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;

int data[22][22];
int vis[22][22];
int cnt = 0;

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

void dfs(int x, int y){
  vis[x][y] = 1;
  cnt++;
  for(int i = 0; i < 4; i++){
    if(data[x + dx[i]][y + dy[i]] == 1 && vis[x + dx[i]][y + dy[i]] == 0){
      dfs(x + dx[i], y + dy[i]);
    }
  }
}

int main(){
  int w, h;
  while(cin >> w >> h && w + h){
    cnt = 0;
    for(int i = 0; i < 22; i++){
      for(int k = 0; k < 22; k++){
	data[i][k] = vis[i][k] = 0;
      }
    }
    pair<int,int> start;
    for(int i = 0; i < h; i++){
      string s;
      cin >> s;
      for(int k = 0; k < w; k++){
	if(s[k] == '.')
	  data[i + 1][k + 1] = 1;
	else if(s[k] == '@')
	  start.first = i + 1,start.second = k + 1;
      }
    }
    dfs(start.first, start.second);
    cout << cnt << endl;
  }
}

C++: #include<iostream>
#include<stdio.h>
#include<math.h>
#include<algorithm>
#include<queue>
#include<deque>
#include<stack>
#include<string>
#include<string.h>
#include<vector>
#include<set>
#include<map>
#include<bitset>
#include<stdlib.h>
#include<cassert>
#include<time.h>
#include<bitset>

int main(){
  while(1){
    int h,w;
    cin>>w>>h;
    if(h==0&&w==0) break;
    char a[h][w];
    queue<pair<int,int>> q;
    bool used[h][w];
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
        cin>>a[i][j];
        used[i][j]=0;
        if(a[i][j]=='@') q.push(make_pair(i,j));
      }
    }
    int ans=0;
    while(!q.empty()){
      pair<int,int> p=q.front();
      q.pop();
      int x=p.first, y=p.second;
      if(used[x][y]) continue;
      used[x][y]=1;
      ans++;
      if(x>0){
        if(!used[x-1][y]&&a[x-1][y]=='.'){
          q.push(make_pair(x-1,y));
        }
      }
      if(x<h-1){
        if(!used[x+1][y]&&a[x+1][y]=='.'){
          q.push(make_pair(x+1,y));
        }
      }
      if(y>0){
        if(!used[x][y-1]&&a[x][y-1]=='.'){
          q.push(make_pair(x,y-1));
        }
      }
      if(y<w-1){
        if(!used[x][y+1]&&a[x][y+1]=='.'){
          q.push(make_pair(x,y+1));
        }
      }
    }
    cout<<ans<<endl;
  }
  return 0;
}

C++: #include<iostream>
#include<vector>
#include<algorithm>

using namespace std;

#define rep(i,a) for(int i = 0 ; i < a ; i ++)
#define loop(i,a,b) for(int i = a ; i < b ; i ++)

int ans;
char maze[27][27];
void dfs(int x,int y){
	int dx[4] = {1,-1, 0, 0};
	int dy[4] = {0, 0, 1, -1};
	maze[x][y]='#';
	rep(i,4){
		int nx = dx[i] + x;
		int ny = dy[i] + y;
		if(maze[nx][ny]=='.'){
			ans++;
			dfs(nx,ny);
		}
	}
}

int main(void){
	int w,h;
	while(cin>>w>>h){
		rep(i,25)rep(j,25)
			maze[i][j]='#';
		ans=0;
		if(w==0&&h==0)return 0;
		rep(i,h)rep(j,w)
			cin>>maze[i][j];
		rep(i,h)rep(j,w){
			if(maze[i][j]=='@'){
				dfs(i,j);
				cout<<ans+1<<endl;
			}
		}
	}
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <cmath>
#include <cstring>
#include <numeric>
#include <algorithm>
#include <functional>
#include <array>
#include <map>
#include <queue>
#include <limits.h>
#include <set>
#include <stack>

using namespace std;

#define Getsign(n) ((n > 0) - (n < 0))

typedef vector<int> Ivec;
typedef pair<int, int> pii;
const pii eight_Dir[4] = { //?????????
	{ 0 ,1 },
	{ -1 ,0 },{ 1 ,0 },
	{ 0 ,-1 }
};
//pii operator+ (pii a, pii b){return { a.first + b.first, a.second + b.second };}
//pii operator- (pii a, pii b) { return { a.first - b.first, a.second - b.second }; }
//pii operator* (pii a, pii b) { return{ a.first * b.first, a.second * b.second }; }
//pii operator/ (pii a, pii b) { return{ a.first / b.first, a.second / b.second }; }
//pii operator% (pii a, pii b) { return{ a.first % b.first, a.second % b.second }; }

int main() {
	int w, h;

	while (scanf("%d %d", &w, &h)) {
		if (w == 0)
			return 0;

		array<array<bool, 22>, 22> map = {}, al = {};
		queue<pii> que;
		for (int i = 1; h >= i; i++) {
			scanf("%*c");
			for (int j = 1; w >= j; j++) {
				char a;
				scanf("%c", &a);

				if (a == '@') {
					que.push({j,i});
					al[j][i] = true;
					map[j][i] = true;
				}
				else if (a == '.') {
					map[j][i] = true;
				}
			}
		}
		int cou = 0;
		while (!que.empty()) {
			pii pos = que.front();
			que.pop();
			cou++;
			for (int i = 0; 4 > i; i++) {
				if (!al[pos.first + eight_Dir[i].first][pos.second + eight_Dir[i].second] && map[pos.first + eight_Dir[i].first][pos.second + eight_Dir[i].second]) {
					al[pos.first + eight_Dir[i].first][pos.second + eight_Dir[i].second] = true;
					que.push({ pos.first + eight_Dir[i].first,pos.second + eight_Dir[i].second});
				}
			}

		}
		printf("%d\n", cou);
	}

	return 0;
}