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++: #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[100][100]; //??§?¨?????????????????????¨???

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])){
				q.push(make_pair(cur.first+dx[i],cur.second+dy[i]));
				tot++;
				P[cur.first+dx[i]][cur.second+dy[i]]='#'; //???????????£?????´???????°???????
			}
		}
		P[cur.first][cur.second]='#';
	}
}

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

C++: #include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

void saiki(int x,int y,vector<vector<char> > &field,vector<vector<char> > &k){
    if(field[y][x] == '#' || k[y][x] == 'x'){
        return;
    }

    field[y][x] = '#';
    k[y][x] = 'x';
    saiki(x-1,y,field,k);
    saiki(x+1,y,field,k);
    saiki(x,y-1,field,k);
    saiki(x,y+1,field,k);
    field[y][x] = '.';
    return;
}

int solve(int w,int h){
    //char field[w+2][h+2];
    vector<vector<char> > field(h+2,vector<char>(w+2));
    vector<vector<char> > k(h+2,vector<char>(w+2));

    int x,y;
    for(int i=0;i<h+2;i++){
        for(int j=0;j<w+2;j++){
            field[i][j] = '#';
        }
    }

    for(int i=1;i<=h;i++){
        for(int j=1;j<=w;j++){
            cin >> field[i][j];
            if(field[i][j] == '@'){
                y = i;
                x = j;
            }
            k[i][j] = field[i][j];
        }
    }

#ifdef DEB
    cout << "X:" << x << " Y:" << y << endl;
    for(int i=0;i<h+2;i++){
        for(int j=0;j<w+2;j++){
            cout << field[i][j];
        }
        cout << endl;
    }
#endif
    saiki(x,y,field,k);

    int m = 0;
    for(int i=0;i<h+2;i++){
        for(int j=0;j<w+2;j++){
            if(k[i][j] == 'x'){
                m++;
            }
        }
    }
    cout << m << endl;
}

int main(){
    int x,y;
    while(cin >> x >> y){
        if(x==0 && y == 0){
            break;
        }
        solve(x,y);
    }
}

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

#define rep(i, j) for(int i = 0; i < j; i++)
#define all(i) i.begin(), i.end()
#define ll long long
#define bl bool
#define nn printf("\n");
#define mod 1000000007

using namespace std;

int w, h, cnt;
void srch(int x, int y, vector<vector<char>> &inp, vector < vector <bool>> &hit)
{
	if (x < 0 || w <= x || y < 0 || h <= y || inp[x][y] == '#') { return; }
	if (hit[x][y]) return;
	hit[x][y] = true;
	cnt += 1;
	srch(x + 1, y, inp, hit);
	srch(x - 1, y, inp, hit);
	srch(x, y + 1, inp, hit);
	srch(x, y - 1, inp, hit);
}
int main()
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	while (1)
	{
		cin >> w >> h;
		if (w == 0 && h == 0) { break; }
		cnt = 0;
		vector<vector<char>> inp(w, vector<char>(h, 0));
		vector<vector<bool>> hit(w, vector<bool>(h, false));
		int x = 0, y = 0;
		rep(i, h)
		{
			rep(j, w)
			{
				cin >> inp[j][i];
				if (inp[j][i] == '@'){ x = j; y = i; }
			}
		}
		srch(x, y, inp, hit);
		printf("%d\n", cnt);
	}
	return 0;
}

C++: #include<iostream>

char map[20][20]={0};
int number=1;
int h,w;

void debug(void) {
	for(int i=0;i<h;i++) { for(int j=0;j<w;j++) putchar(map[i][j]); putchar('\n'); }
}

void trace(int x,int y) {
	if(x-1 >= 0 && map[y][x-1]=='.') {
		map[y][x-1] = '@';
		trace(x-1,y);

		number++;
	}

	if(x+1 < w && map[y][x+1]=='.') {
		map[y][x+1] = '@';
		trace(x+1,y);
		number++;
	}
	if(y-1 >= 0 && map[y-1][x]=='.') {
		map[y-1][x] = '@';
		trace(x,y-1);
		number++;
	}

	if(y+1 < h && map[y+1][x]=='.') {
		map[y+1][x] = '@';
		trace(x,y+1);
		number++;
	}
}

int main(void) {
	char buf[21];

	int x,y;


	for(;;) {

		number=1;
		scanf("%d %d\n",&w,&h);

		if(w==0 || h==0) break;

		for(int i=0;i<h;i++) {
			fgets(buf,1024,stdin);

			memcpy(&map[i],buf,w);
		}


		for(int j=0;j<h;j++) {
			for(int i=0;i<w;i++) {
				if(map[j][i] == '@'){
					x=i;y=j;
				}
			}
		}


		trace(x,y);

		printf("%d\n",number);

	}

	//debug();

	return 0;
}