ny);
  }
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << setprecision(20) << fixed;

  int sx, sy;

  while ( true ) {
    cin >> w >> h;

    if (w == 0 && h == 0) { break; }

    rep(y, h) {
      cin >> grid[y];
    }

    rep(y, h) {
      rep(x, w) {
        if (grid[y][x] == '@') {
          sx = x; sy = y;
        }
      }
    }
    ans = 0;
    dfs(sx, sy);
    cout << ans << endl;
  }

  return 0;
}

C++: // 横型探索

#include <iostream>
#include <string>
#include <vector>
#include <queue>

using namespace std;

int w, h;
int mx[] = {0, 0, -1, 1};
int my[] = {-1, 1, 0, 0};

int main() {
    while ( true ) {
        int x, y, cnt = 0;
        cin >> w >> h;
        if ( w == 0 && h == 0 ) break;

        vector<string> v(h);
        queue<int> s;

        for ( int i = 0; i < h; i++ ) {
            cin >> v[i];

            // @の座標を記録
            for ( int j = 0; j < v[i].size(); j++ ) {
                if ( v[i][j] == '@' ) {
                    y = i;
                    x = j;
                }
            }
        }

        // キューにスタート地点を追加
        s.push(x);
        s.push(y);

        while(!s.empty()) {
            x = s.front(); s.pop();
            y = s.front(); s.pop();

            for ( int i = 0; i < 4; i++ ) {
                int nx = x + mx[i], ny = y + my[i];
                if ( 0 <= nx && nx < w && 0 <= ny && ny < h && v[ny][nx] == '.' ) {
                    v[ny][nx] = '#';
                    s.push(nx);
                    s.push(ny);
                }
            }
            cnt++;
        }
        cout << cnt << endl;
    }
    return 0;
}

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

#ifdef LOCAL
#include "../dump.hpp"
#else
#define dump(...)
#endif

#define rep(i,n) for(ll i=0,i##_cond=(n);i<i##_cond;++i)
#define FOR(i,a,b) for(ll i=(a),i##_cond=(b);i<i##_cond;++i)
#define ROF(i,a,b) for(ll i=(a)-1,i##_cond=(b);i>=i##_cond;--i)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend() //sortで大きい順
#define UNIQUE(v) v.erase(unique(all(v)),v.end())
#define SUM(a) accumulate(all(a),0)
#define sz(x) ((ll)(x).size())
#define pb push_back
#define fst first
#define snd second
#define mp make_pair

typedef long long ll;
typedef vector<ll> vi;
typedef vector<vi> vvi;
typedef vector<string> vs;
typedef pair<ll,ll> pii;

const ll inf = 1ll<<62;
const ll mod = 1e9+7;

#define int ll

vi dx = {1, 0, -1, 0, 1, -1, -1, 1}, dy = {0, 1, 0, -1, 1, 1, -1, -1};

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

    vs maze(h); rep(i,h) cin >> maze[i];
    vvi look(h,vi(w));
    int sx, sy;
    rep(i,h) rep(j,w) if(maze[i][j] == '@'){
      sx = i; sy = j;
    }
    
    queue<pii> q;

    q.push(pii(sx,sy));
    int ans = 1;
    look[sx][sy] = 1;

    while(not q.empty()){
      pii p = q.front();
      q.pop();

      //if(p.fst == gx and p.snd == gy) break;

      rep(i, 4){
	int nx = p.fst + dx[i], ny = p.snd + dy[i];
	if(0 <= nx and nx < h and 0 <= ny and ny < w and maze[nx][ny] != '#' and look[nx][ny] == 0){
	  q.push(pii(nx, ny));
	  look[nx][ny] = 1;
	  ans++;
	}
      }
    }
    cout << ans << endl;
  }
}