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>
using namespace std;
int dx[4]={-1,0,1,0};
int dy[4]={0,1,0,-1};
int dfs(int x, int y, int acc, vector<vector<bool> > &map)
{
    acc=0;
    if(map[x][y]==false)return 0;
map[x][y]=false;
for(int i=0;i<4;i++){
    int nx=x+dx[i];
    int ny=y+dy[i];

    acc+=dfs(nx,ny,acc,map);
}
acc++;
return acc;
}
/*void dfs(int x,int y,vector<vector<bool> > &map,vector<int> &ans){
    if(map[x][y]){
        ans.push_back(x);
    map[x][y]=false;
for(int i=0;i<4;i++){
    int nx=x+dx[i];
    int ny=y+dy[i];
    dfs(nx,ny,map,ans);
}
    }
}*/
int main(void)
{
    int h, w;
    while (1)
    {
        cin >> h >> w;
        if (h == 0)
            break;
        vector<vector<bool> > map(w + 2, vector<bool>(h + 2, false));
        char temp;
        int x, y;
        for (int i = 1; i <= w; i++)
        {
            for (int j = 1; j <= h; j++)
            {
                cin>>temp;
                if(temp!='#')
                map[i][j]=true;
                if(temp=='@'){
                    x=i;
                    y=j;
                 }
            }
        }
        cout<<dfs(x,y,0,map)<<endl;
   /*     vector<int> ans;
dfs(x,y,map,ans);
cout<<ans.size()<<endl;*/

    }
    return 0;
}

C++: #include<iostream>
#include<map>
#include<vector>
#include<algorithm>
#include<cmath>
#include<climits>
#include<ctime>
#include<cstring>
#include<stack>
#include<queue>
#include<sstream>
#include<string>
#include<set>
#include<array>

#define ALL(v) (v).begin(),(v).end()
#define REP(i,p,n) for(int i=p;i<(int)(n);++i)
#define rep(i,n) REP(i,0,n)
#define DUMP(list) cout << "{ "; for(auto nth : list){ cout << nth << " "; } cout << "}" << endl
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i);
#define UNIQUE(a) a.erase(unique(ALL(a)), a.end())

using namespace std;

int dx[4] = {1, 0, -1, 0};//{1,0,-1,0,1,1,-1,-1};//8??????
int dy[4] = {0, 1, 0, -1};//{0,1,0,-1,-1,1,1,-1};//8??????

int px;
int py;
int cnt;
int xr,yr;
vector<string> field;

int dfs(int x, int y) {
  //(x, y) ????????????????????????????????????
    
    if(yr<=y || xr<=x || 0>x || 0>y) return 0;
    if(field[y][x]=='#') return 0;
    if(field[y][x]=='.' || field[y][x]=='@') {field[y][x]='#';cnt++;}
    // ??¨???4??????????????????
    for(int r = 0; r < 4; r++) {
      int nx = x + dx[r];
      int ny = y + dy[r];
      dfs(nx, ny);
    }
  return 0;
}

int main(){

 while(1){
    int x,y;
    cin >> x >> y;
    if(x==0 && y==0) break;
    xr=x; yr=y;
    field.clear(); 
    cnt=0;
    rep(i,y){
      string str;
      cin >> str;
      rep(j,str.size()){
        if(str[j]=='@'){
          px=j;
          py=i;
        }
      }
      field.push_back(str);
    }
   dfs(px,py);
   
   cout << cnt << endl;
  }
}

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

int w, h, black;
char tile[24][24];
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

void dfs(int x, int y){
    black++;
    tile[x][y] = '#';
    for(int i = 0; i < 4; i++){
        if( x+dx[i] >= 0 && x+dx[i] < h &&
            y+dy[i] >= 0 && y+dy[i] < w &&
            tile[ x+dx[i] ][ y+dy[i] ] == '.' ){
            dfs( x+dx[i], y+dy[i] );
        }
    }
}

int main(){
    int i, j;
    while( cin >> w >> h, w||h ){
        for(i = 0; i < h; i++){
            cin >> tile[i];
        }
        black = 0;
        for(i = 0; i < h && !black; i++){
            for(j = 0; j < w && !black; j++){
                if( tile[i][j] == '@' ){
                    dfs(i, j);
                }
            }
        }
        cout << black << endl;
    }
    return 0;
}

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

using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};

int main(){

   while(1){
      int W, H;
      cin>> W>> H;
      if(!(H||W)) break;
      char fld[H][W];
      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;
            }
         }
      }

      bool visited[H][W];
      for(int i=0; i<H; i++)for(int j=0; j<W; j++)visited[i][j]=0;
      queue<pii> Q;
      Q.push(pii(sy, sx));
      while(!Q.empty()){
         pii now=Q.front(); Q.pop();
         visited[now.first][now.second]=1;
         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(visited[ni][nj]) continue;
            Q.push(pii(ni, nj));
         }
      }
      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=visited[i][j];
      }

      cout<< ans<< endl;
   }

   return 0;
}

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

int w , h , count ;
char map[21][21] = {} ;
int dx[] = { -1,0,1,0 } ;
int dy[] = { 0,1,0,-1 } ;

void move( int ny , int nx ){
	map[ny][nx] = '0' ;
	
	for( int i=0 ; i<4 ; i++ ){
		if( ny+dy[i] >= 0 && ny+dy[i] < h &&
			nx+dx[i] >= 0 && nx+dx[i] < w &&
			map[ny+dy[i]][nx+dx[i]] == '.' ){
				count++;
				move( ny+dy[i] , nx+dx[i] ) ;
		}
	}
}

int main(){
	
	int sx , sy ;
	
	while(1){
		cin >> w >> h ;
		if( w==0 && h==0 ) break ;
		
		count = 1 ;
		for( int i=0 ; i<h ; i++ ){
			for( int j=0 ; j<w ; j++ ){
				cin >> map[i][j] ;
				if( map[i][j] == '@' ){
					sx = j ;
					sy = i ;
				}
			}
		}
		
		move(sy,sx) ;
		
		cout << count << endl ;
	}
	
return 0 ;
}