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 <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < (n); i++)

using namespace std;

int d4x[] = {0, 1, 0, -1};
int d4y[] = {-1, 0, 1, 0};

int W, H;
int reachable(vector<string>& map, int x, int y){
  //cout << x << "," << y << endl;
  int res = 0;
  rep(i, 4){
    int sx = d4x[i] + x, sy = d4y[i] + y;
    if(0 <= sx && sx < W && 0 <= sy && sy < H){
      if(map[sy][sx] != '#'){
        map[sy][sx] = '#';
        res += reachable(map, sx, sy) + 1;
      }
    }
  }
  return res;
}

int main(){
  while(cin >> W >> H, W != 0){
    vector<string> map(H);
    rep(i, H) cin >> map[i];
    int gy, gx;
    rep(y, H) {
      rep(x, W) {
        if (map[y][x] == '@'){
          gy = y; gx = x;
          break;
        }
      }
    }
    //cout << "OK" << endl;
    cout << reachable(map, gx, gy) << endl;
  }
}

C++: #include <iostream>
#include <queue>

using namespace std;

struct locate{
  int x;
  int y;
};
 
char graph[20][20];
int cou = 0;
int H,W;

void bfs(locate z){
  queue<locate> Q;
  locate u;
  locate v;
  Q.push(z);
 

  while(!Q.empty()){

    u = Q.front();Q.pop();

    if(graph[u.y + 1][u.x] == '.' && graph[u.y + 1][u.x] != 'N'){
      graph[u.y + 1][u.x] = 'N';
      cou++;
      v.y = u.y + 1;
      v.x = u.x;
      Q.push(v);
    }

    if(graph[u.y][u.x + 1] == '.' && graph[u.y][u.x + 1] != 'N'){
      graph[u.y][u.x + 1] = 'N';
      cou++;
      v.x  = u.x + 1;
      v.y = u.y;
      Q.push(v);
    }

    if(graph[u.y - 1][u.x] == '.' && graph[u.y - 1][u.x] != 'N'){
      graph[u.y - 1][u.x] = 'N';
      cou++;
      v.x = u.x;
      v.y = u.y - 1;
      Q.push(v);
    }

    if(graph[u.y][u.x - 1] == '.' && graph[u.y][u.x - 1] != 'N'){
      graph[u.y][u.x - 1] = 'N';
      cou++;
      v.x = u.x - 1;
      v.y = u.y;
      Q.push(v);
    }
    
  }
  /*  cout << endl;
    for(int i = 0; i< H ;i++){
      for(int j = 0; j < W; j++){
	cout << graph[i][j];
      }
      cout << endl;
      }*/
}

int main(){

  int a1,a2;
  locate a;
  while(1){

    cin >> W >> H;

    if(W == 0 && H == 0) break; 


    for(int i = 0; i< H ;i++){
      for(int j = 0; j < W; j++){
	cin >> graph[i][j];
	if(graph[i][j] == '@'){
	  a1 = i;
	  a2 = j;
	}
      }
    }

    /*
      for(int i = 0; i< H ;i++){
      for(int j = 0; j < W; j++){
      cout << graph[i][j];
      }
      cout << endl;
      }
    */

    a.y = a1;
    a.x = a2;
    bfs(a);

    cout << cou+1 << endl;

    cou = 0;
    for(int i = 0;i < H ;i++){
      for(int j = 0;j < W; j++){
	graph[i][j] = 'N';
      }
    }
    cou = 0;
  }
  return 0;
}

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

#define rep(i,n) rep2(i,0,n)
#define rep2(i,m,n) for(int i=m;i<n;i++)

int w,h,ans,dy[]={-1,0,1,0},dx[]={0,-1,0,1};
char c[22][22];

void dfs(int y,int x){
	ans++;
	c[y][x]='#';
	rep(i,4){
		int ny=y+dy[i],nx=x+dx[i];
		if(c[ny][nx]=='.')dfs(ny,nx);
	}
}

int main(){
	while(cin>>w>>h&&w){
		ans=0;
		int y,x;
		rep2(i,1,h+1)rep2(j,1,w+1){
			cin>>c[i][j];
			if(c[i][j]=='@')y=i,x=j;
		}
		dfs(y,x);
		cout<<ans<<endl;
	}
}

C++: #include <iostream>
#include <map>
#include <set>
#include <vector>
#include <algorithm>
#include <utility>

#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <cstdint>

using namespace std;

typedef long long ll;

int w,h;
char m[22][22];

void rec(int x,int y) {
	if(m[x][y] == '.') {
		m[x][y] = '!';
		rec(x+1,y);
		rec(x-1,y);
		rec(x,y+1);
		rec(x,y-1);
	}
}

int main() {
	while(1) {
		cin >> w >> h;
		if(!(w | h)) break;
		string s;
		for(int i = 0;i < 22;i++)for(int j = 0;j < 22;j++) m[i][j] = '#';
		for(int y = 1;y <= h;y++) {
			cin >> s;
			for(int x = 1;x <= w;x++) {
				m[x][y] = s[x-1];
			}
		}
		for(int x = 0;x < 22;x++)for(int y = 0;y < 22;y++) 
			if(m[x][y] == '@') {
				m[x][y] = '.';
				rec(x,y);
			}
		int cnt = 0;
		for(int x = 0;x < 22;x++)for(int y = 0;y < 22;y++) 
			if(m[x][y] == '!') cnt++;
		cout << cnt << endl;
	}
}