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>
using namespace std;

int serch(int,int);
char tile[20][20];
int W,H;

int main(){
	int posx,posy;

	while(1){
		cin >> W >> H;
		if(W==0&&H==0) break;

		//^CÇÝÝ
		for(int j=0;j<H;j++){
			for(int i=0;i<W;i++){
				cin >> tile[i][j];
			}
		}

		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				if(tile[i][j]=='@'){
					posx=i,posy=j;
				}
			}
		}

		cout << serch(posx,posy)+1 << endl;
		for(int j=0;j<20;j++){
			for(int i=0;i<20;i++){
				tile[i][j]='#';
			}
		}

	}

	return 0;
}

int serch(int x,int y){
	//cout << "x=" << x << " " << "y=" << y << endl;
	int cnt=0;
	if(x==0&&y==0){
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
		}
		//cout << "x==y==0" << endl;
	}else if(x==W&&y==0){
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
		}
		//cout << "x==W&&y==0" << endl;
	}else if(x==0&&y==H){
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
		}
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
		}
		//cout << "x==0&&y==H" << endl;
	}else if(x==W&&y==H){
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
		}
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
		}
		//cout << "x==W&&y==H" << endl;
	}else if(x==0){
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
		}
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
		}
		//cout << "x==0" << endl;
	}else if(x==W){
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
		}
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
		}
		//cout << "x==W" << endl;
	}else if(y==0){
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
		}
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
		}
		//cout << "y==0" << endl;
	}else if(y==H){
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
		}
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
		}
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
		}
		//cout << "y==H" << endl;
	}else {
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
			//cout << "a---------------a" << endl;
		}
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
			//cout << "b---------------b" << endl;
		}
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
			//cout << "c---------------c" << endl;
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
			//cout << "d---------------d" << endl;
		}
	}
	//cout << endl;
	//for(int j=0;j<H;j++){
	//	for(int i=0;i<W;i++){
	//		cout << tile[i][j];
	//	}
	//	cout << endl;
	//}
	//
	return cnt;
}

C++: #include <bits/stdc++.h>

using namespace std;

typedef vector<vector<char> > field;

int search(field &f, int x, int y)
{
    if(f[y][x]=='#') return 0;
    f[y][x] = '#';
    int res = 1;
    if(f[y][x-1]!='#') res += search(f, x-1, y);
    if(f[y][x+1]!='#') res += search(f, x+1, y);
    if(f[y-1][x]!='#') res += search(f, x, y-1);
    if(f[y+1][x]!='#') res += search(f, x, y+1);
    return res;
}

int main()
{
    int w, h;
    while(cin >> w >> h, w|h)
    {
        field f(h+2, vector<char>(w+2, '#'));
        int sx, sy;
        for(int y=0; y<=h+1; y++) f[y].assign(w+2, '#');
        for(int y=1; y<=h; y++)
        {
            string ln;
            cin >> ln;
            for(int i=0; i<w; i++)
            {
                f[y][i+1] = ln[i];
                int spos = ln.find('@');
                if(spos>=0)
                {
                    sy = y;
                    sx = spos + 1;
                }
            }
        }
        cout << search(f, sx, sy) << endl;
    }
}