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 <string>
#include <sstream>
#include <algorithm>
#include <vector>
#include <utility>
#include <functional>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <deque>
#include <ctime>

using namespace std;

#define rep(i,n) REP(i,0,n)
#define REP(i,s,e) for(int i=(s); i<(int)(e); i++)
#define pb push_back
#define mp make_pair
#define all(r) (r).begin(),(r).end()
#define rall(r) (r).rbegin(),(r).rend()
#define fi first
#define se second

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int INF = 1e9;
const ll LINF = 1e18;
const ll MOD = 1e9 + 7;
double EPS = 1e-8;
const double PI = acos(-1);

int dx[]={-1,1,0,0};
int dy[]={0,0,-1,1};
int w,h;
int sx,sy;
int a[30][30];
string str[30];
queue<int> q;

int main(){
	while(cin >> w >> h){
		if(w == 0 && h == 0){
			break;
		}

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

		for(int i=0; i<h; i++){
			cin >> str[i];
		}

		for(int i=0; i<h; i++){
			for(int j=0; j<w; j++){
				if(str[i][j] == '@'){
					sx = j;
					sy = i;
				}
			}
		}
		q.push(sy*30+sx);
		a[sy][sx] = 1;

		while(!q.empty()){
			int tmp = q.front();
			q.pop();

			for(int i=0; i<4; i++){
				int nx = tmp%30 + dx[i];
				int ny = tmp/30 + dy[i];

				if(ny <0 || nx <0 || ny >= h || nx >= w || str[ny][nx] == '#' || a[ny][nx] == 1){
					continue;
				}

				q.push(ny*30+nx);

				a[ny][nx] = 1;
			}
		}

		int count=0;

		for(int i=0; i<h; i++){
			for(int j=0; j<w; j++){
				if(a[i][j] == 1){
					count++;
				}
			}
		}

		cout << count << endl;
	}


}

C++: #include "bits/stdc++.h"

using namespace std;

typedef vector<int> vi;
typedef pair<int,int> pii;
typedef long long ll;

#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
#define all(a)  (a).begin(),(a).end()
#define pb push_back

string fie[30]={};
int w,h;
int c;
bool visited[30][30];

void dfs(int y,int x){
    visited[y][x]=true;
    c++;
    int dx[]={1,0,-1,0};
    int dy[]={0,1,0,-1};
    rep(i,4){
        if(0<=x+dx[i] && x+dx[i]<w  &&  0<=y+dy[i] && y+dy[i]<h  &&  fie[y+dy[i]][x+dx[i]]=='.'  &&  visited[y+dy[i]][x+dx[i]]==false){
            dfs(y+dy[i],x+dx[i]);
        }
    }
}


int main(){
    while(cin>>w>>h){
        c=0;
        if(w==0 && h==0)break;
        rep(i,30)rep(j,30)visited[i][j]=false;
        int sy,sx;
        rep(i,h){
            cin>>fie[i];
        }
        rep(i,h)rep(j,w){
            if(fie[i][j]=='@') sy=i,sx=j;
        }
        
        dfs(sy,sx);
             
        cout<<c<<endl;
    }
}

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;
}