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>
using namespace std;

int i,j,w,h;
string tile[21];
int cnt=0;
int result=0;
int si,sj;

void rec(int i,int j){
  if(tile[i][j]=='.'){
    tile[i][j]='#';
    cnt++;
  }

  if(i+1 < h && tile[i+1][j]=='.')  rec(i+1,j);
  if(j+1 <w && tile[i][j+1]=='.')  rec(i,j+1);
  if(j-1 >=0 && tile[i][j-1]=='.')  rec(i,j-1);
  if(i-1 >=0 && tile[i-1][j]=='.' )  rec(i-1,j);
}

int main(){

  while(1){
    cnt=0;
    result=0;
  cin >> w >> h;
  if(w==0 && h==0)break;
  for(i = 0; i < h; i++){
    cin >> tile[i];
    for(j = 0; j < w; j++){
     
      if(tile[i][j]=='@'){
	si=i;
	sj=j;
      }
    }
  }
 
  rec(si,sj);
 
 
  cout << cnt+1 << endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef unsigned int uint;
typedef long long int lint;
typedef unsigned long long int ulint;

//#define DEBUG

#ifdef DEBUG
#	define dump(i) cout << "[*] " #i " : " << i << endl;
#	define debug(i) i
void print(char map[20][20], int w, int h) {
	for (int i = 0; i < h; ++i) {
		for (int j = 0; j < w; ++j) {
			cout << static_cast<int>(map[i][j]);
		}
		cout << endl;
	}
}
void print(int* arr, int size) {
	for (int i = 0; i < size; ++i) {
		cout << arr[i] << endl;
	}
}
#else
#	define dump(i)
#	define debug(i)
# define print(i, j, k)
# define print(j, k)
#endif
#define rep(i, MAX) for (int i = 0; i < (MAX); ++i)

struct UnionFind
{
	vector<int> par;

	UnionFind(int n): par(n) {
		for (int i = 0; i < n; ++i) {
			par[i] = i;
		}
	}

	int find(int x) {
		if (x == par[x]) return x;
		return par[x] = find(par[x]);
	}

	bool same(int x, int y) {
		return find(x) == find(y);
	}

	void unite(int x, int y) {
		x = find(x);
		y = find(y);
		if (x == y) return;
		par[y] = x;
	}
};

inline void solve() {
	int w, h;
	constexpr int MAX = 20;
	char tile[MAX][MAX];
	dump(MAX);

	while (true) {
		pair<int, int> at;
		cin >> w >> h;
		if (!w && !h) return;
		int table[MAX] = {};
		UnionFind uf = UnionFind(200);

		const int asdf = MAX * MAX;
		dump(asdf);
		rep(i, h) {
			rep(j, w) {
				cin >> tile[i][j];
				if (tile[i][j] == '@') {
					at.first = i;
					at.second = j;
					tile[i][j] = '.';
				}
			}
		}
		int csi = 1;
		if (tile[0][0] == '.') {
			tile[0][0] = csi++;
		} else {
			tile[0][0] = 0;
		}
		table[tile[0][0]]++;
		for (int i = 1; i < w; ++i) {
			if (tile[0][i] == '.') {
				tile[0][i] = tile[0][i-1] != 0 ? tile[0][i-1] : csi++;
			} else {
				tile[0][i] = 0;
			}
			table[tile[0][i]]++;
		}
		rep(i, h) {
			if (i == 0) continue;
			rep(j, w) {
				if (j == 0) {
					if (tile[i][j] == '.') {
						tile[i][j] = tile[i-1][j] != 0 ? tile[i-1][j] : csi++;
					} else {
						tile[i][j] = 0;
					}
				} else {
					if (tile[i][j] == '.') {
						int tmp = tile[i-1][j];
						if (tmp == 0) {
							tmp = tile[i][j-1];
						} else if (tile[i][j-1] != 0) {
							uf.unite(tmp, tile[i][j-1]);
							if (tmp > tile[i][j-1]) {
								tmp = tile[i][j-1];
							}
						}
						tile[i][j] = tmp != 0 ? tmp : csi++;
					} else {
						tile[i][j] = 0;
					}
				}
				table[tile[i][j]]++;
			}
		}
		int ans = 0;
		int iv = tile[at.first][at.second];
		rep(i, csi) {
			if (uf.same(iv, i)) {
				ans += table[i];
			}
		}
		dump("tile");
		print(tile, w, h);
		dump("table");
		print(table, csi);
		dump("ans");
		cout << ans << endl;
	}
}

int main() {
	//*
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	// */
#ifdef DEBUG
	const auto start = chrono::high_resolution_clock::now();
#endif
	solve();
#ifdef DEBUG
	const auto stop = chrono::high_resolution_clock::now();
	cerr << "[*] time: " << chrono::duration_cast<chrono::milliseconds>(stop-start).count()
		<< " [ms]" << endl;
#endif
	return 0;
}