Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

type Node struct {
	ID    int
	X, Y  int
	Color Color
	Nexts []*Node
}

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])

		// Initialize map
		m := map[int]map[int]*Node{}
		for x := 1; x <= w; x++ {
			m[x] = map[int]*Node{}
			for y := 1; y <= h; y++ {
				m[x][y] = &Node{ID: x + y*w, X: x, Y: y}
			}
		}
		for x := 1; x <= w; x++ {
			for y := 1; y <= h; y++ {
				nexts := []*Node{}
				if n, ok := m[x-1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y-1]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x+1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y+1]; ok {
					nexts = append(nexts, n)
				}
				m[x][y].Nexts = nexts
			}
		}

		// Set colors
		queue := []*Node{}
		touchedIDs := map[int]struct{}{}
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					n := m[i+1][y]
					n.Color = ColorBlack
					if c == '@' {
						queue = append(queue, n)
						touchedIDs[n.ID] = struct{}{}
					}
				case '#':
					m[i+1][y].Color = ColorRed
				}
			}
		}

		// Search
		visitedIDs := map[int]struct{}{}
		for {
			if len(queue) == 0 {
				break
			}
			n := queue[0]
			queue = queue[1:]
			visitedIDs[n.ID] = struct{}{}
			for _, next := range n.Nexts {
				if next.Color == ColorRed {
					continue
				}
				if _, ok := touchedIDs[next.ID]; !ok {
					queue = append(queue, next)
					touchedIDs[n.ID] = struct{}{}
				}
			}
		}
		fmt.Println(len(visitedIDs))
	}
}

C++: #include<iostream>
#include<vector>
#include<algorithm>
#include<cctype>
#include<utility>
#include<string>
#include<cmath>
#include<cstring>
#include<queue>
#include<map>
#include<set>

#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define llong long long
#define pb(a) push_back(a)

using namespace std;

typedef long long int ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;


template<typename T>
vector<T> make_v(size_t a) { return vector<T>(a); }
template<typename T, typename... Ts>
auto make_v(size_t a, Ts... ts) {
	return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}
template<typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type
fill_v(T& t, const V& v) { t = v; }
template<typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type
fill_v(T& t, const V& v) {
	for (auto& e : t) fill_v(e, v);
}


#define ARRAY_MAX 100005
const ll INF = 1e9 + 7;
const ll MOD = 1e9 + 7;

int dx[4] = { 1,0,0,-1 };
int dy[4] = { 0,1,-1,0 };


struct Combination {

	vector<ll> fact, rfact;

	Combination(ll sz) : fact(sz + 1), rfact(sz + 1) {

		fact[0] = 1;
		for (ll i = 1; i < fact.size(); i++) {
			fact[i] = fact[i - 1] * i % MOD;
		}
		//逆元
		rfact[sz] = inv(fact[sz]);
		for (ll i = sz - 1; i >= 0; i--) {
			rfact[i] = rfact[i + 1] * (i + 1) % MOD;
		}
	}

	ll inv(ll x) {
		//modpow
		return pow(x, MOD - 2);
	}
	ll pow(ll x, ll n) {
		//累乗
		ll ret = 1;
		while (n > 0) {
			if (n & 1) (ret *= x) %= MOD;
			(x *= x) %= MOD;
			n >>= 1;
		}
		return (ret);
	}
	ll P(ll n, ll r) {
		//順列
		if (r < 0 || n < r) return (0);
		return (fact[n] * rfact[n - r] % MOD);
	}

	ll C(ll p, ll q) {
		//組み合わせ
		if (q < 0 || p < q) return (0);
		return (fact[p] * rfact[q] % MOD * rfact[p - q] % MOD);
	}

	ll H(ll n, ll r) {
		//重複組み合わせ
		if (n < 0 || r < 0) return (0);
		return (r == 0 ? 1 : C(n + r - 1, r));
	}
};


/******************************************************************************************/

char mp[25][25];
int flag[25][25];

int main() {

	int h, w;
	int sx, sy;

	while (cin >> w >> h, h | w) {

		int ans = 0;


		for (int i = 0; i < 25; i++)
		{
			for (int j = 0; j < 25; j++) {

				flag[i][j] = -1;
			}
		}


		for (int i = 0; i < h; i++)
		{
			for (int j = 0; j < w; j++) {

				cin >> mp[i][j];
				if (mp[i][j] == '@') {
					sy = i;
					sx = j;
				}
				if (mp[i][j] == '#') {
					flag[i][j] = 0;
				}
			}
		}

		queue<pii> que;

		que.push(pii(sy, sx));
		while (!que.empty()) {

			pii hoge = que.front();
			que.pop();
			flag[hoge.first][hoge.second] = 1;
			
			//cout << hoge.second << " " << hoge.first << endl;

			for (int i = 0; i < 4; i++)
			{
				int ny = hoge.first + dy[i];
				int nx = hoge.second + dx[i];

				if (0 <= ny && ny < h && 0 <= nx && nx < w && mp[ny][nx] != '#' && flag[ny][nx] < 0) {
					que.push(pii(ny, nx));
				}

			}

		}
		for (int i = 0; i < h; i++)
		{
			for (int j = 0; j < w; j++) {

				if (flag[i][j] == 1) {
					ans++;
				}
			}
		}
		cout << ans << endl;

	}

	



	return 0;
}

C++: #include<iostream>
#include<queue>
using namespace std;

int point(int x,int y,int w,int h){
  int s;
  s= x*w*y*h;
  return s;
}

int pointx(int x,int w){
  int s;
  s=x/w;
  return s;
}

int pointy(int x,int w){
  int s;
  s=x%w;
  return s;
}

int main(){
  int w,h;
  w = 1;
  h = 1;
  cin>>w>>h;
  while(w!=0){
    int a[w][h];
    int sx,sy;
    for(int i=0;i<w;i++){
      for(int j=0;j<h;j++){
	char s;
	cin>>s;
	if(s=='.'){
	  a[i][j]=1;
	}else if(s=='#'){
	  a[i][j]=-1;
	}else{
	  a[i][j]=2;
	  sx=i;
	  sy=j;
	}
      }
    }

    int d[21][21];
    for(int i=0;i<21;i++){
      for(int j=0;j<21;j++){
	d[i][j]=-1;
      }
    }
    queue<int>q;
    d[sx][sy]=0;
    int str;
    str=point(sx,sy,w,h);
    q.push(str);
    while(! q.empty()){
      int cur =q.front();
      q.pop();
      int x,y;
      x=pointx(cur,w);
      y=pointy(cur,w);
      if(x>=1&&a[x-1][y]==1&&d[x-1][y]==-1){
	d[x-1][y]=0;
	q.push(point(cur-1,y,w,h));
      }
      if(x<=w-2&&a[x+1][y]==1&&d[x+1][y]==-1){
	d[x+1][y]=0;
	q.push(point(cur+1,y,w,h));
      }
      if(y>=1&&a[x][y-1]==1&&d[x][y-1]==-1){
	  d[x][y-1]=0;
	  q.push(point(x,cur-w,w,h));
      }
      if(y<=h-2&&a[x][y+1]==1&&d[x][y+1]==-1){
	d[x][y-1]=0;
	q.push(point(x,cur+w,w,h));
      }
    }

    int t=0;
    for(int i=0;i<w;i++){
      for(int j=0;j<h;j++){
	if(d[i][j]==0){
	  t+=1;
	}
      }
    }
    cout << t<< endl;
    cin>>w>>h;
  }
}

C++: #include <bits/stdc++.h>
using namespace std;

int H, W;
char board[30][30];
bool visited[30][30];

int dx[] = {0, 0, 1, -1};
int dy[] = {1, -1, 0, 0};

void dfs(int x, int y) {
   for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= H || ny < 0 || ny >= W) continue;
      if (board[nx][ny] == '#') continue;
      if (visited[nx][ny]) continue;
      visited[nx][ny] = true;
      dfs(nx, ny);
   }
}

int main() {
   while (cin >> W >> H, W || H) {
      memset(visited, false, sizeof(visited));

      int sx = 0, sy = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            cin >> board[i][j];
            if (board[i][j] == '@') {
               sx = i, sy = j;
            }
         }
      }

      visited[sx][sy] = true;
      dfs(sx, sy);

      int cnt = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (visited[i][j]) cnt++;
         }
      }
      cout << cnt << endl;

   }
   return 0;
}