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 <cstdio>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <queue>
using namespace std;

struct state { int y, x, step; };
static const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};

int main() {
        int w, h;
        while (scanf("%d%d", &w, &h), w) {
                vector<string> s(h);
                for (int i = 0; i < h; i ++) cin >> s[i];
                int sy, sx;
                for (int i = 0; i < h; i ++) {
                        for (int j = 0; j < w; j ++) {
                                if (s[i][j] == '@') {
                                        sy = i, sx = j;
                                }
                        }
                }
                vector<vector<bool>> used(h, vector<bool>(w, false));
                queue<state> q;
                q.push({sy, sx, 0});
                used[sy][sx] = true;
                while (!q.empty()) {
                        state p = q.front(); q.pop();
                        for (int d = 0; d < 4; d ++) {
                                int xx = p.x + dx[d], yy = p.y + dy[d];
                                if (xx < 0 || xx >= w || yy < 0 || yy >= h) continue;
                                if (used[yy][xx] || s[yy][xx] == '#') continue;
                                used[yy][xx] = true;
                                q.push({yy, xx, p.step + 1});
                        }
                }
                int ans = 0;
                for (int i = 0; i < h; i ++) {
                        for (int j = 0; j < w; j ++) {
                                ans += used[i][j];
                        }
                }
                printf("%d\n", ans);
        }
        return 0;
}

C++: #include<stdio.h>
#include<stdlib.h>
#include<iostream>
#include<string.h>
#include<math.h>
#include<cstring>
using namespace std;

int main(){
	while(1){
		int i,j,k,w,h,a;
		char c[20][20];
		cin>>w>>h;
		if(w==0&&h==0)break;
		for(i=0;i<20;i++){
			for(j=0;j<20;j++){
				c[i][j]='\0';
			}
		}
		
		
		for(i=0;i<h;i++){
			cin>>c[i];
		}
		for(k=0;k<=w*h;k++){
			for(i=0;i<h;i++){
				for(j=0;j<w;j++){
					if(c[i][j]=='.'){
						if(c[i][j+1]=='@'||c[i][j-1]=='@'||c[i-1][j]=='@'||c[i+1][j]=='@'){
							c[i][j]='@';
						}
					}	
				}
			}
		}
		

		a=0;
		for(i=0;i<h;i++){
			for(j=0;j<w;j++){
				if(c[i][j]=='@')a++;	
			}
		}
		cout<<a<<endl;
	}
    return 0;
}

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

char map[20][20];

int W, H, start_x, start_y, ans = 0;

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

void dfs(int x, int y)
{
  ans++;
  if(map[x][y] != '#'){
    map[x][y] = '#';
    for(int i = 0; i < 4; i++){
      int next_x = x + dx[i], next_y = y + dy[i];
      if(next_x < W && next_x >= 0 && next_y < H && next_y >= 0){
	if(map[next_x][next_y] == '#')
	  continue;
	dfs(next_x, next_y);
      }else{
	continue;
      }
    }
  }
}

int main()
{
  while(1){
    cin >> W >> H;
    if(W == 0 && H == 0){
      break;
    }else{
      for(int i = 0; i < H; i++){
	for(int j = 0; j < W; j++){
	  cin >> map[j][i];
	  if(map[j][i] == '@'){
	    start_x = j;
	    start_y = i;
	  }
	}
      }
      dfs(start_x, start_y);
      cout << ans << endl;
      ans = 0;
    }
  }
}

C++: #include<iostream>
#include<string>
#include<queue>
#include<vector>
using namespace std;
#define frp(i,n) for((i)=0;(i)<(n);i++)
int vx[4]={0,1,0,-1};
int vy[4]={1,0,-1,0};
int H,W;
int check(int n1,int n2,int rednum,vector<int> red1,vector<int> red2){
  for(int i=0;i<rednum;i++){
    if(n1==red1[i]&&n2==red2[i]||n1<0||n2<0||n1>=H||n2>=W){
      return 0;
    }
  }
  return 1;
}
int main(){
  int i,j,now1,now2;
  while(cin>>W>>H,W+H!=0){
    int num=1;
    string str2;
    vector<string> str;
    vector<int> red1;
    vector<int> red2;
    frp(i,H){
      cin>>str2;
      str.push_back(str2);
    }
    frp(i,H){
      frp(j,W){
	if(str[i][j]=='#'){
	  red1.push_back(i);
	  red2.push_back(j);
	}
      }
    }
    queue<pair<int,int> > qii;
    frp(i,H){
      frp(j,W){
	if(str[i][j]=='@'){
	  qii.push(make_pair(i,j));
	  red1.push_back(i);
	  red2.push_back(j);
	}
      }
    }
    while(!qii.empty()){
      now1=qii.front().first;
      now2=qii.front().second;
      qii.pop();
      for(i=0;i<4;i++){
	if(check(now1+vx[i],now2+vy[i],red1.size(),red1,red2)==1){
	  red1.push_back(now1+vx[i]);
	  red2.push_back(now2+vy[i]);
	  qii.push(make_pair(now1+vx[i],now2+vy[i]));
	  num++;
	}
      }
    }
    cout<<num<<endl;
  }
  return 0;
}