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>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <string>
#include <set>
#include <map>
#include <queue>
#include <deque>
#include <bitset>
#include <numeric>
#include <sstream>
#include <cmath>
#include <cstdlib>
#include <cstring>
#include <stack>
#include <tuple>

using namespace std;

#define int long long

#define rep(i,n) for(int i = 0; i<n ; i++)
#define FOR(i,a,n) for(int i=a; i<n; i++)
#define REPSTR(i,s) for(int i=0; (s)[i]; i++)
#define REPITR(i,s) for(auto i=(s).begin(); i!=(s).end();i++)

#define RANGE(a,i,b) ((a)<=(i)&&(i)<=(b))

#define pb push_back
#define mp make_pair

#define ALL(a) begin(a), end(a)

#define EXIST(s,x) ((s).find(x)!=(s).end())

#define SET(a,c) memset(a,c,sizeof(a))
#define CLR(a) memset(a,0,sizeof(a))

#ifdef _DEBUG
#define DEBUG(x) cerr<<#x<<":"<<x<<endl
#else
#define DEBUG(x)
#endif

#define PRINTJOIN(x,n) rep(i,n){if(i)cout<<" ";cout<<x[i];}cout<<endl 
#ifdef int
#define INF 1001001001001001001LL
#else
#define INF 1001001001
#endif

typedef long long ll;
typedef pair<int, int> PII;
typedef vector<int> VI;
typedef stringstream SS;

#define PQ priority_queue

#define FST first
#define SEC second

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

int dfs (int i, int j, const vector<vector<bool>> & field, vector<vector<bool>> & done) {
    if(!field[i][j]) {
        return 0;
    }
    if(done[i][j]) {
        return 0;
    }
    int ans=0;
    done[i][j]=true;
    rep(k,4){
        if(dx[k]+i<0||dx[k]+i>=h||dy[k]+j<0||dy[k]+j>=w) continue;
        ans+=dfs(dx[k]+i, dy[k]+j, field, done);
    }

    return ans+1;
}


signed main(void) {
	cin.tie(0);
	ios::sync_with_stdio(false);
    while(cin>>w>>h,w||h){
        vector<vector<bool>> field(h,vector<bool>(w,false));
        vector<vector<bool>> done(h,vector<bool>(w,false));
        int si, sj;
        rep(i,h)rep(j,w){
            char c;
            cin>>c;
            field[i][j] = c=='#'?false:true;
            if(c == '@') {
                si=i;
                sj=j;
            }
        }

        cout<<dfs(si, sj, field, done)<<endl;
    }

	return 0;
}

C++: 
#include<iostream>
#include<cstdio>
#include<string.h>
#define rep(i,n) for(int i =0;i<n;i++)
using namespace std;

string t[21];


void dfs(int y,int x){
int dx[4] = {1,-1, 0, 0};
int dy[4] = {0, 0, 1, -1};


	if((x<0||y<0)||t[x][y]!='.') return;
	t[x][y]='@';	
	rep(k,4){
	dfs(y+dy[k],x+dx[k]);
}
return;
}


int main(void){

	int w,h,sx,sy,cnt;
	rep(i,20)rep(j,20){
t[i][j]=='#';	
	}

while(true){
scanf("%d %d",&w,&h);
if(!w&&!h) {break;}
rep(i,h){
cin >> t[i];
}
rep(i,h)rep(j,w){

	if(t[i][j]=='@'){	
	sx =i;
	sy =j;

t[i][j]='.';
	}

}
dfs(sy,sx);
cnt =0;

rep(i,h){ 
	rep(j,w)if(t[i][j]=='@') cnt++;	
}

cout <<endl<<cnt;;


}

return 0;
}

C++: #include<iostream>
#include<cstdio>
#include <string>
#define rep(i,n) for(int i =0;i<n;i++)
using namespace std;

string t[21];


void dfs(int y,int x){
int dx[4] = {1,-1, 0, 0};
int dy[4] = {0, 0, 1, -1};


	if(t[x][y]!='.') return;
	t[x][y]='@';	
	rep(k,4){
	dfs(y+dy[k],x+dx[k]);
}
return;
}


int main(void){

	int w,h,sx,sy,cnt;
	rep(i,20)rep(j,21){
t[i][j]=='#';	
	}

while(true){
scanf("%d %d",&w,&h);
if(!w&&!h) return 0;
for(int i=1;i<=h;i++)
{
cin >> t[i];
}	
rep(i,h)rep(j,w){

	if(t[i][j]=='@'){	
	sx =i;
	sy =j;

t[i][j]='.';
	}

}
dfs(sy,sx);
cnt =0;

rep(i,h){
	cout << t[i]<<endl;
	rep(j,w)if(t[i][j]=='@') cnt++;	
}

cout <<cnt<< endl;;


}



return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <set>
#include <queue>
#include <stack>
#include <string>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cmath>

#define rep(i, n) for(int i = 0; i < n; i ++)
#define ALL(T) T.begin(), T.end()
#define mp make_pair
#define pb push_back
#define X first
#define Y second

using namespace std;

typedef pair<int, int> pii;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;

const int INF = 1 << 24;
const int dx[4] = {1, -1, 0, 0};
const int dy[4] = {0, 0, 1, -1};

int cnt, w, h;
char m[20][20];

void dfs(pii);

int main(void){
    while(cin >> w >> h, w){
        cnt = 0;
        pii p;

        rep(y, h){
            cin >> m[y];
            rep(x, w) if(m[y][x] == '@') m[y][x] = '.', p = mp(x, y);
        }

        dfs(p);
        cout << cnt << endl;
    }

    return 0;
}

void dfs(pii p){
    if(p.X < 0 || w <= p.X ||
            p.Y < 0 || h <= p.Y) return;

    if(m[p.Y][p.X] == '.'){ cnt ++; m[p.Y][p.X] = '#';}
    else return;
    rep(i, 4){
        pii next = mp(p.X + dx[i], p.Y + dy[i]);
        dfs(next);
    }
}