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<cstdio>
using namespace std;

char maps[22][22];
int count;
int me[2];
int w,h;

void init(){
  for(int i = 0; i < 22; i++){
    for(int j = 0; j < 22; j++){
      maps[i][j] = '#';
    }
  }
}

void where(int w,int h){
  for(int i = 0; i < h; i++){
    for(int j = 0; j < w; j++){
      if(maps[i][j] == '@'){
	me[0] = i;
	me[1] = j;
	maps[i][j] = '.';
	return;
      }
    }
  }
}

void search(int y, int x){
  maps[y][x] = '#';
  count++;
  if(maps[y - 1][x] == '.' && y > 0){
    search(y - 1, x);
  }
  if(maps[y + 1][x] == '.' && y < h){
    search(y + 1, x);
  }
  if(maps[y][x - 1] == '.' && x > 0){
    search(y, x - 1);
  }
  if(maps[y][x + 1] == '.' && x < w){
    search(y, x + 1);
  }
  return;
}
  
int main(){
  while(true){
    // input 
    cin >> w >> h;
    if(!w && !h){
      break;
    }
    init();
    for(int i = 0; i < h; i++){
      cin >> maps[i];
    }
    count = 0;
    where(w, h);
    search(me[0], me[1]);

    cout << count << endl;
  }
  return 0;
}

C++: #include<iostream>
#include<string>
#include<queue>
#include<map>
using namespace std;
int main(){
	int w,h;
	while(cin>>w>>h,w+h!=0){
		string ban[100];
		int sx=0,sy=0;
		queue< pair<int,int> >bfs;
		bool table[100][100];
		for(int i=0;i<100;i++)for(int j=0;j<100;j++)table[i][j]=false;
		
		for(int i=0;i<h;i++){
			cin>>ban[i];
			for(int j=0;j<ban[i].size();j++){
			if(ban[i][j]=='@')sy=i,sx=j;
			}
		}
		pair<int,int> s(sx,sy);
		bfs.push(s);
		table[sy][sx]=true;
		int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
		while(bfs.empty()==false){
			pair<int,int>now=bfs.front();
			bfs.pop();
			int nx=now.first,ny=now.second;
			for(int i=0;i<4;i++){
				if(nx+dx[i]>=0&&nx+dx[i]<w&&ny+dy[i]>=0&&ny+dy[i]<h){
					if(table[ny+dy[i]][nx+dx[i]]==false&&ban[ny+dy[i]][nx+dx[i]]=='.'){
					pair<int,int>tmp(nx+dx[i],ny+dy[i]);
					bfs.push(tmp);
					table[tmp.second][tmp.first]=true;
				}
				}
			}
		}
		int cnt=0;
		for(int i=0;i<h;i++)for(int j=0;j<w;j++)if(table[i][j])cnt++;
		cout<<cnt<<endl;
		for(int i=0;i<100;i++)ban[i].clear();
	}
	return 0;
}

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

using namespace std;

int w,h;
int ans=1;
char tile[21][21];

void saiki(int x,int y){
	if(x<0 | x>w | y<0 | y>h)return;
	tile[x][y]='#';
	if(tile[x-1][y]=='.')ans++,saiki(x-1,y);
	if(tile[x+1][y]=='.')ans++,saiki(x+1,y);
	if(tile[x][y-1]=='.')ans++,saiki(x,y-1);
	if(tile[x][y+1]=='.')ans++,saiki(x,y+1);
}

int main(void){
	int a,b; 
	while(1){
		scanf("%d %d",&w,&h);
		if(w==0 && h==0)break;
		ans=1;
		for(a=0;a<h;a++){
			scanf("%s",tile[a]);
		}
		for(a=0;a<h;a++){
			for(b=0;b<w;b++){
				if(tile[a][b]=='@')saiki(a,b);
			}
		}
		printf("%d\n",ans);
	}
	return 0;
}

C++: #include <vector>
#include <iostream>
#include <utility>
#include <algorithm>
#include <string>
#include <deque>
#include <tuple>
#include <queue>
#include <functional>
#include <cmath>
#include <iomanip>
#include <map>
//cin.sync_with_stdio(false);
//streambuf
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
char m[22][22];
int main() {
	int w, h;
	pii pos;
	while (cin>>w>>h,w) {
		fill((char*)m, (char*)m + 22 * 22,'#');
		for (int i = 1; i <= h; i++) {
			for (int j = 1; j <= w; j++) {
				cin >> m[i][j];
				if (m[i][j] == '@')pos = pii(i, j);
			}
		}
		int sum = 1;
		queue<pair<int, int>> que;
		que.push(pos);
		while (!que.empty()) {
			int dx[] = { 1,0,-1,0 }, dy[] = { 0,1,0,-1 };
			for (int i = 0; i < 4; i++) {
				int x = que.front().second+dx[i], y = que.front().first+dy[i];
				if (m[y][x] == '.') {
					m[y][x] = '@';
					sum++;
					que.push(pii(y, x));
				}
			}
			que.pop();
		}
		cout << sum << endl;
	}
	return 0;
}