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++: #define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
//bool c[22][22] = {};
int cnt,w, h, p[22][22] = {};

int main() {
	int j, f,cx,cy;
	char in[21];
	while (1) {
/*		for (int i = 0; i < 484; i++) {
			c[i / 22][i % 22] = p[i / 22][i % 22] = 0;
		}
*/		cnt = 0;
		f = 1;
		scanf("%d %d", &w, &h);
		if (w ==0&& h == 0)
			break;
		scanf("\n", &j);
		for (int i = 0; i < h; i++) {
			scanf("%s", &in);
			for (j = 0; j < w; j++) {
				switch (in[j]) {
				case '#':
//					c[j+1][i+1] = 0;
					p[j+1][i+1] = -1;
					break;
				case '@':
					cx = j;
					cy = i;
					p[j+1][i+1] = 1;
//				case '.':
//					c[j+1][i+1] = 1;
				}
			}
		}

		/*for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++)
				printf("%3d", c[j][i]);
			printf("\n");
		}*/

		while (f == 1) {
			f = 0;
			for (int i = 0; i < h; i++) {
				for (int j = 0; j < w; j++)
					printf("%3d", p[j+1][i+1]);
				printf("\n");
			}
			printf("\n");
			//if (w != 1 && h != 1) {
				for (int i = 0; i < w*h; i++) {
					if (p[i / h+1][i%h+1] == 0 && (p[i / h + 2][i % h+1] == 1 || p[i / h][i % h+1] == 1 || p[i / h+1][i % h + 2] == 1 || p[i / h+1][i % h] == 1)) {
						p[i / h+1][i%h+1] = 1;
						f = 1;
					}
				}
			//}
			/*if (w == 1 && h != 1) {
				for (int i = 0; i < h; i++) {
					if (p[1][i+1] == 0 && (p[1][i + 2] == 1 || p[0][i] == 1)) {
						p[1][i+1] = 1;
						f = 1;
						printf("a");
					}
				}
			}

			if (w != 1 && h == 1) {
				for (int i = 0; i < w; i++) {
					if (p[i+1][1] == 0 && (p[i + 2][1] == 1 || p[i][1] == 1)) {
						p[i+1][1] = 1;
						f = 1;
					}
				}
			}*/
		}

		for (int i = 0; i < w*h; i++) {
			if (p[i / h+1][i%h+1] == 1)
				cnt++;
		}
		printf("%d\n", cnt);
	}
	return 0;
}

C++: #include <cstdio>

using namespace std;

#define rep(i,x) for(int i=0;i<(int)(x);++i)

int dx[] = {-1, 1, 0, 0}, dy[] = {0, 0, -1, 1};

char mp[20][22];
int H, W;

void dfs(int y, int x)
{
    if (y < 0 || y >= H || x < 0 || x >= W || mp[y][x] == '#' || mp[y][x] == ' ') return;

    mp[y][x] = ' ';

    rep(i, 4) dfs(y + dy[i], x + dx[i]);
}

int main()
{
    while (scanf("%d %d", &W, &H), H || W) {
        int sy, sx;

        rep(i, H) rep(j, W) {
            scanf("\n%c", &mp[i][j]);
            if (mp[i][j] == '@') sy = i, sx = j, mp[i][j] = '.';
        }

        dfs(sy, sx);

        int cnt = 0;

        rep(i, H) rep(j, W) if (mp[i][j] == ' ') cnt++;

        printf("%d\n", cnt);
    }
}

C++: #include <bits/stdc++.h>
using namespace std;

#define int				long long
#define FOR( i, m, n )  for( int (i) = (m); (i) < (n); (i)++ )
#define REP( i, n )     FOR( i, 0, n )
#define REPR( i, m )	for( int (i) = (m); (i) >= 0; (i)-- )
#define REPONE( i, n )	FOR( i, 1, n + 1 )
#define ALL( a )        (a).begin(), (a).end()
#define MP				make_pair
#define X				first.first
#define Y				first.second

typedef pair<int, int> P;

template<class T>bool chmax( T& a, const T& b ) { if( a < b ) { a = b; return 1; } return 0; }
template<class T>bool chmin( T& a, const T& b ) { if( a > b ) { a = b; return 1; } return 0; }

const int INF = 1e9;
const int MOD = 1e9 + 7;

int dx[4] = { 1, 0, -1, 0 };
int dy[4] = { 0, 1, 0, -1 };

char tile[21][21];
int W, H;
int ans;

void dfs( int x, int y ) {
	REP( i, 4 ) {
		int nx = x + dx[i];
		int ny = y + dy[i];

		if( nx >= 0 && nx < W && ny >= 0 && ny < H && tile[nx][ny] == '.' ) {
			tile[nx][ny] = '#';
			ans++;
			dfs( nx, ny );
		}
	}
}

signed main() {
	while( cin >> W >> H, W, H ) {
		
		int sx, sy;
		REP( i, H ) {
			REP( j, W ) {
				cin >> tile[j][i];
				if( tile[j][i] == '@' ) {
					sx = j;
					sy = i;
					tile[j][i] = '#';
				}
			}
		}

		
		ans = 1;
		dfs( sx, sy );
		cout << ans << endl;
		
	}
}

C++: #include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
const ll MOD=1000000007;
const ll INF=1000000000LL;
const int MAX=100001;
const double EPS=1e-10;
struct edge{int to,cost;};
typedef pair<int,int> P;
int w,h;
char c[21][21];
bool visited[21][21];
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};
bool field(int x,int y){
    if(x>=0&&y>=0&&x<w&&y<h&&(!visited[y][x])&&(c[y][x]=='.')){
        return true;
    }
    return false;
}
void dfs(int x,int y){
    visited[y][x]=true;
    for(int i=0;i<4;i++){
        int nx=x+dx[i];
        int ny=y+dy[i];
        if(field(nx,ny)){
            dfs(nx,ny);
        }
    }
    return;
}
int main(){
    while(cin>>w>>h,w+h){
        int sx,sy;
        for(int i=0;i<h;i++){
            cin>>c[i];
            for(int j=0;j<w;j++){
                if(c[i][j]=='@'){
                    sx=j;
                    sy=i;
                }
            }
        }
        int ans=0;
        dfs(sx,sy);
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                if(visited[i][j]){
                    ans++;
                    visited[i][j]=false;
                }
            }
        }
        cout<<ans<<endl;
    }
}