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 <vector>
#include <string>
#include <utility>
#include <queue>
#include <algorithm>
#include <map>
#include <set>
#include <cmath>
#include <climits>
//#include "h.h"
using namespace std;

#define ll long long
#define pint pair<int, int>
#define pll pair<ll, ll>
#define pb push_back
#define mp make_pair
#define pm(first, second) pb(mp(first, second))
#define SPACE " "
#define fpf first.first
#define fps first.second
#define spf second.first
#define sps second.second

#define all(X) (X).begin(), (X).end()

int main(){

	while(true){
		int W, H;
		cin >> W >> H;
		if(W == 0 && H == 0)break;
		vector<string> S(H);
		queue<pair<int, int>> Q;
		for(int i = 0; i < H; i ++){
			cin >> S[i];
			for(int j = 0; j < W; j ++){
				if(S[i][j] == '@')Q.push(mp(i, j));
			}
		}
		pair<int, int> P;
		while(!Q.empty()){
			P = Q.front();
			Q.pop();
			S[P.first][P.second] = '@';
			if(P.first > 0 && S[P.first - 1][P.second] == '.')Q.push(mp(P.first - 1, P.second));
			if(P.first < H - 1 && S[P.first + 1][P.second] == '.')Q.push(mp(P.first + 1, P.second));
			if(P.second > 0 && S[P.first][P.second - 1] == '.')Q.push(mp(P.first, P.second - 1));
			if(P.second < W - 1 && S[P.first][P.second + 1] == '.')Q.push(mp(P.first, P.second + 1));
		}
		int ans = 0;
		for(int i = 0; i < H; i ++){
			for(int j = 0; j < W; j ++){
				if(S[i][j] == '@')ans ++;
			}
		}
		cout << ans << endl;
	}

	return 0;
}

C++: #include <iostream>
#include <string>
#include <queue>
#include <stack>
#include <algorithm>
#include <list>
#include <vector>
#include <complex>
#include <utility>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <bitset>
#include <ctime>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <cassert>
#include <cstddef>
#include <iomanip>
#include <numeric>
#include <tuple>
#include <sstream>
#include <fstream>

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 RREP(i, a) for(int (i) = (a) - 1; (i) >= 0; (i)--)
#define FORR(i, a, b) for(int (i) = (a) - 1; (i) >= (b); (i)--)
#define DEBUG(C) cerr << #C << " = " << C << endl;
using LL = long long;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<LL>;
using VVL = vector<VL>;
using VD = vector<double>;
using VVD = vector<VD>;
using PII = pair<int, int>;
using PDD = pair<double, double>;
using PLL = pair<LL, LL>;
using VPII = vector<PII>;
#define ALL(a) begin((a)), end((a))
#define RALL(a) rbegin((a)), rend((a))
#define SORT(a) sort(ALL((a)))
#define RSORT(a) sort(RALL((a)))
#define REVERSE(a) reverse(ALL((a)))
#define MP make_pair
#define FORE(a, b) for (auto &&a : (b))
#define FIND(s, e) ((s).find(e) != (s).end())
#define EB emplace_back

const int INF = 1e9;
const int MOD = INF + 7;
const LL LLINF = 1e18;

vector<string> C;
vector<vector<bool>> used;
int MH, MW;
#define H first
#define W second
bool inside(int h, int w) {
    return 0 <= h && h < MH && 0 <= w && w < MW;
}
bool inside(PII p){return inside(p.H, p.W);}
int dh[] = {-1, 0, 0, 1};
int dw[] = {0, -1, 1, 0};
int dfs(PII now) {
    int res = 1;
    if (used[now.H][now.W]) return 0;
    used[now.H][now.W] = true;
    REP(i, 4) {
        auto buf = MP(now.H + dh[i], now.W + dw[i]);
        if (!inside(buf)) continue;
        if (C[buf.H][buf.W] == '.') res += dfs(buf);
    }
    return res;
}
void solve(int w, int h) {
    MH = h; MW = w;
    C.resize(h);
    used.resize(h, vector<bool>(w, false));
    PII start;
    REP(i, h) {
        cin >> C[i];
        REP(j, w) {
            if (C[i][j] == '@') start = MP(i, j);
        }
    }
    //cout << start.H << " " << start.W << endl;
    cout << dfs(start) << endl;
    used.clear();
    return;
}

int main(void) {
    int n, p, m, w, h;
    string s;
    int a, b, c;
    while (cin >> w >> h) {
        if (w == 0 && h == 0) return 0;
        solve(w, h);
        //return 0;
    }
}