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;
typedef pair<int, int> p;

int field[22][22];
bool visited[22][22];
int dx[] = {0, 1, 0, -1};
int dy[] = {1, 0, -1, 0};
int sx, sy;

int main(){
  int w, h;
  char c;
  while(cin >> w >> h, w|h){
    memset(field, 0, sizeof(field));
    memset(visited, false, sizeof(visited));
    for(int i = 0; i < h; i++){
      for(int j = 0; j < w; j++){
        cin >> c;
        if(c == '.') field[i][j] = 1;
        if(c == '@') sx = i, sy = j;
      }
    }
    queue<p> que;
    int sum = 1;
    visited[sx][sy] = true;
    que.push(p(sx, sy));
    while(!que.empty()){
      p cur = que.front(); que.pop();
      for(int i = 0; i < 4; i++){
        int nx = cur.first + dx[i];
        int ny = cur.second + dy[i];
        if(nx < 0 || nx > 20) continue;
        if(ny < 0 || ny > 20) continue;
        if(visited[nx][ny]) continue;
        if(field[nx][ny] == 0) continue;
        visited[nx][ny] = true;
        sum++;
        que.push(p(nx, ny));
      }
    }
    cout << sum << endl;
  }
  return 0;
}

C++: #include<iostream>
#include<vector>
#include<map>
#include<set>
#include<algorithm>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<string>
#include<queue>
#include<complex>
#include<numeric>
#include<bitset>
#define INF 1001001001
#define EPS 0.000000001
#define x first
#define y second

using namespace std;
typedef vector<int> vint;
typedef vector<vint> vvint;
typedef pair<int,int> pint;

int dx[4] = {0,1,0,-1};
int dy[4] = {1,0,-1,0};
//int dx[8] = {0,1,1,1,0,-1,-1,-1};
//int dy[8] = {1,1,0,-1,-1,-1,0,1};

int main(){
	int i,j,fx,fy;
	int w,h;
	char m[30][30];
	while(1){
		cin >> w >> h;
		if(w==0 && h==0) break;
		for(i=0;i<h;i++){
			for(j=0;j<w;j++){
				cin >> m[i][j];
				if(m[i][j]=='@'){
					fx=i;
					fy=j;
				}
			}
		}
		
		int ans=1;
		queue<pint> q;
		q.push(pint(fx,fy));
		while(!q.empty()){
			pint c = q.front(); q.pop();
			for(i=0;i<4;i++){
				int nx = c.x + dx[i];
				int ny = c.y + dy[i];
				if(0 <= nx && nx < h && 0 <= ny && ny < w && m[nx][ny]=='.'){
					m[nx][ny] = '@';
					q.push(pint(nx,ny));
					ans++;
				}
			}
		}
		cout << ans << endl;
	}
}

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

int h,w;
string str[30];

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

int dfs(int y,int x){
    str[y][x] = '#';
    int res = 0;
    for(int i=0;i<4;i++){
        if(y+dy[i]<0 || y+dy[i]>=h || x+dx[i]<0 || x+dx[i]>=w || str[y+dy[i]][x+dx[i]]!='.') continue;
        res += dfs(y+dy[i],x+dx[i])+1;
    }
    return res;
}

int main()
{
    
    while(cin >> w >> h,w!=0&&h!=0){
        for(int i=0;i<h;i++) cin >> str[i];
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                if(str[i][j] == '@'){
                    cout << dfs(i,j)+1 << endl;
                }
            }
        }
    }

    return(0);
}

C++: #include<bits/stdc++.h>
#define rep(i,n) for(int i=0; i<n; i++)
using namespace std;
char field[21][21];
char visit[21][21];
int dx[]={1,-1,0,0};
int dy[]={0,0,1,-1};
int W,H;
void dfs(int y, int x){
  visit[y][x] = 1;
  rep(i,4){
    int ny = y+dy[i];
    int nx = x+dx[i];
    if(field[ny][nx] == '.'
       && visit[ny][nx] == 0
       && nx >= 0
       && nx < W
       && ny >= 0
       && ny < H){
          dfs(ny,nx);
        }
  }
}
int main(){
  while(cin >> W >> H && W){
    int y,x;
    rep(i,H){
      rep(j,W){
        cin >> field[i][j];
        if(field[i][j] == '@') y=i,x=j;
      }
    }
    memset(visit,0,sizeof(visit));
    dfs(y,x);
    int ans = 0;
    rep(i,H){
      rep(j,W){
        ans += visit[i][j];
      }
    }
    cout << ans << endl;
  }
  return 0;
}