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 map[20][20];


int solve(int x,int y){
	int count = 0;
	if(map[y][x] == 0){
		count++;
		map[y][x] = 1;
		if(x-1 >= 0){
			count += solve(x-1,y);
		}
		if(y-1 >= 0){
			count += solve(x,y-1);
		}
		if(20 > x+1){
			count += solve(x+1,y);
		}
		if(20 > y+1){
			count += solve(x,y+1);
		}
	}
	return count;
}

int main(){
	int w,h;
	int startX,startY;

	while(true){
		cin >> w >> h;

		if(w == 0 && h == 0){
			break;
		}

		for(int i=0;i<20;i++){
			for(int j=0;j<20;j++){
				map[i][j] = 1;
			}
		}

		char c;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				cin >> c;
				switch(c){
				case '.':
					map[i][j] = 0;
					break;
				case '#':
					map[i][j] = 1;
					break;
				case '@':
					map[i][j] = 0;
					startX = j;
					startY = i;
					break;
				}
			}
		}

		cout << solve(startX,startY) << endl;
	}
}

C++: #include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <string>
#include <vector>
#include <list>
#include <set>
#include <queue>
#include <map>

using namespace std;

#define F first
#define S second
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define INT(a) int((a)+1e-9)
#define SUPMaxX 22
#define SUPMaxY 22

typedef pair<int,int> Pii;

int DX[4]={1,0,-1,0};
int DY[4]={0,1,0,-1};

int maxX,maxY;
char tizu[SUPMaxX][SUPMaxY];
int re;
int initX,initY;

int dfs(int px, int py){
	re++;
	FOR(i,0,4){int qx=px+DX[i]; int qy=py+DY[i];
		if(tizu[qx][qy]=='.'){
			tizu[qx][qy]='@';
			dfs(qx,qy);
		}
	}
}

int main(){
	while(true){
		cin >> maxY >> maxX;			if(!maxX) return 0;
		FOR(x,0,maxX+2) FOR(y,0,maxY+2) tizu[x][y]='#';
		FOR(x,1,maxX+1) FOR(y,1,maxY+1){
			cin>>tizu[x][y];
			if(tizu[x][y]=='@'){initX=x; initY=y;}
		}
		re=0;
		dfs(initX,initY);
		cout<<re<<endl;
	}
}

C++: #include <iostream>
using namespace std;
int w,h;
char a[21][21];
int dfs(int i,int j){
    if(i==h||j==w||i<0||j<0)return 0;
    if(a[i][j]=='#')return 0;
    int s=1;
    a[i][j]='#';
    s+=dfs(i+1,j);
    s+=dfs(i-1,j);
    s+=dfs(i,j+1);
    s+=dfs(i,j-1);
    return s;
}
int main(){
    int i,j;
    while(cin>>w>>h,w!=0&&h!=0){
    for(i=0;i<h;i++)cin>>a[i];
    for(i=0;i<h;i++)for(j=0;j<w;j++)if(a[i][j]=='@')cout<<dfs(i,j)<<endl;
}}

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

int W, H;
char b[20][20];
int ans;
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};

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

int main() {
	while (cin >> W >> H , W) {
		for (int y = 0; y < H; y++) {
			for (int x = 0; x < W; x++) {
				cin >> b[y][x];
			}
		}

		ans = 0;

		for (int y = 0; y < H; y++) {
			for (int x = 0; x < W; x++) {
				if (b[y][x] == '@') {
					dfs(x, y);
				}
			}
		}
		cout << ans << endl;
	}
}

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=0,sx,sy;//finded
string m[20],t,s;

void find(int x,int y){
	if(x>=W || x<0 || y>=H || y<0)return;
	t = m[x][y];
	if(t=="#")return;
	if(f[x][y]!=0)return;
	f[x][y] = 1;
	ans++;
	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;
		rep(i,H){
			cin>>s;
			rep(j,W){
				m[j][i]=s[j];
				t=m[j][i];
				if(t=="@"){
					sx=j;
					sy=i;
				}
			}
		}
		find(sx,sy);
		cout<<ans<<endl;
	}
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int w, h, cnt;
int vx[] = {0, 1, 0, -1};
int vy[] = {1, 0, -1, 0};
char field[20][20];
void solve(int x, int y){
    ++cnt;
    field[y][x] = '#';
    for(int i = 0; i < 4; ++i){
        int nx = x + vx[i];
        int ny = y + vy[i];
        if(nx >= 0 && nx < w && ny >= 0 && ny < h && field[ny][nx] == '.')
            solve(nx, ny);
    }
}
int main(){
    int sx, sy;
    while(cin >> w >> h){

        if(!(w || h)) break;
        for(int i = 0; i < h; ++i){
            for(int j = 0; j < w; ++j){
                cin >> field[i][j];
                if(field[i][j] == '@'){
                    sx = j;
                    sy = i;
                }
            }
        }
        cnt = 0;

        solve(sx, sy);
        cout << cnt << endl;
    }
}