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 <stdio.h>
#include <queue>
#include <string.h>
using namespace std;	
int W,H;
char c[101][101];
bool visited[100][100];
 
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
 	
void compute();//????????????????????£?¨? 
	
int main(){
	
	while(scanf("%d %d",&W,&H),W |H){
	    int i;
		
		for(i=0;i<H;i++){
			scanf("%d",&c[i][0]);
		}
	}
	return 0;
}

void compute(){
	
	memset(visited,false,sizeof visited);  //{????????????????????????1bit??????????????????1bit????????§?????????} 
	
	queue<int> que;
	int i,j;
	for(j=0;j<W;j++){ 
	 for(i=0;i<H;i++){
	  if(c[i][j]=='@'){
	  	que.push(j+i*H);
	  	visited[i][j]==true;
	  	 break;
	  }
	 }
	}
  int answer=1;
  while(!que.empty()){
	int cur=que.front();
	        que.pop();
	        int x=cur%W;
	        int y=cur/W;
    

  for(i=0;i<4;i++){
	int nx=x+dx[i],ny=y+dy[j];
	
  
  
  if(!(0<=nx&&nx<W&&0<=ny))continue;
  if(c[ny][nx]=='#'||visited[ny][nx])continue;

  que.push(nx+ny*W);
  visited[ny][nx]=true;

    }
   }
 
 }

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

int moved = 0;
int w, h;
char myMap[20][20];

bool safe(int x, int y){
    return (x < h && y < w) || (x >= 0 && y >= 0);
}

void move(int x, int y){
    myMap[x][y] = '#';
    moved++;
    if (myMap[x+1][y] == '.' && safe(x+1, y)) move(x+1, y);
    if (myMap[x-1][y] == '.' && safe(x-1, y)) move(x-1, y);
    if (myMap[x][y+1] == '.' && safe(x, y+1)) move(x, y+1);
    if (myMap[x][y-1] == '.' && safe(x, y-1)) move(x, y-1);
    return;
}

int main(){
    while (1){
	cin >> w >> h;
	if (w == 0 && h == 0) break;

	moved = 0;
	int initX, initY;

	for (int i = 0; i < h; i++){
	    for (int j = 0; j < w; j++){
		cin >> myMap[i][j];
		if (myMap[i][j] == '@'){
		    initX = i;
		    initY = j;
		}
	    }
	}
	
	move(initX, initY);
	
	cout << moved << endl;
    }
}

C++: #include<cmath>
#include<iostream>
#include<algorithm>
#include<iomanip>
#include<vector>
#include<string>
#include<queue>
#include<stack>
#include<cstdio>
#include<map>
#include<utility>
#include<list>
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define fori(i,k,n) for(int i=k;i<(int)n;i++)
using namespace std;
const int dx[4] = { 0, 1, 0, -1 }, dy[4] = { 1, 0, -1, 0 };
const int dX[8] = { 0, 1, 1, 1, 0, -1, -1, -1 }, dY[8] = { 1, 1, 0, -1, -1, -1, 0, 1 };
struct XY
{
	int x;
	int y;
};


/*
(char)'1' - (int)48 = (int)1;


*/
int main()
{
	int h, w;
	cin >> w>> h;
	while (w != 0 || h != 0)
	{
		queue<XY> m;
		XY fis;
		int a[200][200] = {};
		for (int i = 0; i < h; i++)
		{
			for (int l = 0; l < w; l++)
			{
				char r;
				cin >> r;
				if (r == '.')
					a[i + 1][l + 1] = 1;
				else if (r == '@')
				{
					fis.x = l + 1;
					fis.y = i + 1;
				}

			}
		}


		int ans = 1;
		m.push(fis);
		while (!m.empty())
		{
			XY gg;
			int x, y;
			x = m.front().x;
			y = m.front().y;
			m.pop();

			for (int i = 0; i < 4; i++)
			{
				if (a[y + dy[i]][x + dx[i]] == 1)
				{
					ans++;
					a[y + dy[i]][x + dx[i]] = 0;
					gg.x = x + dx[i];
					gg.y = y + dy[i];
					m.push(gg);
				}
			}

		}

		cout << ans << endl;
		cin >> w >> h;
	}
}

C++: #include <iostream>
#include <stdio.h>
#include <algorithm>
#include <math.h>
#include <vector>
#include <string>
#include <sstream>
#include <cstring>

#define rep(i, j) for(int i = 0; i < j; i++)
// sp_rep
#define sp_rep(i, j) for(int i = 1; i <= j; i++)
#define all(i) i.begin(), i.end()
#define ll long long

using namespace std;

string Num2Str(int num)
{
	stringstream ss;
	ss << num;
	return ss.str();
}

int w, h, cnt;
char maze[20][20];
bool reached[20][20];

void serch(int x, int y) {
	if (x < 0 || w <= x || y < 0 || h <= y || maze[y][x] == '#') return;
	if (reached[y][x])return;
	reached[y][x] = true;
	cnt++;

	serch(x + 1, y);
	serch(x - 1, y);
	serch(x, y + 1);
	serch(x, y - 1);
}

int main()
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	// code
	while (true) {
		cin >> w >> h;
		if (w == 0 && h == 0)break;
		cnt = 0;
		memset(reached, false, sizeof(reached));
		int sx, sy;
		rep(y, h) {
			rep(x, w) {
				cin >> maze[y][x];
				if (maze[y][x] == '@') {
					sx = x;
					sy = y;
				}
			}
		}
		serch(sx, sy);
		cout << cnt << endl;
	}

	return 0;
}