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 <string>
#include <stack>
using namespace std;

int main(){
  int W, H;
  while(cin >> W >> H && W>0 && H>0){
    int map[400];
    for(int i=0;i<400;i++)
      map[i]=0;
    int at[2];

    for(int i=0;i<H;i++){
      string s;
      cin >> s;
      for(int j=0;j<W;j++){
        map[20*i+j]=(s[j]=='.'?1:(s[j]=='@'?2:0));
        if(s[j]=='@'){
          at[0]=i; at[1]=j;
        }
      }
    }

    vector<int> list[400];  //??£??\?????????
    for(int i=0;i<400;i++){
      if(i>=20) //???
        if(map[i-20]>0) list[i].push_back(i-20);
      if(i%20>0) //???
        if(map[i-1]>0) list[i].push_back(i-1);
      if(i%20<19) //???
        if(map[i+1]>0) list[i].push_back(i+1);
      if(i<380) //???
        if(map[i+20]>0) list[i].push_back(i+20);
    }

    bool d[400];
    for(int i=0;i<400;i++) d[i]=false;

    stack<int> cur;
    cur.push(at[0]*20+at[1]);
    d[at[0]*20+at[1]]=true;

    while(!cur.empty()){
      bool flg=false;
      for(vector<int>::iterator it=list[cur.top()].begin();it!=list[cur.top()].end();it++){
        if(!d[*it]){
          flg=true;
          cur.push(*it);
          d[*it]=true;
          break;
        }
      }
      if(flg) continue;
      cur.pop();
    }

    int cnt=0;
    for(int i=0;i<400;i++)
      if(d[i]) cnt++;

    cout << cnt << endl;
  }

  return 0;  
}

C++: //
//  RedAndBlack.cpp
//  
//
//  Created by ??°????????? on 2015/10/30.
//
//

#include <iostream>
using namespace std;

int w, h, num = 0, room[20][20];

void dfs(int x, int y) {
    if (room[y][x] == 0) return;
    
    num++;
    room[y][x] = 0;
    
    // ??????????????¢?´¢
    if (y-1 > -1) dfs(x, y-1);
    if (y+1 < h) dfs(x, y+1);
    if (x-1 > -1) dfs(x-1, y);
    if (x+1 < w) dfs(x+1, y);

    return;
}

int main() {
    while (cin >> w >> h) {
        if (w == 0 && h == 0) break;
        
        int manX, manY;
        
        for (int i = 0; i < h; i++) {
            string s;
            cin >> s;
            for (int j = 0; j < w; j++) {
                if (s[j] == '.') {
                    room[i][j] = 1;
                } else if (s[j] == '#') {
                    room[i][j] = 0;
                } else if (s[j] == '@') {
                    room[i][j] = 1;
                    manX = j;
                    manY = i;
                }
            }
        }
        
        dfs(manX, manY);
        
        cout << num << endl;
        
        num = 0;
    }
    
    return 0;
}

C++: #include <stdio.h>
char c[1000][1000];
bool went[1000][1000];
int ans,n,m;
void go(int x,int y){
    if(x>=n||y>=m||x<0||y<0)return ;
    if(c[x][y]=='#')return;
    if(went[x][y])return;
    went[x][y]=true;
    go(x+1,y);
    go(x,y+1);
    go(x-1,y);
    go(x,y-1);
    ans++;
    return ;
}
int main(){
    bool f;
    while(1){
        scanf("%d%d",&m,&n);
        if(n==0&&m==0)return 0;
        f=true;
        ans=0;
        for(int i=0;i<n;i++)scanf("%s",c[i]);
        for(int i=0;i<n;i++)for(int j=0;j<m;j++)went[i][j]=false;
        for(int i=0;i<n&&f;i++)for(int j=0;j<m&&f;j++)if(c[i][j]=='@'){
            go(i,j);
            f=false;
        }
        printf("%d\n",ans);
    }
}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;

int cnt;
int w, h;
int dx[] = {0, 1, 0, -1};
int dy[] = {1, 0, -1, 0};
char tile[20][20];

void dfs(int x, int y);

int main()
{
	int x, y;

	while (true){
		scanf("%d %d", &w, &h);

		if (w == 0){
			break;
		}

		for (int i = 0; i < h; i++){
			scanf("%s", tile[i]);

			for (int j = 0; j < w; j++){
				if (tile[i][j] == '@'){
					y = i;
					x = j;
				}
			}
		}

		cnt = 0;
		dfs(x, y);

		printf("%d\n", cnt);
	}

	return (0);
}

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

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

		if (nx >= 0 && nx < w && ny >= 0 && ny < h && tile[ny][nx] == '.'){
			dfs(nx, ny);
		}
	}
}

C++: #include <iostream>
#include <algorithm>
#include <cstdlib>
#include <cstring>
#include <cstdio>
#include <vector>
#include <queue>
using namespace std;
int dx[]={-1,0,1,0},dy[]={0,-1,0,1};
int H,W;
char fld[20][20];
int main(){
	queue<pair<int,int> > p;
	while(cin>>W>>H,H,W){
		for(int i=0;i<H;i++){
			for(int j=0;j<W;j++){
				cin>>fld[j][i];
				if(fld[j][i]=='@'){
					fld[j][i]='#';
					p.push(make_pair(j,i));
				}
			}
		}
		int cnt=1;
		while(!p.empty()){
			int x=p.front().first,y=p.front().second;
			p.pop();
			for(int i=0;i<4;i++){
				int nx=x+dx[i],ny=y+dy[i];
				if(nx>=0 && nx<W && ny>=0 && ny<H && fld[nx][ny]=='.'){
					fld[nx][ny]='#';
					p.push(make_pair(nx,ny));
					cnt++;
				}
			}
		}
		cout<<cnt<<endl;
	}
}

C++: #include<iostream>
#include<queue>

using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
char fld[20][20];
int d[20][20];
const int inf=1e8;

int main(){

   while(1){
      int W, H;
      cin>> W>> H;
      if(!(H||W)) break;
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
            }
         }
      }

      for(int i=0; i<H; i++)for(int j=0; j<W; j++)d[i][j]=inf;
      queue<pii> Q;
      Q.push(pii(sy, sx));
      d[sy][sx]=0;
      while(!Q.empty()){
         pii now=Q.front(); Q.pop();
         for(pii dd: dxdy){
            int ni=now.first+dd.first;
            int nj=now.second+dd.second;
            if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
            if(fld[ni][nj]=='#') continue;
            if(d[ni][nj]<=d[now.first][now.second]+1) continue;
            d[ni][nj]=d[now.first][now.second]+1;
            Q.push(pii(ni, nj));
         }
      }

      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=(d[i][j]!=inf);
      }

      cout<< ans<< endl;
   }

   return 0;
}