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 <fstream>
#include <stdio.h>

using namespace std;




void map_ser(int x, int y, int w, int h, int map[][30], int *cnt){
    
    if(map[y][x] == 1){
        *cnt+=1;
        map[y][x] = 2;
        if(0<=x-1 && x-1<w && 0<=y && y<h){
            map_ser(x-1, y, w, h, map, cnt);
        }
        if(0<=x+1 && x+1<w && 0<=y && y<h){
            map_ser(x+1, y, w, h, map, cnt);
        }
        if(0<=x && x<w && 0<=y-1 && y-1<h){
            map_ser(x, y-1, w, h, map, cnt);
        }
        if(0<=x && x<w && 0<=y+1 && y+1<h){
            map_ser(x, y+1, w, h, map, cnt);
        }
    }
    else if(map[y][x] != 1){
        return;
    }
}

int main(int argc, const char * argv[]) {
    // insert code here...
    
    
    /////////
    //write//
    /////////
    
    int map[30][30];
    int cnt;
    char tmp[100];
    int at_x,at_y;
    
    while(1){
        int w,h; cin >> w >> h;
        if(w==0 && h==0)break;
        cnt = 0;
        
        for(int i=0; i<h; i++){
            cin >> tmp;
            for(int j=0; j<w; j++){
                if(tmp[j] == '.')map[i][j] = 1;
                else if(tmp[j] == '#')map[i][j] = 0;
                else if(tmp[j] == '@'){
                    at_x = j;
                    at_y = i;
                    map[i][j] = 1;
                }
            }
        }
        map_ser(at_x, at_y, w, h, map, &cnt);
        cout << cnt << endl;
    }
    
    
    return 0;
}

C++: #include <cstdio>
#include <cstring>

char map[21][21];
int res;
int w,h;

void dfs(int sx, int sy) {
    if(sx<0 || sx>w-1 || sy<0 || sy>h-1) return;
    if(map[sy][sx] == '#') return;

    res++;
    map[sy][sx] = '#';

    dfs(sx+1,sy);
    dfs(sx,sy+1);
    dfs(sx-1,sy);
    dfs(sx,sy-1);
    return;
}

int main(void)
{
    int px,py;

    while(true)
    {
        res = 0;
        scanf("%d%d", &w, &h);
        if(!w && !h) break;
        for(int j=0; j < h; j++)
            scanf("%s", map[j]);

        for(int j=0; j < h; j++)
            for(int i=0; i < w; i++)
                if(map[j][i] == '@')
                    py = j, px =i;
        dfs(px, py);
        printf("%d\n", res);
    }

    return 0;
}

C++: #include <iostream>

int W, H;
int sx, sy;
char map[20][20];
int dx[4] = {-1, 0, 1, 0}, dy[4] = {0, 1, 0, -1};
int cnt = 0;

void dfs(int y, int x){
  map[y][x] = '#';
  cnt++;
  for(int i=0; i < 4; i++){
    if(0 <= y + dy[i] && y + dy[i] < H && 0 <= x + dx[i] && x + dx[i]< W && map[y + dy[i]][x + dx[i]] == '.'){
      dfs(y + dy[i], x + dx[i]);
    }
  }
}



int main(void){
  while(1){
    std::cin >> W >> H;
    if(W == 0 && H == 0)  return 0;

    for(int i=0; i < H; i++){
      for(int j=0; j < W; j++){
        std::cin >> map[i][j];
        if(map[i][j] == '@'){
          sx = j; sy = i;
        }
      }
    }

    cnt = 0;
    dfs(sy, sx);

    std::cout << cnt << std::endl; 
  }
}

C++: #if 0
08-162015?????????????????°???????????????????????????
pair???
#endif

#include <iostream>
#include <queue>
#include <string.h>
#include <utility>
using namespace std;

int w,h;
int tot=1; //?????£????????????(?????????????????????)
const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1}; //??????????§??????¨
char P[30][30]; //??§?¨?????????????????????¨???

bool valid(int x,int y){ //??????????????????
	return (x>=0)&&(x<w)&&(y>=0)&&(y<h)&&(P[x][y]!='#');
}

void bfs(int x,int y){ //???????????¢?´¢
	queue<pair<int,int> > q;
	pair<int,int> p;
	p = make_pair(x,y); //pair??????????????¨??????????????????
	q.push(p);
	while(!q.empty()){
		pair<int,int> cur=q.front(); //?????¨??°?????????????????????
		q.pop();
		for(int i=0;i<4;++i){
			if(valid(cur.first+dx[i],cur.second+dy[i]))
				tot+=1;
				P[cur.first+dx[i]][cur.second+dy[i]]='#'; //???????????£?????´???????°???????
				q.push(make_pair(cur.first+dx[i],cur.second+dy[i]));
				cur.first+=dx[i]; cur.second+=dy[i];
				q.push(cur);
		}
	}
}

int main(){
	int x1,y1;
	while((cin>>w>>h)&&(w!=0&&h!=0)){
		for(int i=0;i<h;++i){
			for(int j=0;j<w;++j){
				cin>>P[i][j];
				if(P[i][j]=='@')
					x1=i; y1=j; 
			}
		}
		bfs(x1,y1); //??????????????°????????£??\????????¢?´¢????§?
		cout << tot << endl;
	}
}

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

int w, h;
char tile[20][20];
int cnt = 0;
int dx[4] = { 1, -1, 0, 0 };
int dy[4] = { 0, 0, 1, -1 };

void f(int x, int y){
	tile[x][y] = '#';
	cnt++;
	for (int i = 0; i < 4; i++){
		int X = x + dx[i];
		int Y = y + dy[i];
		if (tile[X][Y] == '.' && 0 <= x && x < w && 0 <= y && y < h){
			f(X, Y);
		}
	}
}

int main(){

	while (cin >> w >> h, w + h != 0){
		for (int y = 0; y < h; y++){
			for (int x = 0; x < w; x++){
				cin >> tile[x][y];
			}
		}

		for (int y = 0; y < h; y++){
			for (int x = 0; x < w; x++){
				if (tile[x][y] == '@') f(x, y);
			}
		}

		cout << cnt << endl;
		cnt = 0;
	}

	return 0;
}