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;

#define MAX_H 22
#define MAX_W 22

int W, H, X, Y, A;

char tile[MAX_H][MAX_W];

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

void dfs(int x, int y)
{
	tile[y][x] = '@';

	for (int i = 0; i < 4; i++)
	{
		if (0 <= x + dx[i] <= W - 1 && 0 <= y + dy[i] <= H - 1)
		{
			if (tile[y + dy[i]][x + dx[i]] == '.')
			{
				dfs(x + dx[i], y + dy[i]);
			}
		}
	}
}

int main()
{
	while (true)
	{
		cin >> W >> H;

		if (W == 0 && H == 0) { break; }

		for (int i = 0; i < H; i++)
		{
			for (int j = 0; j < W; j++)
			{
				cin >> tile[i][j];

				if (tile[i][j] == '@')
				{
					X = j;
					Y = i;
				}
			}
		}

		dfs(X, Y);

		A = 0;

		for (int i = 0; i < H; i++)
		{
			for (int j = 0; j < W; j++)
			{
				if (tile[i][j] == '@')
				{
					A++;
				}
			}
		}

		cout << A << endl;
	}

	return 0;
}

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

typedef struct{
	int x;
	int y;
}point;

int main(){
	while(1){
		int w, h;
		char field[30][30];
		int visit[30][30]={0};
		int ans=0;
		int dx[4]={1,0,-1,0}, dy[4]={0,1,0,-1};
		point start;
		
		scanf(" %d %d", &w, &h);
		if(w==0) break;	
		for(int i=0; i<h; ++i){
			for(int j=0; j<w; ++j){
				scanf(" %c", &field[j][i]);		
				if(field[j][i] == '@'){
					start.x=j;
					start.y=i;	
				}
			}	
		}
		
		queue<point> que;
		visit[start.x][start.y]=1;
		que.push(start);
		++ans;
		
		while(!que.empty()){
			point now=que.front();
			que.pop();
			
			for(int i=0; i<4; ++i){
				int nx=now.x+dx[i], ny=now.y+dy[i];
				
				if(nx>=0 && nx<w && ny>=0 && ny<h && field[nx][ny]=='.' && visit[nx][ny]==0){
					visit[nx][ny]=1;
					++ans;
					
					point pp;
					pp.x=nx; pp.y=ny;
					que.push(	pp);
				}
			}
		}
		
		printf("%d\n", ans);
	}
	
	return 0;
}

C++: //#define _GRIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;

// ?????¬???????????¬??????
#pragma region MACRO
#define P(x) cout << (x) << endl
#define p(x) cout << (x)
#define PED cout << "\n"
#define rep(i,n) for(int i=0; i<(int)n; ++i)
#define REP(i,x,n) for(int i=x; i<(int)n; ++i)
#define repi(i,n) for(int i=0; i<=(int)n; ++i)
#define REPI(i,x,n) for(int i=x; i<=(int)n; ++i)
#define ILP while(true)
#define FOR(i,c) for(__typeof((c).begin())!=(c).begin(); i!=(c).end(); ++i)
#define ALL(c) (c).begin(), (c).end()
#define mp make_pair
#pragma endregion

// ???
#pragma region TYPE_DEF
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<string, string> pss;
typedef pair<string, int> psi;
typedef pair<int, string> pis;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<long double> vld;
typedef vector<long> vl;
typedef vector<long long> vll;
typedef vector<string> vs;
#pragma endregion

// Effective std
#pragma region ESTD
template<typename C, typename T> int count(C& c, T t) { return count(ALL(c), t); }
template<typename C, typename F> int count_if(C& c, F f) { return count_if(ALL(c), f); }
template<typename C, typename T> void erase(C& c, T t) { remove(ALL(c), t), c.end(); }
template<typename C> void remove(vector<C>& c, unsigned int index) { c.erase(c.begin()+index); }
template<typename C, typename T, typename U> void replace(C& c, T t, U u) { replace(ALL(c), t, u); }
template<typename C, typename F, typename U> void replace_if(C& c, F f, U u) { (ALL(c), f, u); }
template<typename C> void reverse(C& c) { reverse(ALL(c)); }
template<typename C> void sort(C& c) { sort(ALL(c)); }
template<typename C, typename Pred> void sort(C& c, Pred p) { sort(ALL(c), p); }
#pragma endregion

// ?????°
#pragma region CONST_VAL
constexpr int PI = (2*acos(0.0));
constexpr int EPS = (1e-9);
constexpr int MOD = (int)(1e9+7);
constexpr int INF = 100000000;
#pragma endregion

int w, h;
char a[21][21];
int dx[]{1,-1,0,0};
int dy[]{0,0,1,-1};
int cnt;

void dfs(int x, int y) {
    a[x][y] = 0;
    rep(i,4) {
        int nx = x + dx[i], ny = y + dy[i];
        if(0<=nx&&nx<h&&0<=ny&&ny<w&&a[nx][ny]=='.') {
            cnt++;
            dfs(nx, ny);
        }
    }
}

int main() {
    while(cin>>w>>h, w) {
        cnt = 1;
        rep(i,h) cin >> a[i];
        rep(i, h) rep(j,w) {
            if(a[i][j]=='@') {
                dfs(i,j);
                break;
            }
        }
        cout << cnt << endl;
    }
}