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++: //C++14 (Clang 3.8.0)

#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <string>
#define rp(i,a,b) for(int (i)=(int)(a);i<(int)(b);++i)

typedef long long ll;

int mx[]={-1,1,0,0};
int my[]={0,0,-1,1};
using namespace std;
int n=0;
void dfs(int startx,int starty,vector<vector<bool> > &map){
    for(int i=0;i<4;i++){
        int x=startx+mx[i],y=starty+my[i];
        if(x>=0 and y>=0 and y<map.size() and x<map[0].size() and map[y][x]==true){
        map[y][x]=false;
        n++;
        dfs(x,y,map);}
    }
    
}

int main(){
    int x,y;
    while(cin>>x>>y,x|y){
        int startx,starty;
        vector<vector<bool> > map;

        if(x==0 and y==0) return 0;
        for(int i=0;i<y;i++){
            vector<bool> tmp;
            string s;
            cin>>s;
            for(int j=0;j<x;j++){

                if(s[j]=='@') {startx=j;starty=i;tmp.push_back(true);}
                else if(s[j]=='.') tmp.push_back(true);
                else tmp.push_back(false);    
            }
            map.push_back((tmp));
        }
        n=0;
        dfs(startx,starty,map);
        cout<<n<<endl;
        
    }

    return 0;
};

C++: #include<iostream>
#include<iomanip>
#include<algorithm>
#include<bitset>
#include<cctype>
#include<cmath>
#include<cstdio>
#include<cstring>
#include<functional>
#include<limits>
#include<list>
#include<map>
#include<set>
#include<stack>
#include<string>
#include<sstream>
#include<queue>
#include<vector>
using namespace std;

#define MOD 1000000007
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL

#define MAX 20
#define NIL -1

static char M[MAX][MAX];
static int color[MAX][MAX];
int W, H;
int cnt;

void dfs(int h, int w) {
	cnt++;
	color[h][w] = 0;
	if (h != H - 1 && M[h + 1][w] == '.'&&color[h + 1][w] == NIL)
		dfs(h + 1, w);
	if (h != 0 && M[h - 1][w] == '.'&&color[h - 1][w] == NIL)
		dfs(h - 1, w);
	if (w != W - 1 && M[h][w + 1] == '.'&&color[h][w + 1] == NIL)
		dfs(h, w + 1);
	if (w != 0 && M[h][w - 1] == '.'&&color[h][w - 1] == NIL)
		dfs(h, w - 1);
}

struct { int x; int y; } s;

int main() {
	while (cin >> W >> H&&H&&W) {
		for (int h = 0; h < H; h++) {
			for (int w = 0; w < W; w++) {
				cin >> M[h][w];
				if (M[h][w] == '@')s.x = w, s.y = h;
			}
		}
		fill(color[0], color[MAX], NIL);
		cnt = 0;
		dfs(s.y, s.x);
		cout << cnt << endl;
	}
	return 0;
}

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

int dfs(int x,int y);
int w,h;
int dx[4]={0,1,0,-1};
int dy[4]={-1,0,1,0};
int sum=1;
char maps[21][21];

int main(){
  int sx,sy;
  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 >> maps[i][j];
	if(maps[i][j]=='@'){
	  sy=i;
	  sx=j;
	}
      }
    }
    
    dfs(sx,sy);
    cout << sum << endl;
    sum=1;
  }
  return(0);
}

int dfs(int x,int y){
  int nx,ny;
  for(int i=0;i<4;i++){
    nx=x+dx[i];
    ny=y+dy[i];
    maps[y][x]='#';
    if(0<=nx && nx<w && 0<=ny && ny<h && maps[ny][nx]!='#'){
      dfs(nx,ny);
      sum++;
    } 
  }
  return 0;
}

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

char c[25][25];
void dfs(int x,int y,int w,int h) {
	c[x][y] = '!';

	for (int i = -1;i <= 1;i++) {
		int ax = x + i;
		if (ax >= 0 && ax < h&&c[ax][y] == '.')dfs(ax, y, w, h);
	}
	for (int i = -1;i<= 1;i++) {
		int ay = y + i;
		if (ay >= 0 && ay < w&&c[x][ay] == '.')dfs(x, ay, w, h);
	}
}

int main() {
	int w, h;
	while (cin >> w >> h,w,h) {
		for (int i = 0;i < h;i++) {
			for (int j = 0;j < w;j++)cin >> c[i][j];
		}

		for (int i = 0;i < h;i++) {
			for (int j = 0;j < w;j++) {
				if (c[i][j] == '@')dfs(i,j,w,h);
			}
		}
		int al = 0;
		for (int i = 0;i < h;i++) {
			for (int j = 0;j < w;j++) {
				if (c[i][j] == '!')al++;
			}
		}
		cout << al << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
int nx[]={0,1,0,-1};
int ny[]={-1,0,1,0};
int w,h;
char mp[101][101];
int dfs(int y,int x);
int a,b;
int main()
{
  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 >> mp[i][j];
	if(mp[i][j]=='@'){
	  a=i;
	  b=j;
	}
      }
    }
    cout << dfs(a,b) << endl;
  }
  return(0);
}

int dfs(int y,int x){
  if(0>y||0>x||y>=h||x>=w){
    return(0);
  }
  if(mp[y][x]=='#'){
    return(0);
  }
  mp[y][x]='#';
  int ans=0;
  for(int i=0;i<4;i++){
    ans=ans+dfs(y+ny[i],x+nx[i]);
  }
  return ans+1;
}