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 <string>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <utility>
#include <stack>
#include <numeric>
#include <algorithm>
#include <bitset>
#include <complex>
#include <iomanip>
using namespace std;
typedef long long ll;
typedef long long int llint;
#define MM 1000000000
#define MOD MM+7
#define pi pair<int,int>
#define pl pair<ll,ll>
#define chmax(a,b) (a<b ? a=b:0)
#define chmin(a,b) (a>b ? a=b:0)
const long double PI = acos(-1);
const long long INF = 1e15;
int dx[8] = {-1,0,1,0,-1,-1,1,1};
int dy[8] = {0,-1,0,1,-1,1,1,-1};
template<typename T>
T GCD(T u,T v){
    return v ? GCD(v,u%v) : u;
}
char s[22][22];
bool visited[22][22];
void init(int H, int W){
    for(int i = 0; i < H; i++){
        for(int j = 0; j < W; j++){
            s[i][j] = 'a';
            visited[i][j] = false;
        }
    }
}
void bfs(int sh, int sw, int h, int w){
    queue<pair<int,int> > que;
    que.push(make_pair(sh,sw));
    while(que.size()){
        pair<int,int> F = que.front();
        visited[F.first][F.second] = true;
        que.pop();
        for(int i = 0; i < 4; i++){
            int ny = F.first + dy[i];
            int nx = F.second + dx[i];
            if (s[ny][nx] == '#') continue;
            if (visited[ny][nx] == false && 0 <= nx && 0 <= ny && nx < w && ny < h && (s[ny][nx] == '.' || s[ny][nx] == '@')){
                que.push(make_pair(ny,nx));
                visited[ny][nx] = true;
            }
        }
    }
}
int check(int h, int w){
    int cnt = 0;
    for(int i = 0; i < h; i++){
        for(int j = 0; j < w; j++){
            if (visited[i][j] == true && (s[i][j] == '.' || s[i][j] == '@')){
                cnt++;
            }
        }
    }
    return cnt;
}
void display(int h, int w){
    for(int i = 0; i < h; i++){
        for(int j = 0; j < w; j++){
            cout << visited[i][j] << " ";
        }
        cout << endl;
    }
}


int main(){
    vector<int> v(0);
    while(true){
        int h,w; cin >> w >> h;
        if (h == 0 && w == 0) break;
        init(h,w);
        pair<int,int> start;
        for(int i = 0; i < h; i++){
            for(int j = 0; j < w; j++){
                cin >> s[i][j];
                if (s[i][j] == '@'){
                    start.first = i;
                    start.second = j;
                }
            }
        }
        bfs(start.first, start.second, h, w);
        v.push_back(check(h, w));
        //display(h,w);
    }
    for(int i = 0; i < v.size(); i++){
        cout << v[i] << endl;
    }
}