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 <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <functional>
using namespace std;
#define MOD 1000000007
#define ADD(X,Y) ((X) = ((X) + (Y)%MOD) % MOD)
typedef long long i64; typedef vector<int> ivec; typedef vector<string> svec; typedef pair<int, int> pi;
int dd[4] = { 0, 1, 0, -1 };


int main()
{
	while (true)
	{
		int W, H;
		cin >> W >> H;
		if (W == 0 && H == 0) break;

		char C[30][30];
		int B[30][30];
		for (int i = 0; i < H; i++)
		{
			cin >> C[i];
		}
		int sx = -1; int sy = -1;
		for (int i = 0; i < H; i++)
		{
			for (int j = 0; j < W; j++)
			{
				B[i][j] = 0;
				if (C[i][j] == '@')
				{
					sy = i;
					sx = j;
				}
			}
		}

		queue<pi> Q;
		Q.push(pi(sy, sx));
		B[sy][sx] = 1;
		int ans = 1;
		while (!Q.empty())
		{
			int ty = Q.front().first; int tx = Q.front().second;
			Q.pop();

			for (int r = 0; r < 4; r++)
			{
				int y = ty + dd[r];
				int x = tx + dd[r + 1];
				if (y < 0 || y >= H || x < 0 || x >= W)continue;
				if (C[y][x] == '#')continue;
				if (B[y][x])continue;
				ans++;
				B[y][x] = 1;
				Q.push(pi(y, x));
			}
		}

		cout << ans << endl;
	}
	return 0;
}

C++: #ifndef _WIN32
#include<iostream>
#endif
#include<string>
#include<vector>
#include<algorithm>
#include<queue>
using namespace std;

#define FOR(i,bg,ed) for(int i=(bg);i<(ed);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
typedef vector<string> VS;
VS wrap(VS v,char c) {
	int R = v.size(), C = v[0].size();
	VS ret(R + 2, string(C + 2, c));
	REP(i, R)REP(j, C)ret[i + 1][j + 1] = v[i][j];
	return ret;
}
int res ;
const int dr[] = { 0,0,1,-1 };
const int dc[] = { 1,-1,0,0 };

void f(int r, int c,VS&x) {
	if (x[r][c] == '#')return;
	res++;
	x[r][c] = '#';
	REP(i, 4) {
		f(r + dr[i], c + dc[i], x);
	}
}
int main() {
	int R, C;
	while (cin >> C >> R, R + C) {
		VS in(R);
		REP(i, R)cin >> in[i];
		in = wrap(in, '#');
		int sr, sc;
		REP(i, R + 2)REP(j, C + 2)if (in[i][j] == '@')sr = i, sc = j;
		res = 0;
		f(sr, sc, in);
		cout << res << endl;

	}
}

C++: #include<iostream>
#include<string>
#include<vector>
#include<algorithm> 
#include<functional> 
#include<cassert>
#include<numeric> // std::accumulate(vec.begin(),vec.end(),0) sum of element
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<queue>
#include<map>
#include<set>


using namespace std;


typedef long long ll;
typedef std::pair<int, int> P;
typedef vector<int> vi;


#define rep(i,n) for(int i=0;i<n;i++)
#define rep1(i,n) for(int i=1;i<=(int)(n);i++)
#define rrep(i,n) for(int i=n-1;i>=0;i--)
#define all(c) c.begin(),c.end()
#define show(x) cout << #x << " = " << x << endl

#define fi first
#define se second
#define pb push_back



#define DEBUG


// void print_vector(const std::vector<int> &t);
// void unique_vector(std:: vector<int> &t);

template <class X>
void print_vector(const std::vector<X> &t);
template <class X>
void unique_vector(std:: vector<X> &t);


int w, h;

vector<int> xl={-1, 0, 1, 0};
vector<int> yl={0, 1, 0, -1};

int main(){

  vi ans;
  while(true){
    cin >> w >> h;
    if(w == 0 && h == 0) break;
    vector<vector<char>> f(h, vector<char>(w));
    queue<P> ser;
    for(int i = 0; i < h; i++){
      for(int j = 0; j < w; j++) {
	cin >> f[i][j];
	if(f[i][j] == '@')
	  ser.push(make_pair(i, j));
      }
    }
    int c = 0;
    while(!ser.empty()){
      P t = ser.front();
      ser.pop();
      int y = t.fi, x = t.se;
      // show(x); show(y);
      if(f[y][x] == '@') f[y][x] = '#';
      for(int i = 0; i < 4; i++) {
	int dx = x+xl[i], dy=y+yl[i];
	// cout << dx << ' ' << dy << endl;
	// show(dx); show(dy);
	if(dy < 0 || dy >= h || dx < 0 || dx >= w || f[dy][dx] == '#')
	  continue;
	
	if(f[dy][dx] == '.'){
	  c++;
	  ser.push(make_pair(dy, dx));
	  f[dy][dx] = '#';
	}
      }	
      // for(int i = 0; i < h; i++) {
      // 	for(int j = 0; j < w; j++) {
      // 	  cout << f[i][j] << ' ';
      // 	}
      // 	cout << endl;
      // }
      // cout << endl;
    }
    ans.pb(c+1);
  }
  for(auto &i : ans) cout << i << endl;
}

template <class X>
void unique_vector(std::vector<X> &t)
{
  std::sort(t.begin(), t.end());
  t.erase( std::unique(t.begin(), t.end()), t.end() );
}

template <class X>
void print_vector(const std::vector<X> &t){
  for(auto itr=t.begin(); itr != t.end(); itr++){
    std::cout << *itr;
    if(itr != t.end()-1) std::cout << ' ';
  }
  std::cout << std::endl;
}