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;
char mp[21][21];
int alred[21][21];
int hai = 0;
void init(){
  for(int i = 0; 20 > i; i++){
    for(int j = 0; 20 > j; j++){
      mp[i][j] = ' ';
      alred[i][j] = 0;
    }
  }
  hai = 0;
}
int tansa(int w, int h, int sw, int sh,int ret){
  if(mp[sw][sh] == '.')ret++,alred[sw][sh] = 1;
  if(((sw+1)!=w) && (mp[sw+1][sh] == '.' && alred[sw+1][sh] == 0))ret+=tansa(w,h,sw+1,sh,0);
  if((sw!=0) &&(mp[sw-1][sh] == '.' && alred[sw-1][sh] == 0))ret+=tansa(w,h,sw-1,sh,0);
  if(((sh+1)!=h) && (mp[sw][sh+1] == '.' && alred[sw][sh+1] == 0))ret+=tansa(w,h,sw,sh+1,0);
  if((sh!=0) && (mp[sw][sh-1] == '.' && alred[sw][sh-1] == 0))ret+=tansa(w,h,sw,sh-1,0);
  return ret;
}
int main(){
  while(true){
    int w,h;cin>>w>>h;
    init();
    int sw,sh;
    if(w == 0)break;
    for(int i = 0; h > i; i++){
      for(int j = 0; w > j; j++){
        cin >> mp[i][j];
        if(mp[i][j] == '@'){
          sw = i;
          sh = j;
        }
      }
    }
    cout << tansa(h,w,sw,sh,0)+1 << endl;
  }
}

C++: #include<bits/stdc++.h>
#define endl '\n';
#define rep(i,n) for(int i=0; i<(n); i++)
using namespace std;
using ll = long long;
using P = pair<int,int>;
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#pragma GCC optimize("Ofast")
constexpr ll INF = 1e18;
constexpr int inf = 1e9;
constexpr double INFD = 1e100;
constexpr ll mod = 1000000007;
constexpr ll mod2 = 998244353;
const double PI = 3.1415926535897932384626433832795028841971;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
// ios::sync_with_stdio(false);
// cin.tie(nullptr);
// ---------------------------------------------------------------------------

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int W,H;
    cin >> W >> H;
    while(W!=0){
        vector<string> G(H);
        for(int i=0; i<H; i++){
            cin >> G[i];
        }
        vector<vector<bool>> dist(H,vector<bool> (W,false));
        for(int i=0; i<H; i++){
            for(int j=0; j<W; j++){
                if(G[i][j] == '@'){
                    int ans = 1;
                    queue<P> que;
                    que.push(P(i,j));
                    dist[i][j] = true;
                    while(que.size()){
                        P v = que.front();
                        que.pop();
                        for(int i=0; i<4; i++){
                            int ny = v.first + dy[i];
                            int nx = v.second + dx[i];
                            if(ny<0 or ny>=H or nx<0 or nx>=W) continue;
                            if(G[ny][nx] == '#') continue;
                            if(dist[ny][nx]) continue;
                            ans++;
                            dist[ny][nx] = true;
                            que.push(P(ny,nx));
                        }
                    }
                    cout << ans << endl;
                }
            }
        }
        cin >> W >> H;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
bool mp[50][50];
int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
int h,w;
int dfs(int y,int x){
  if(mp[y][x])return 0;
  int ret=1;
  mp[y][x]=1;
  for(int i=0;i<4;i++){
    int nx=x+dx[i],ny=y+dy[i];
    if(nx<0 || nx>=w || ny<0 || ny>=h) continue;
    ret+=dfs(ny,nx);
  }
  return ret;
}
  
int main(){
  string s;
  int si,sj;
  while(cin>>w>>h && w&&h){
    memset(mp,0,sizeof(mp));
    for(int i=0;i<h;i++){
      cin>>s;
      for(int j=0;j<w;j++){
        if(s[j]=='#') mp[i][j]=1;
        else if(s[j]=='@') {si=i;sj=j;}
      }
    }
    cout<<dfs(si,sj)<<endl;
  }
  return 0;
}