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>
#include <cmath>
#include <vector>
#include <map>
#include <stack>
#include <queue>
#include <algorithm>
#include <set>

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,j) FOR(i,0,j)
#define mp std::make_pair

typedef long long ll;
typedef unsigned long long ull;
typedef std::pair<int,int> P;
typedef std::pair<int,P> State;

const int INF = 1001001001;

// S N E W(南北東西)
const int dx[8] = {0, 0, 1, -1, 1, 1, -1, -1}, dy[8] = {1, -1, 0, 0, 1, -1, 1, -1};

int width, height;
char map[20][20];
int sx, sy;
bool used[20][20];

int dfs(int x, int y){
    used[y][x] = true;

    int res = 1;
    REP(i, 4){
        int nx = x + dx[i], ny = y + dy[i];
        if(0 <= nx && nx < width && 0 <= ny && ny < height &&
           !used[ny][nx] && map[ny][nx] == '.'){
            res += dfs(nx, ny);
        }
    }
    
    return res;
}

int main(){
    while(std::cin >> width >> height, width || height){
        REP(i, height){
            REP(j, width){
                std::cin >> map[i][j];
                if(map[i][j] == '@'){
                    sx = j; sy = i;
                    map[i][j] = '.';
                }
            }
        }

        REP(i, 20){
            REP(j, 20){
                used[i][j] = false;
            }
        }
        std::cout << dfs(sx, sy) << std::endl;
    }
}

C++: #include <iostream>
#include <vector>
#include <string>
using namespace std;

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

int W, H, ans;
char maze[20][20];

bool check(int x, int y){
	return x >= 0 && x < W && y >= 0 && y < H;
}

void dfs(int x, int y){
	
	
	for(int i = 0; i < 4; ++i){
		int nx = x + dx[i], ny = y + dy[i];
		if(check(nx, ny) && maze[ny][nx] != '#'){
			maze[ny][nx] = '#';
			ans++;
			dfs(nx, ny);
		}
	}
}

int main() {
	while(cin >> W >> H && (W || H)){
		for(int i = 0; i < H; ++i)
			for(int j = 0; j < W; ++j)
				cin >> maze[i][j];
		
		ans = 0;
		for(int i = 0; i < H; ++i){
			for(int j = 0; j < W; ++j){
				if(maze[i][j] == '@'){
					dfs(j, i);
					break;
				}
			}
		}
		cout << ans << endl;
	}
	return 0;
}

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

int used[23][23];
int answer;
int h,w;
string s[23];

void dfs(int y,int x){
  answer++;
  used[y][x] = 1;
  
  if( y-1 >= 0 && used[y-1][x]==0 && s[y-1][x]!='#') dfs( y-1,x );
  
  if( y+1 <  h && used[y+1][x]==0 && s[y+1][x]!='#') dfs( y+1,x );
  
  if( x-1 >= 0 && used[y][x-1]==0 && s[y][x-1]!='#') dfs( y,x-1 );
  
  if( x+1 <  w && used[y][x+1]==0 && s[y][x+1]!='#') dfs( y,x+1 );
}

int main(){
  
  while( cin>>w>>h,w){
    
    int y,x;

    answer = 0;
    
    memset( used,0,sizeof(used) );
    for(int i=0;i<h;i++) cin>>s[i];
    
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	if(s[i][j]=='@'){
	  y = i;
	  x = j;
	}
      }
    }    
    dfs(y,x);
    cout<< answer <<endl;
  }
}

C++: #include <bits/stdc++.h>
using namespace std;
string place[20];
int dx[4]={0,1,0,-1},dy[4]={-1,0,1,0};
int serch(int n,int m,int y,int x)
{
	int ans=1;
	place[y][x]='#';
	for(int i=0,ax,ay;i<4;i++)if((ay=dy[i]+y)<m&&(ax=dx[i]+x)<n&&ay>=0&&ax>=0&&place[ay][ax]=='.')ans+=serch(n,m,ay,ax);
	return ans;
}
int main() {
	int n,m,y,x;
	while(cin>>n>>m,n,m){
		for(int i=0;i<m;i++){
			cin>>place[i];
			for(int j=0;j<n;j++){
				if(place[i][j]=='@'){
					x=j;
					y=i;
				}
			}
		}
		cout<<serch(n,m,y,x)<<endl;
	}
}