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 <cstring>
#include <cstdlib>
#include <cassert>
#include <climits>
#include <algorithm>
#include <string>
#include <sstream>
#include <complex>
#include <vector>
#include <list>
#include <queue>
#include <deque>
#include <stack>
#include <map>
#include <set>
#include <fstream>

using namespace std;
#define SZ(x) (int)(x).size()
#define REP(i,n) for(int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REPR(i,n) for(int i=(n)-1;i>=0;i--)
#define ALL(s) (s).begin(), (s).end()
#define so(V) sort(ALL(V))
#define rev(V) reverse(ALL(V))
#define uni(v) v.erase( unique(ALL(v)) , v.end());

typedef long long unsigned int llu;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vll;
typedef vector<bool> vb;
const double EPS = 1e-9;
const int MOD = 1e9 + 7;
const int INF = (1 << 29);
const ll LINF = 1e18;
const double PI = acos(-1);

template<typename T>vector<T> make_v(size_t a) { return vector<T>(a); }
template<typename T, typename... Ts>auto make_v(size_t a, Ts... ts) { return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...)); }
template<typename T, typename U, typename... V>
typename enable_if<is_same<T, U>::value != 0>::type
fill_v(U & u, const V... v) { u = U(v...); }
template<typename T, typename U, typename... V>
typename enable_if<is_same<T, U>::value == 0>::type
fill_v(U & u, const V... v) { for (auto& e : u) fill_v<T>(e, v...); }
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T& a, const T& b) { if (a > b) { a = b; return 1; } return 0; }

int W, H;
bool check(int y, int x) {
	if (y < 0 || y >= H)return false;
	if (x < 0 || x >= W)return false;
	return true;
}

const int BLACK = 1;
const int RED = -1;
vector<vb> Ans;
vvi Map;
int dx[] = { -1,0,0,1 };
int dy[] = { 0,-1,1,0 };

void dfs(int x,int y){
	REP(i, 4) {
		int X = x + dx[i];
		int Y = y + dy[i];
		if (check(Y, X)) {
			if (Map[Y][X] == BLACK && (!Ans[Y][X])) {
				Ans[Y][X] = true;
				dfs(X,Y);
			}
		}
	}
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	cin >> W >> H;
	while (W != 0) {
		Ans = vector<vb>(H, vb(W, false));
		Map = vvi(H, vi(W, 0));
		int x = 0, y = 0;
		REP(i, H) {
			string S;
			cin >> S;
			REP(j, W) {
				if (S[j] == '.') {
					Map[i][j] = BLACK;
				}
				else if (S[j] == '#') {
					Map[i][j] = RED;
				}
				else {
					Map[i][j] = BLACK;
					x = j;
					y = i;
					Ans[i][j] = true;
				}
			}
		}
		dfs(x, y);
		int ans = 0;
		REP(i, H) {
			REP(j, W) {
				if (Ans[i][j])ans++;
			}
		}
		cout << ans << endl;
		cin >> W >> H;
	}
	return 0;
}

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

#define REP(i, n) for(int i = 0;i < (n);i++)
#define FOR(i, a, b) for(int i = (a);i < (b);i++)
#define all(v) v.begin(),v.end()

using int64 = long long;

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

int main(void){
	int H, W;
	while(cin >> H >> W && H && W){
		vector<string> f(H+2, string(W+2, '#'));
		int sy, sx;
		REP(i, H){
			cin >> f[i+1];
			REP(j, W+2)
				if(f[i+1][j] == '@'){
					sy = i+1;
					sx = j;
				}
			f[i+1][0] = f[i+1][W+1] = '#';
		}
		bool visited[30][30] = {};
		int cnt = 0;
		
		function<void(int, int)> dfs = [&](int y, int x){
			visited[y][x] = 1;
			cnt++;
			REP(i, 4){
				int yy = y+dy[i], xx = x+dx[i];
				if(f[yy][xx] != '#' && !visited[yy][xx]){
					dfs(yy, xx);
				}
			}
		};
		dfs(sy, sx);
		cout << cnt << endl;
	}
}