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>
using namespace std;
int w,h;
char ma[100][100];
int sx,sy; 
int dx[] = {0,0,1,-1},dy[] = {1,-1,0,0};
int cnt;
int dfs(int px,int py){
  ma[py][px] = '#';
  cnt++;
  for(int i = 0 ; i < 4 ; i++){
    int nx = px + dx[i],ny = py + dy[i];
    if(0 <= nx && nx < w && 0 <= ny && ny < h &&
       ma[ny][nx] != '#'){
      if(ma[ny][nx] == '.')dfs(nx,ny);
    }
  }
  return cnt;
}

int main(){
  while( cin >> w >> h, w + h){
    cnt = 0;
    for(int i = 0 ; i < h ; i++){
      for(int j = 0 ; j < w ; j++){
	cin >> ma[i][j];
	if(ma[i][j] == '@'){sx = j;sy = i;}
      }
    }
    cout << dfs(sx,sy) << endl;
  }
}

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

int moved = 0;
int w, h;
char myMap[20][20];

bool safe(int x, int y){
    return (x < h && y < w) && (x >= 0 && y >= 0);
}

void move(int x, int y){
    myMap[x][y] = '#';
    moved++;
    if (myMap[x+1][y] == '.' && safe(x+1, y)) move(x+1, y);
    if (myMap[x-1][y] == '.' && safe(x-1, y)) move(x-1, y);
    if (myMap[x][y+1] == '.' && safe(x, y+1)) move(x, y+1);
    if (myMap[x][y-1] == '.' && safe(x, y-1)) move(x, y-1);
    return;
}

int main(){
    while (1){
	cin >> w >> h;
	if (w == 0 && h == 0) break;

	moved = 0;
	int initX, initY;

	for (int i = 0; i < h; i++){
	    for (int j = 0; j < w; j++){
		cin >> myMap[i][j];
		if (myMap[i][j] == '@'){
		    initX = i;
		    initY = j;
		}
	    }
	}
	
	move(initX, initY);
	
	cout << moved << endl;
    }
}

C++: #if 0
????????£?????????????´?1??????
pdf??????????????????????????????????????????????????§?????????????????????
???????????§??????????????§?????????????????°?????????????????£??¨????????§?????????
??¢?????\????????????????????????????????£??¨??????????????????
#endif

#include <iostream>
#include <queue>
using namespace std;
class pos
{
	public:
	int x;
	int y;
	pos(int,int);
};

pos::pos(int a,int b)
{
	x=a;y=b;
}

int main()
{
	int w,h;
	int g[21][21];
/*	for(int i=0;i<21;i++)
		for(int j=0;j<21;j++)
			g[i][j]=-1;
*/	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++)
				g[i][j]=-1;
		int psx,psy;
		bool visited[21][21];
		for(int i=0;i<h;i++)
			for(int j=0;j<w;j++)
			{
				char s;
				cin>>s;
				switch(s)
				{
					case '.':
						g[i][j]=0;break;
					case '#':
						g[i][j]=1;break;
					case '@':
						g[i][j]=2;psx=i;psy=j;break;
				}
				visited[i][j]=false;
			}

/*		for(int i=0;i<h;i++)
		{
			for(int j=0;j<w;j++)
				cout<<g[i][j];
			cout<<endl;
		}
*/
		int num=0;
		queue<pos> Q;
		Q.push(pos(psx,psy));
		num++;
		visited[psx][psy]=true;
		while(!Q.empty())
		{
			pos now=Q.front();
			Q.pop();
	
			for(int i=-1;i<=1;i+=2)
			{
				int newx=now.x+i;
				int newy=now.y;
				if(newx>=0&&newx<h&&newy>=0&&newy<w&&g[newx][newy]==0&&visited[newx][newy]==false)
				{
					Q.push(pos(newx,newy));		
					num++;
					visited[newx][newy]=true;
				}
			}
			for(int i=-1;i<=1;i+=2)
			{                
				int newx=now.x;
				int newy=now.y+i;
				if(newx>=0&&newx<h&&newy>=0&&newy<w&&g[newx][newy]==0&&visited[newx][newy]==false)
				{
					Q.push(pos(newx,newy));
					num++;
					visited[newx][newy]=true;
				}
			}

		}
		cout<<num<<endl;
								


	}
}

C++: #include<iostream>
#include<string>
#include<stack>
using namespace std;
int w,h;
string map[40];
int vec[4][2]={{0,1},{0,-1},{-1,0},{1,0}};

struct pos{
	pos(int X,int Y){
		x=X,y=Y;
	}
	int x,y;
};

int move(int x,int y){
	int ans=0;
	map[x][y]='#';
	stack<pos> bfs;
	bfs.push(pos(x,y));
	while(!bfs.empty()){
		pos n=bfs.top();bfs.pop();
		ans++;
		for(int i=0;i<4;i++){
			if((n.x+vec[i][0]>=0)&&(n.y+vec[i][1]>=0)&&(n.x+vec[i][0]<h)&&(n.y+vec[i][1]<w)){
				if(map[n.x+vec[i][0]][n.y+vec[i][1]]=='.'){
					map[n.x+vec[i][0]][n.y+vec[i][1]]='#';
					bfs.push(pos(n.x+vec[i][0],n.y+vec[i][1]));
				}
			}
		}
		
	}
	
	
	return ans;
}

int main(){
	while(cin>>w>>h,w){
		int fx,fy;
		for(int i=0;i<h;i++){
			cin>>map[i];
		}
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(map[i][j]=='@'){
					fx=i;
					fy=j;
					break;
				}
			}
		}
		int ans=move(fx,fy);
		cout<<ans<<endl;
		
	}
}