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>
#include <functional>
#include<algorithm>
#include <cmath>
#include <string>
#include <map>
using namespace std;

int W,H;
char a[30][30];
bool re[30][30];

void se(int x,int y,int *c){
	if(x<0||W<=x||y<0||H<=y||a[y][x]=='#')
		return;
	if(re[y][x])
		return;
	
	re[y][x]=true;
		*c=*c+1;
	
	se(x+1,y,c);
	se(x,y+1,c);
	se(x-1,y,c);
	se(x,y-1,c);
}

int main(){
	int x,y;
	int x1,y1;
	while(cin>>W>>H){
		if(W!=0&&H!=0){
			int k=0;
		for(int i=0;i<H;i++){
			for(int j=0;j<W;j++){
			cin>>a[i][j];
				if(a[i][j]=='@'){
				x1=j;
				y1=i;}
			}
		}
		se(x1,y1,&k);
		cout<<k<<endl;
		}
		else
			break;
	}

	
}

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

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

char field[20][20];
bool flag[20][20];
int W, H, ans;

void dfs(int x, int y) {
  //field[x][y] = '.';
  //cout << x << ' ' << y << endl;
  //cout << flag[x][y] << endl;
  //cout << ans << endl;
  for(int i = 0 ; i < 4 ; i++) {
    int nx = x + dx[i], ny = y + dy[i];
    if(0 <= nx && nx < H && 0 <= ny && ny < W && field[nx][ny] == '.' &&
       flag[nx][ny] == false) {
      ans++; flag[nx][ny] = true; dfs(nx, ny);
      //cout << "OK" << endl;
    }
    //else dfs(nx, ny);
  }
}

int main(void) {
  int x, y;
  int res;
  
  while(cin >> W >> H, W != 0 && H != 0) {
    memset(flag, false, sizeof(flag));
    ans = 1;
    for(int i = 0 ; i < H ; i++) {
      for(int j = 0 ; j < W ; j++) {
	cin >> field[i][j];
	if(field[i][j] == '@') x = i, y = j;
      }
    }
    /*for(int i = 0 ; i < H ; i++) {
      for(int j = 0 ; j < W ; j++) {
	cout << field[i][j];
      }
      cout << endl;
      }*/
    //printf("@x = %d, @y = %d\n", x, y);
    dfs(x, y);
    cout << ans << endl;
  }
  return 0;
}

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

#define REP(i, n) for(int i = 0;i < (n);i++)
#define FOR(i, a, b) for(int i = (a);i < (b);i++)
#define all(v) v.begin(),v.end()

using int64 = long long;

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

int main(void){
	int H, W;
	while(cin >> W >> H && H && W){
		vector<string> f(H+2, string(W+2, '#'));
		int sy, sx;
		REP(i, H){
			cin >> f[i+1];
			REP(j, W)
				if(f[i+1][j] == '@'){
					sy = i+1;
					sx = j+1;
				}
			f[i+1] = '#'+f[i+1]+'#';
		}
		bool visited[30][30] = {};
		int cnt = 0;
		
		function<void(int, int)> dfs = [&](int y, int x){
			visited[y][x] = 1;
			cnt++;
			REP(i, 4){
				int yy = y+dy[i], xx = x+dx[i];
				if(f[yy][xx] != '#' && !visited[yy][xx]){
					dfs(yy, xx);
				}
			}
		};
		dfs(sy, sx);
		cout << cnt << endl;
	}
}

C++: #include<iostream>
#include<algorithm>
#include<string>
#include<cstring>
using namespace std;
char mas[30][30];
int W,H,sum;
int dx[4] = {0,-1,0,1};
int dy[4] = {1,0,-1,0};
void dfs(int x, int y);
int main(){
  int sx,sy;
  while(1){
    sum = 0;
    cin >> W >> H;
    if(W == 0 && H == 0)break;
    for(int i = 0; i < H; i++){
      for(int j = 0; j < W; j++){
	cin >> mas[i][j];
	if(mas[i][j] == '@'){
	  sx = j;
	  sy = i;
	}
      }
    }
    dfs(sx,sy);
    cout << sum << endl;
  }
}
void dfs(int x, int y){
  sum++;
  mas[y][x] = '#';
  for(int i = 0; i < 4; i++){
    int nx = x + dx[i];
    int ny = y + dy[i];
    if(nx >= 0 && nx < W && ny >= 0 && ny < H && mas[ny][nx] != '#'){
      dfs(nx,ny);
    }
  }
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <numeric>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <utility>
#include <sstream>
#include <complex>

using namespace std;

#define FOR(i,a,b) for(long long i=(a);i<(b);i++)
#define REP(i,N) for(long long i=0;i<(N);i++)
#define ALL(s) (s).begin(),(s).end()
#define fi first
#define se second

#define PI acos(-1.0)
#define INF 1000000007
#define EPS 1e-10

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<double, double> PD;
typedef pair<string, ll> PS;
typedef vector<ll> V;
typedef pair<P, char> PC;

int w, h;
char field[21][21];
bool used[21][21];

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

void solve(int y,int x){

	REP(i, 4){
		int ny = y + dy[i], nx = x + dx[i];
		if (ny < h&&ny >= 0 && nx < w&&nx >= 0){
			if (field[ny][nx] == '.'&&used[ny][nx] == 0){
				used[ny][nx] = 1;
				solve(ny, nx);
			}
		}
	}

	return;
}

int main(){
	while (cin >> w >> h&&w + h){
		REP(i, 21)REP(j, 21)used[i][j] = 0;
		REP(i, h){
			REP(j, w){
				cin >> field[i][j];
				if (field[i][j] == '@'){
					sy = i, sx = j;
				}
			}
		}

		solve(sy, sx);
		used[sy][sx] = 1;

		int ans = 0;
		REP(i, h){
			REP(j, w){
				if (used[i][j])ans++;
			}
		}
		cout << ans << endl;
	}
}