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<cstdio>
char table[20][20];
int n,m,si,sj;
int solve(int a,int b){
	int xx[]={a,a+1,a,a-1},yy[]={a+1,a,a-1,a},res=1;
	if(table[a][b]!='.')return 0;
	table[a][b]='#';
	for(int i=0;i<4;i++){
		if(xx[i]<0 || xx[i]>=m || yy[i]<0 || yy[i]>=n)continue;
		res+=solve(xx[i],yy[i]);
	}
	return res;
}
int main(){
	scanf("%d%d",&n,&m);
	for(int i=0;i<m;i++){
		for(int j=0;j<n;j++){
			scanf(" %c",&table[i][j]);
			if(table[i][j]=='@')si=i,sj=j,table[i][j]='.';
		}	
	}
	printf("%d\n",solve(si,sj));
	return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<vector>
#include<queue>
#include<algorithm>
#include<cmath>
#include<climits>
#include<string>
#include<set>
#include<numeric>
#include<map>
#include<iostream>
using namespace std;
#define rep(i,n) for(int i = 0;i<((int)(n));i++)
#define reg(i,a,b) for(int i = ((int)(a));i<=((int)(b));i++)
#define irep(i,n) for(int i = ((int)(n)-1);i>=0;i--)
#define ireg(i,a,b) for(int i = ((int)(b));i>=((int)(a));i--)
typedef long long ll;
typedef pair<ll, ll> mp;

int W,H,f[20][20],ans[1000],cnt=0,sx,sy;
string m[20];

void find(int x,int y){
	if(x>=W || x<0 || y>=H || y<0)return;
	string t;
	t = m[x][y];
	if(t=="#")return;
	if(f[x][y]!=0)return;
	f[x][y] = 1;
	ans[cnt]++;
	find(x+1,y);
	find(x-1,y);
	find(x,y+1);
	find(x,y-1);
}

int main(void){
	while(1){
		cin>>W>>H;
		if(W==0 && H==0)break;
		ans[cnt]=0;
		rep(i,H){
			string s;
			cin>>s;
			rep(j,W){
				f[j][i]=0;
				m[j][i]=s[j];
				string t;
				t=m[j][i];
				if(t=="@"){
					sx=j;
					sy=i;
				}
			}
		}
		find(sx,sy);
		cnt++;
	}
	rep(i,cnt)cout<<ans[i]<<endl;
	return 0;
}

C++: /*
15 1
##@........##..
*/
#include <iostream>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <bitset>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
using namespace std;
typedef long long ll;
typedef pair<int,int> pint;
typedef vector<int> vint;
typedef vector<pint> vpint;
#define mp make_pair
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define REP(i,n) for (int i=(n)-1;i>=0;i--)
#define reps(i,f,n) for(int i=(f);i<(n);i++)
#define REPS(i,f,n) for (int i=(f)-1;i>=(n);i--)
int dx[4] = {1, 0, 0, -1};
int dy[4] = {0, 1, -1, 0};

char board[21][21];
int w, h;
int x, y;// ?????¨?????????

void slove(void){
	int cnt = 0;
	queue<pint> q;
	q.push(make_pair(y, x));//?????¨????????????queue?????\??????

	while(!q.empty()){
		pint now = q.front(); q.pop();
		rep(i, 4){
			int nowy = now.fi + dy[i];
			int nowx = now.se + dx[i];
			if (0 <= nowy && nowy <= h - 1 && 0 <= nowx && nowx <= w - 1){
				if (board[nowy][nowx] == '.'){
					cnt++;
					board[nowy][nowx] = 0;
					//printf("board %c\n", board[nowy][nowx]);
					q.push(make_pair(nowy, nowx));
				}
			}
		}
		/*
		rep(i, h){
			rep(j, w){
				printf("%c", board[i][j]);
			}
			printf("\n");
		}
		*/
	}
	printf("%d\n", cnt + 1);//?§?????????´????????\??????
	return;
}

void scan(int w, int h){
	rep(i, h){
		rep(j, w){
			cin >> board[i][j];
			if (board[i][j] == '@'){
				y = i; x = j;
			}
		}
		getchar();
	}

	slove();
	return;
}

int main(void){
	rep(i, 1000){
		cin >> w >> h;

		if (w == 0) break;
		scan(w, h);
	}
	return 0;
}

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);
    }
}