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++: #if 0
#endif

#include <iostream>
#include <string>
#include <set>
using namespace std;

struct pos
{
    int x;
    int y;
};
bool state[25][25];
int result = 1, width, height;

void tansaku(pos cur)
{
    // 右へ
    if (cur.x < width - 1 && state[cur.x + 1][cur.y])
    {
        state[cur.x + 1][cur.y] = false;
        result++;
        tansaku({.x = cur.x + 1, .y = cur.y});
    }
    // 左へ
    if (0 < cur.x && state[cur.x - 1][cur.y])
    {
        state[cur.x - 1][cur.y] = false;
        result++;
        tansaku({.x = cur.x - 1, .y = cur.y});
    }
    // 上へ
    if (0 < cur.y && state[cur.x][cur.y - 1])
    {
        state[cur.x][cur.y - 1] = false;
        result++;
        tansaku({.x = cur.x, .y = cur.y - 1});
    }
    // 下へ
    if (cur.y < height - 1 && state[cur.x][cur.y + 1])
    {
        state[cur.x][cur.y + 1] = false;
        result++;
        tansaku({.x = cur.x, .y = cur.y + 1});
    }
}

int main()
{
    while (cin >> width >> height && (width != 0 || height != 0))
    {
        result = 1;
        pos cur;

        for (int y = 0; y < height; y++)
        {
            string word;
            cin >> word;
            for (int x = 0; x < width; x++)
            {
                char c = word[x];
                if ('.' == c)
                {
                    state[x][y] = true;
                }
                else if ('@' == c)
                {
                    state[x][y] = false;
                    cur.x = x;
                    cur.y = y;
                }
                else if ('#' == c)
                {
                    state[x][y] = false;
                }
            }
        }
        tansaku(cur);
        cout << result << endl;
    }
}

C++: #include<iostream>
#include<vector>
#include<string>
#include<queue>
#include<map>
using namespace std;
typedef pair<int,int> P;
int dx[]={0,0,1,-1},dy[]={1,-1,0,0};
int h,w;
vector<string> s;
bool inf(int y,int x){
	return (y>=0&&y<h&&x>=0&&x<w)?true:false;
}
int bfs(int sy,int sx){
	int ret=0;
	queue<P> que;
	que.push(P(sy,sx));
	vector<vector<bool> > used(h,vector<bool>(w,false));
	while(!que.empty()){
		P t=que.front();
		que.pop();
		int y=t.first,x=t.second;
		if(used[y][x]){
			continue;
		}
		used[y][x]=true;
		ret++;
		for(int i=0;i<4;i++){
			int ny=dy[i]+y,nx=dx[i]+x;
			if(inf(ny,nx)&&!used[ny][nx]&&s[ny][nx]!='#'){
				que.push(P(ny,nx));
			}
		}
	}
	return ret;
}
int main(){
	
	
	while(cin>>w>>h,h||w){
		s=vector<string>(h);
		for(int i=0;i<h;i++){
			cin>>s[i];
		}
		int ans=0;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(s[i][j]=='@'){
					ans=bfs(i,j);
				}
			}
		}
		cout<<ans<<endl;
	}
	return 0;
}

C++: #include <iostream>

using namespace std;


void search(int h, int w, int tairu[22][22], int check[22][22]){
    if(check[h][w]==1)return;
    if(tairu[h][w]==0)return;
    
    check[h][w]=1;
    search(h+1,w,tairu,check);
    search(h-1,w,tairu,check);
    search(h,w+1,tairu,check);
    search(h,w-1,tairu,check);
}






int main()
{

    int H,W;
    while(cin>>W>>H){
        if(W==0&&H==0)break;
        int tairu[22][22]={0};
        int start_w, start_h;
        for(int i=1; i<=H; i++){
            for(int j=1; j<=W; j++){
                char input;
                cin>>input;
                if(input=='.')tairu[i][j]=1;
                if(input=='@'){
                    tairu[i][j]=1;
                    start_w=j;
                    start_h=i;
                }
            }
        }
        //
        
        int check[22][22]={0};
        search(start_h, start_w, tairu, check);
        int ans=0;
        for(int i=1; i<=H; i++){
            for(int j=1; j<=W; j++){
                ans+=check[i][j];
            }
        }
        cout<<ans<<endl;
        
        
        
    }
}