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

void dfs(int x,int y);
int w,h;
 
int count;
bool flag;
char maze[21][21];
bool reached[501][501];
 
int main(){
        int w,h;

    while(cin>>w>>h){
        
            if(w==0&&h==0){
                break;
            }

            for(int i=0;i<h;i++){
                for(int j=0;j<w;j++){
                    cin>>maze[i][j];
                }
            }

            //  for(int i=0;i<h;i++){
            //     for(int j=0;j<w;j++){
            //         cout<<maze[i][j];
            //     }
            //     cout<<endl;
            // }

            // cout<<endl

            for(int i=0;i<h;i++){
                for(int j=0;j<w;j++){
                    if(maze[i][j]=='@'){
                        dfs(j,i);
                    }
                }
            }

            // for(int i=0;i<h;i++){
            //     for(int j=0;j<w;j++){
            //         cout<<maze[i][j];
            //     }
            //     cout<<endl;
            // }

            count=0;
            for(int i=0;i<h;i++){
                for(int j=0;j<w;j++){
                    if(maze[i][j]=='!')count++;
                }
            }

            cout<<count<<endl;




        }
    
    // int s_x=0;
    // int s_y=0;
    // int g_x=0;
    // int g_y=0;
 
    // cin>>w>>h;
 
 
    // for(int i=0;i<w;i++){
    //     for(int j=0;j<h;j++){
    //         if(maze[i][j]=='s'){
    //             s_x=i;
    //             s_y=j;
    //             dfs(i,j);
    //         }
    //     }
    // }

    // for(int i=0;i<h;i++){
    //     for(int j=0;j<w;j++){
    //         if(reached[i][j])count++;
    //     }
    // }
    

 
    return 0;
}
 
 
void dfs(int x,int y){
    maze[y][x]='!';

    for(int i=0;i<4;i++){
        int mx=x+dx[i];
        int my=y+dy[i];
        if(maze[my][mx]=='.'){
            dfs(mx,my);
        }
        if(mx<0||w<=mx||my<0||h<=my){
            continue;
        }//??????????????´?????´???->???????????????
        
    } 
}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;

int cnt;
int w, h;
int dx[] = {0, 1, 0, -1};
int dy[] = {1, 0, -1, 0};
char tile[200][200];

void dfs(int x, int y);

int main()
{
	int x, y;

	while (true){
		scanf("%d %d", &w, &h);

		if (w == 0){
			break;
		}

		for (int i = 0; i < h; i++){
			scanf("%s", tile[i]);

			for (int j = 0; j < w; j++){
				if (tile[i][j] == '@'){
					y = i;
					x = j;
				}
			}
		}

		cnt = 0;
		dfs(x, y);

		printf("%d\n", cnt);
	}

	return (0);
}

void dfs(int x, int y)
{
	tile[y][x] = '#';
	cnt++;

	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 && tile[ny][nx] == '.'){
			dfs(nx, ny);
		}
	}
}

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

int explorer(vector<string> *map,int i, int j){
	if(map->at(i)[j]=='#') return 0;
	
	int cnt=1;
	vector<int>vi;
    vi.reserve(4);
	vector<int>vj;
    vj.reserve(4);
	
	map->at(i)[j]='#';
	
	if(i!=0&&map->at(i-1)[j]=='.'){
		vi.push_back(i-1);
		vj.push_back(j);
	}if(i+1!=map->size()&&map->at(i+1)[j]=='.'){
		vi.push_back(i+1);
		vj.push_back(j);		
	}if(j!=0&&map->at(i)[j-1]=='.'){
		vi.push_back(i);
		vj.push_back(j-1);		
	}if(j+1!=map->at(i).size()&&map->at(i)[j+1]=='.'){
		vi.push_back(i);
		vj.push_back(j+1);		
	}

	while(!vi.empty()){
    cout<<i<<j<<endl;
		cnt += explorer(map,vi.back(),vj.back());
		vi.pop_back();
		vj.pop_back();
	}

	return cnt;
}

int main() {
	int w,h;
	
	while(cin >> w >> h,w){
	vector<string> map;
	int psx=0,psy=0;
	map.resize(h);
	for(int i=0;i<h;i++){
		cin >> map[i];
		
		if(psx==0){
			if(map[i].find('@')!=-1){
				psx=map[i].find('@');
				psy=i;
			}
		}
	}
	
	cout << explorer(&map,psy,psx) << endl;
	}
	return 0;
}