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<algorithm>
#include<functional>
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<sstream>
#include<iostream>
#include<iomanip>
#include<vector>
#include<list>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<bitset>

#define all(c) (c).begin(), (c).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define pb(e) push_back(e)
#define mp(a, b) make_pair(a, b)
#define fr first
#define sc second

typedef unsigned long long ull;
typedef long long ll;

const int INF=100000000;
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};

using namespace std;

typedef pair<int ,int > P;

int ans;
int w,h;
char maze[27][27];

void dfs(int x,int y)
{
	//printf("[debug]:%d,%d\n",x,y);
	maze[x][y]='#';
	rep(i,4)
	{
		int nx=dx[i]+x;
		int ny=dy[i]+y;
		if(maze[nx][ny]=='.')
		{
			ans++;
			dfs(nx,ny);
		}
	}

}

void solve()
{
	/*
	rep(i,25)
	{
		rep(j,25)
		{
			cout<<maze[i][j];
		}
		cout<<endl;
	}
	*/
	rep(i,h) rep(j,w)
	{
		if(maze[i][j]=='@')
		{
			dfs(i,j);
			cout<<ans+1<<endl;
			return;
		}
	}


}

int main()
{
	rep(i,25) rep(j,25) maze[i][j]='#';
	
	while(cin>>w>>h)
	{
		if(w==0&&h==0) return 0;
		rep(i,h) rep(j,w) 
		{
			cin>>maze[i][j];
		}
		solve();
		ans=0;
		rep(i,25) rep(j,25) maze[i][j]='#';
	}


	return 0;
}

C++: #include <string>
#include <iostream>
#include <string.h>

using namespace std;

int w, h;
bool b[20][20];
bool ok[20][20];

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

void dfs(int x, int y){
	for(int i=0;i<4;i++){
		if(0<=x+dx[i]&&x+dx[i]<w&&0<=y+dy[i]&&y+dy[i]<h){
			if(!b[y+dy[i]][x+dx[i]]) continue;
			if(ok[y+dy[i]][x+dx[i]]) continue;
			ok[y+dy[i]][x+dx[i]] = true;
			dfs(x+dx[i],y+dy[i]);
		}
	}
}

int main(){
	while(cin >> w >> h){
		if(!w) break;
		int x, y;
		string s;
		memset(b,false,sizeof(b));
		for(int i=0;i<h;i++){
			cin >> s;
			for(int j=0;j<w;j++){
				b[i][j] = (s[j] !='#');
				if(s[j]=='@') x = j, y = i;
			}
		}
		memset(ok,false,sizeof(ok));
		ok[y][x] = true;
		dfs(x,y);
		int count = 0;
		for(int i=0;i<h;i++)
			for(int j=0;j<w;j++)
				count += ok[i][j];
		cout << count << endl;
	}
}

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(){
    while(1){
        int x,y,startx,starty;
        vector<vector<bool> > map;
        cin>>x>>y;

        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<utility>
#include<queue>
using namespace std;

void RB(int x,int y){
    int W,H;
    long count = 0;
    char area[22][22]={};
    char temp;
    using P = pair <int,int>;
    queue <P> que;
    const int direction[4][2] = {{1,0},{-1,0},{0,1},{0,-1}}; 

    W = x;
    H = y;
    for(int i=1;i<=H;i++){
        for(int j=1;j<=W;j++){
            cin >> temp;
            switch (temp){
                case '.':
                    area[i][j] = 1;
                    break;
                case '#':
                    break;
                case '@':
                    area[i][j] = 0;
                    que.push(P(i,j));
                    break;
            }
        }
    }

    while(!que.empty()){
        P temppair = que.front();
        que.pop();
        for(int i=0;i<4;i++){
            int tempx = temppair.first + direction[i][0];
            int tempy = temppair.second + direction[i][1];
            if(area[tempx][tempy]==1){
                que.push(P(tempx,tempy));
                area[tempx][tempy] = 0;
            }
        }
        count++;
    }

    cout << count << endl;

}

int main(){
    int x,y;
    while(1){
        cin >> x >> y;
        if(x == 0 && y == 0) break;
        RB(x,y);
    }
    return 0;
}