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 <stdio.h>
#include <queue>
#include <string.h>
using namespace std;	
int W,H;
char c[101][101];
bool visited[100][100];
 
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
 	
void compute();//????????????????????£?¨? 
	
int main(){
	
	while(scanf("%d %d",&W,&H),W |H){
	    int i;
		
		for(i=0;i<H;i++){
			scanf(" %s",&c[i][0]);
		}
compute();
	}
	return 0;
}

void compute(){
	
	memset(visited,false,sizeof visited);  //{????????????????????????1bit??????????????????1bit????????§?????????} 
	
	queue<int> que;
	int i,j;
	for(j=0;j<W;j++){ 
	 for(i=0;i<H;i++){
	  if(c[i][j]=='@'){
	  	que.push(j+i*H);
	  	visited[i][j]=true;
	  	 break;
	  }
	 }
	}
  int answer=1;
  while(!que.empty()){
	int cur=que.front();
	        que.pop();
	        int x=cur%H;
	        int y=cur/H;
    

  for(i=0;i<4;i++){
	int nx=x+dx[i],ny=y+dy[i];
	
  
  
  if(!(0<=nx&&nx<W&&0<=ny&&ny<H))continue;
  if(c[ny][nx]=='#'||visited[ny][nx])continue;

  que.push(nx+ny*H);
  visited[ny][nx]=true;
++answer;
    }
   }
 //cout << answer << endl;
 printf("%d\n",answer);
 }

C++: #include<iostream>
#include<cstdio>
using namespace std;
char field[22][22];
int cnt=0;
void tansaku(int y,int x){
	if(field[y][x+1]=='.'){
		field[y][x+1]='#';
		cnt++;
		tansaku(y,x+1);
	}
	if(field[y][x-1]=='.'){
		field[y][x-1]='#';
		cnt++;
		tansaku(y,x-1);
	}
	if(field[y+1][x]=='.'){
		field[y+1][x]='#';
		cnt++;
		tansaku(y+1,x);
	}
	if(field[y-1][x]=='.'){
		field[y-1][x]='#';
		cnt++;
		tansaku(y-1,x);
	}
}
int main(){
	while(1){
		int w,h;
		cnt=0;
		cin >> w >> h;
		if(!w&&!h)	break;
		for(int i=1;i<=h;i++)
			scanf("%s",(field[i]+1));
		for(int i=0;i<=w+1;i++){
			field[0][i]='#';
			field[h+1][i]='#';
		}
		for(int i=0;i<=h+1;i++){
			field[i][0]='#';
			field[i][w+1]='#';
		}
		for(int y=1;y<=h;y++){
			for(int x=1;x<=w;x++){
				if(field[y][x]=='@'){
					tansaku(y,x);
					break;
				}
			}
		}
		cout << cnt+1 << endl;
	}
}

C++: #include <iostream>
using namespace std;
void func(int x,int y);
int h,w;
char a[111][111];
int dx[4]={0,-1,0,1};
int dy[4]={-1,0,1,0};
int nx,ny;
int cnt=0;
int main(){
  int fx,fy;
  while(1){
    cin >> w >> h;
    if(h==0 && w==0)break;
    for(int i=1;i<=h;i++){
      for(int j=1;j<=w;j++){
	cin >> a[i][j];
	if(a[i][j]=='@'){
	  fx=i;
	  fy=j;
	}
      }
    }
    func(fx,fy);
    cout << cnt << endl;
    cnt=0;
  }
}


void func(int x,int y){
  cnt++;
  a[x][y]='0';
  for(int i=0;i<4;i++){
    nx=dx[i]+x;
    ny=dy[i]+y;
    if(nx<=h && nx>=1 && ny<=w && ny>=1 && a[nx][ny]!='#' && a[nx][ny]!='0'){
      func(nx,ny);
    }
  }
}

C++: 
#include <iostream>

using namespace std;

int w, h;
string f[20];
int cnt;

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

void rec(int x, int y) {
  f[y][x] = '#';
  cnt++;

  for (int i=0; i<4; i++) {
    int mx = x + dx[i];
    int my = y + dy[i];

    if (mx < 0 || w <= mx) { continue; }
    if (my < 0 || h <= my) { continue; }
    if (f[my][mx] != '.') { continue; }

    rec(mx, my);
  }
}

int main() {
  while (cin >> w >> h, (w||h)) {
    for (int i=0; i<h; i++) {
      cin >> f[i];
    }

    cnt = 0;
    for (int y=0; y<h; y++) {
      for (int x=0; x<w; x++) {
        if (f[y][x] == '@') { rec(x, y); }
      }
    }

    cout << cnt << endl;
  }

  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int (i)=(0);(i)<(int)(n);++(i))
using ll = long long;
using P = pair<int, int>;
using namespace std;

template<class T> void vin(vector<T>& v, int n) {
    v.resize(n);
    for (int i = 0; i < n; ++i) {
        cin >> v[i];
    }
}

int w, h;
char ch[50][50];
int dx[] = { 0, 1, 0, -1 };
int dy[] = { 1, 0, -1, 0 };

int bfs(int y, int x) {
    int res = 0;
    rep(i, 4) {
        int ny = y + dy[i], nx = x + dx[i];
        if (0 > ny or 0 > nx or ny >= h or nx >= w) continue;
        if (ch[ny][nx] == '.') {
            ch[ny][nx] = 'a';
            res += bfs(ny, nx) + 1;
        }
    }
    return res;
}

int main() {
    while (cin >> w >> h and w and h) {
        int py = 0, px = 0;
        rep(i, h) rep(j, w) {
            cin >> ch[i][j];
            if (ch[i][j] == '@') {
                py = i, px = j;
                ch[i][j] = '.';
            }
        }
        cout << bfs(py, px) << endl;
    }
}

C++: #include<iostream>
#include<string>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
using namespace std;
typedef long long ll;
const ll MOD = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
typedef pair<int, int> P;
typedef unsigned int ui;
int main(){
	int w, h;
	string s;
	int i, j;
	while (cin >> w >> h) {
		if (w == 0)break;
		char tile[22][22]; int used[22][22];
		for (i = 0; i < 22; i++) {
			tile[i][0] = '#';
			tile[i][h + 1] = '#';
		}
		for (j = 0; j < 22; j++) {
			tile[0][j] = '#';
			tile[w + 1][j] = '#';
		}
		for (i = 0; i < 22; i++) {
			for (j = 0; j < 22; j++) {
				used[i][j] = 0;
			}
		}
		vector<int> q1,q2;
		for (j = 1; j <= h; j++) {
			cin >> s;
			for (i = 1; i <= w; i++) {
				tile[i][j] = s[i - 1];
				if (s[i - 1] == '@') {
					q1.push_back(i); q2.push_back(j);
				}
			}
		}
		while (!q1.empty()) {
			int x = q1[0]; int y = q2[0]; used[x][y]=1;
			q1.erase(q1.begin() + 0); q2.erase(q2.begin() + 0);
			if (used[x - 1][y] == 0 && tile[x - 1][y]=='.') {
				q1.push_back(x - 1); q2.push_back(y);
			}
			if (used[x + 1][y] == 0 && tile[x + 1][y] == '.') {
				q1.push_back(x + 1); q2.push_back(y);
			}
			if (used[x][y-1] == 0 && tile[x][y-1] == '.') {
				q1.push_back(x); q2.push_back(y-1);
			}
			if (used[x][y+1] == 0 && tile[x][y+1] == '.') {
				q1.push_back(x); q2.push_back(y+1);
			}
		}
		int count = 0;
		for (i = 0; i < 22; i++) {
			for (j = 0; j < 22; j++) {
				if (used[i][j] == 1)count++;
			}
		}
		cout << count << endl;
	}
	return 0;
}