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 <vector>
#include <string>
#include <algorithm>
#include <stdio.h>
using namespace std;
typedef long long ll;
#define rep(i,n) for (int i=0; i < (n); i++)

const int MAX_W = 23;
bool visited[MAX_W][MAX_W];
char map[MAX_W][MAX_W] = {};
int dx[]={0,1,0,-1};
int dy[]={-1,0,1,0};

int dfs(int x, int y) {
  if (map[y][x] != '.') return 0;
  int ret = 1;
  map[y][x] = '#';
  rep(i,4) {
    ret += dfs(x + dx[i], y + dy[i]);
  }
  return ret;
}

void solve(int w, int h) {
  int sx, sy;
  rep(i, MAX_W) rep(j, MAX_W) map[i][j] = 0;
  rep(i,h) {
    string s;
    cin >> s;
    rep(j,w) {
      if (s[j] == '@') {
        sx = j+1;
        sy = i+1;
        s[j] = '.';
      }
      map[i+1][j+1] = s[j];
    }
  }
  cout << dfs(sx, sy) << endl;
}

int main() {
  int w, h;
  while(true) {
    cin >> w >> h;
    if (w == 0) return 0;
    solve(w, h);
  }
}

C++: #include <iostream>
#include <queue>
#include <utility>
using namespace std;

const int dx[] = {1,0,-1,0};
const int dy[] = {0,-1,0,1};
queue<pair<int,int> > M;
int W,H;
int q[20+5][20+5] = {};	
char p[20+5][20+5];
int counter;
int x,y;

int main() {
	while(cin >> W >> H && W != 0){
		counter = 0;
		for (int i = 1; i <= 22; i++){
			for (int j = 1; j <= 22; j++){
					q[i][j] = 0;
					p[i][j] = ' ';
				
			}
		}
		for (int i = 1; i <= H; i++){
			for (int j = 1; j <= W; j++){
				cin >> p[i][j];
				if (p[i][j] == '@' ){
					M.push(make_pair(i,j));
					counter = 1;
					q[i][j] = 1;
				}
			}
		}
		while(!M.empty()){
			x = M.front().first;
			y = M.front().second;
			M.pop();
			for (int k = 0; k <= 3; k++)
			{		
				if (p[x+dx[k]][y+dy[k]] == '.' && q[x+dx[k]][y+dy[k]] == 0)
				{
					M.push(make_pair(x+dx[k], y+dy[k]));
					q[x+dx[k]][y+dy[k]] = 1;
					counter += 1;
				}
			}
		}
		cout << counter << endl;
	}
}

C++: #include<bits/stdc++.h>
#define  INF 1100000000
#define	 MOD 1000000007
using namespace std;

int vis[22][22];
char mp[22][22];

int dfs(int sh, int sw, int ans){
	if(mp[sh+1][sw] == '.' && vis[sh+1][sw] == 0){
		vis[sh+1][sw] = 1;
		ans++;
		ans = dfs(sh+1, sw, ans);
	}
	if(mp[sh-1][sw] == '.' && vis[sh-1][sw] == 0){
		vis[sh-1][sw] = 1;
		ans++;
		ans = dfs(sh-1, sw, ans);
	}
	if(mp[sh][sw-1] == '.' && vis[sh][sw-1] == 0){
		vis[sh][sw-1] = 1;
		ans++;
		ans = dfs(sh, sw-1, ans);
	}
	if(mp[sh][sw+1] == '.' && vis[sh][sw+1] == 0){
		vis[sh][sw+1] = 1;
		ans++;
		ans = dfs(sh, sw+1, ans);
	}
	return ans;
}

int main(){
	int W, H, ans, sw, sh;
	
	while(1){
		ans = 0;
		cin >> W >> H;
		if(W == 0 && H == 0) break;
		for(int i=0;i<22;i++){
			for(int j=0;j<22;j++){
				vis[i][j] = 1;
			}
		}
			
		for(int i=1;i<=H;i++){
			for(int j=1;j<=W;j++){
				cin >> mp[i][j];
				if(mp[i][j] == '@'){
					sh = i;
					sw = j;
					vis[i][j] = 1;
				}else{
					vis[i][j] = 0;
				}
			}
		}
		
		ans = dfs(sh, sw, ans);
		ans++;
		cout << ans << endl; 
	}
	return 0;
}

C++: #include<iostream>
#include<queue>
#include<utility>

using namespace std;

queue<pair<int,int> > Q;
pair<int,int> P;
int sum=0;
int W,H;
int ai,aj;
char x[25][25];
int y[25][25];
const int dx[]={1,0,-1,0},dy[]={0,-1,0,1};

bool valid(int a,int b){
    if(0<=a && a<H && 0<=b && b<W && x[a][b]=='.'){
        return 1;
    }
    else{
        return 0;
    }
}

int solve( ){
    sum++;y[ai][aj]=1;
    while(!Q.empty()){
        pair<int,int> P2=Q.front();
        Q.pop();
        for(int i=0;i<4;i++){
            if(valid(P2.first+dx[i],P2.second+dy[i])==1&&y[P2.first+dx[i]][P2.second+dy[i]]==0){
                sum++;
                y[P2.first+dx[i]][P2.second+dy[i]]=1;
                P=make_pair(P2.first+dx[i],P2.second+dy[i]);
                Q.push(P);
            }
        }
    }
    return sum;
}
void inits(){
    sum=0;
    for(int i=0;i<25;i++ ){
        for(int j=0;j<25;j++){
            x[i][j]=0;
            y[i][j]=0;
        }
    }
}

int main(){
    while(cin>>W>>H&&W!=0){
        inits();
        for(int i=0;i<H;i++)
            for(int j=0;j<W;j++){
                cin>>x[i][j];
                if(x[i][j]=='@'){
                    ai=i;aj=j;
                }
            }
        P=make_pair(ai,aj);
        Q.push(P);
        cout<<solve()<<endl;
    }
    return 0;
}