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>
using namespace std;
void move(const vector<vector<char>> &board, vector<vector<bool>> &reached, int x, int y, int w, int h) {
    if (x < 0 || w <= x || y < 0 || h <= y) {
        return;
    } else if (board[x][y] == '#') {
        return;
    } else if (reached[x][y]) {
        return;
    }
    reached[x][y] = true;
    int dxAry[] = {0, 1, 0, -1};
    int dyAry[] = {-1, 0, 1, 0};
    for (int i = 0; i < 4; ++i) {
        move(board, reached, x + dxAry[i], y + dyAry[i], w, h);
    }
}
int main(void){
    int w, h;
    while (cin >> w >> h && w && h) {
        vector<vector<char>> board(w, vector<char>(h));
        int sx, sy;
        for (int i = 0; i < h; ++i) {
            string row;
            cin >> row;
            for (int j = 0; j < row.length(); ++j) {
                board[j][i] = row[j];
                if (row[j] == '@') {
                    sx = j;
                    sy = i;
                }
            }
        }
        vector<vector<bool>> reached(w, vector<bool>(h, false));
        move(board, reached, sx, sy, w, h);
        int count = 0;
        for (int y = 0; y < h; ++y)
            for (int x = 0; x < w; ++x)
                count += reached[x][y];
        cout << count << endl;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int w,h;
char a[100][100];
int dfs(int i,int j){
    if(i==h||j==w||i<0||j<0)return 0;
    if(a[i][j]=='#')return 0;
    int s=1;
    a[i][j]='#';
    s+=dfs(i+1,j);
    s+=dfs(i-1,j);
    s+=dfs(i,j+1);
    s+=dfs(i,j-1);
    return s;
}
int main(){
    int i,j;
    while(cin>>w>>h,w!=0&&h!=0){
    //char a[h][w];
    for(i=0;i<h;i++)cin>>a[i];
    for(i=0;i<h;i++)for(j=0;j<w;j++)if(a[i][j]=='@')cout<<dfs(i,j)<<endl;
}}

C++: // clang-format off
#include <bits/stdc++.h>
#define int long long
#define main signed main()
// #define main int main()
#define loop(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n) loop(i, 0, n)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define prec(n) fixed << setprecision(n)
#define stlice(from, to) substr(from, (to) - (from) + 1)
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define fi first
#define se second
using namespace std;
using pii = pair<int, int>;
using vi = vector<int>;
using vd = vector<double>;
using vc = vector<char>;
using vb = vector<bool>;
using vs = vector<string>;
using vpii = vector<pii>;
using vvi = vector<vi>;
using vvb = vector<vb>;
using vvpii = vector<vpii>;
template<typename A> using fn = function<A>;
constexpr int INF = sizeof(int) == sizeof(long long) ? 1000000000000000000LL : 1000000000;
constexpr int MOD = 1000000007;
constexpr double PI = acos(-1);
template<typename A, typename B> bool cmin(A &a, const B &b) { return a > b ? (a = b, true) : false; }
template<typename A, typename B> bool cmax(A &a, const B &b) { return a < b ? (a = b, true) : false; }
constexpr bool odd(const int &n) { return n & 1; }
constexpr bool even(const int &n) { return !odd(n); }
void solve();
main { solve(); return 0; }
// clang-format on

void solve() {
  int w, h;
  while (cin >> w >> h, w) {
    int si, sj;
    vs m(h);
    rep(i, h) {
      cin >> m[i];
      rep(j, m[i].size()) {
        if (m[i][j] == '@') {
          si = i;
          sj = j;
        }
      }
    }
    int c = 0;
    fn<void(int, int)> f = [&](int i, int j) {
      if (i < 0 || j < 0 || i >= h || j >= w || m[i][j] == '#') return;
      c++;
      m[i][j]  = '#';
      int di[] = {1, 0, -1, 0}, dj[] = {0, 1, 0, -1};
      rep(k, 4) f(i + di[k], j + dj[k]);
    };
    f(si, sj);
    cout << c << endl;
  }
}

C++: #include<cstdio>
#include<cstring>
#include<vector>
#include<queue>
#include<algorithm>
#include<cmath>
#include<climits>
#include<string>
#include<set>
#include<numeric>
#include<map>
#include<iostream>
using namespace std;
#define rep(i,n) for(int i = 0;i<((int)(n));i++)
#define reg(i,a,b) for(int i = ((int)(a));i<=((int)(b));i++)
#define irep(i,n) for(int i = ((int)(n)-1);i>=0;i--)
#define ireg(i,a,b) for(int i = ((int)(b));i>=((int)(a));i--)
typedef long long ll;
typedef pair<ll, ll> mp;

//?????????WA????????????
//????????????????????£??????

int W,H,f[21][21],ans[1000],cnt=0,sx,sy;
string s,t;

void find(int x,int y){
	if(x>=W || x<0 || y>=H || y<0)return;
	if(f[x][y]==0)return;
	f[x][y]=0;
	ans[cnt]++;
	find(x+1,y);
	find(x-1,y);
	find(x,y+1);
	find(x,y-1);
}

int main(void){
	while(1){
		cin>>W>>H;
		if(W==0 && H==0)break;
		ans[cnt]=0;
		rep(i,H){
			cin>>s;
			rep(j,W){
				t = s[j];
				if(t==".")f[j][i]=1;
				if(t=="#")f[j][i]=0;
				if(t=="@"){
					f[j][i]=1;
					sx=j;
					sy=i;
				}
			}
		}
		find(sx,sy);
		cnt++;
	}
	rep(i,cnt)cout<<ans[i]<<endl;
	return 0;
}