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 <cstdio>
#include <string>
#define rep(i, n) for (int i = 0; i < n; ++i)
using namespace std;
typedef long long ll;

const ll MAX_W = 20, MAX_H = 20;
int tiles[MAX_H][MAX_W];
int d[4][2] = {{-1, 0}, {0, -1}, {0, 1}, {1, 0}};

void dfs(ll x, ll y, ll w, ll h) {
  tiles[y][x] = 0;

  rep(i, 4) {
      int dx = d[i][0];
      int dy = d[i][1];
      if (x + dx < 0 || x + dx >= w) continue;
      if (y + dy < 0 || y + dy >= h) continue;

      if (tiles[y + dy][x + dx] == -1) {
        dfs(x + dx, y + dy, w, h);
      }
  }
}


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

    string line;
    int startX;
    int startY;
    rep(i, h) {
      cin >> line;
      rep(j, w) {
        if (line[j] == '.') {
          tiles[i][j] = -1;
        } else if (line[j] == '#') {
          tiles[i][j] = 1;
        } else {
          tiles[i][j] = -1;
          startX = j;
          startY = i;
        }
      }
    }

    dfs(startX, startY, w, h);
    ll cnt = 0;
    rep(i, h) {
      rep(j, w) {
        if (tiles[i][j] == 0) ++cnt;
      }
    }
    cout << cnt << endl;
  }


  return 0;
}

C++: #include <iostream>
#include <algorithm>
#define MAX_INT  100005
using namespace std;

//??\???????????§???
#define MAX_X  22
#define MAX_Y  22

//?????°
char field[MAX_X][MAX_Y];
int X, Y;
int px, py;                     // players position


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


void print_field() {
  for (int y = 0; y < Y; y++ ) {
    for (int x = 0; x < X; x++ ) {
      cout << field[x][y];
    }
    cout << endl;
  }
  cout << endl;
}


void dfs(int x, int y) {

  field[x][y] = '$';            // ?¨????????????¨?????°

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

    if(field[nx][ny] == '.') {
      dfs(nx, ny);
    }
  }

}


int main() {

  while(true) {

    cin >> X >> Y;

    if(X == 0 && Y == 0) break;

    for(int y = 0; y < Y; y++ ){
      for(int x = 0; x < X; x++ ){
        cin >> field[x][y];
        if( field[x][y] == '@' ) { px = x; py = y; }
      }
    }

    dfs(px, py);

    // ?¨?????????´????????°??????
    int res = 0;
    for(int y = 0; y < Y; y++ ){
      for(int x = 0; x < X; x++ ){
        if ( field[x][y] == '$' ) { res++; }
      }
    }

    cout << res << endl;
  }
  return 0;

}

C++: #include<bits/stdc++.h>
#define rep(i,N) for(int i=0;i<(N);i++)
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
using namespace std;
const long long MOD = 1e9 + 7;
const long long INF = 1e12;
const int inf = 1e9;
typedef long long ll;
typedef pair<int,int> P;
typedef set<int> S;
ll n;
char maze[25][25];
int dist[25][25];
int sx,sy;
int h,w;
int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
vector<int> ans;
void bfs(){
    queue<P> que;
    for(int i = 0;i < 25; i++){
        for(int j = 0;j < 25; j++){
            dist[i][j]=-1; //距離の初期化
        }
    }
    que.push(P(sy,sx));
    dist[sy][sx]=0;
    while(que.size()){
        P p=que.front();
        que.pop(); //キューの先頭を取り出す
        for(int i=0;i<4;i++){
            int ny = p.first + dy[i] , nx = p.second + dx[i]; 
            if(0<=nx && nx <= (w-1) && 0<=ny && ny <= (h-1) && maze[ny][nx]!='#' && dist[ny][nx] == -1){
            que.push(P(ny,nx));
            dist[ny][nx]=dist[p.first][p.second]+1;
            }
        }
    }
    int cnt=0;
    for(int i=0;i<25;i++){
        for(int j=0;j<25;j++){
            if(dist[i][j]!=-1) cnt++;
        }
    }
    ans.push_back(cnt);
}
int main(){
    while(1){
        cin>>w>>h;
        if(h==0 && w==0){
            for(int i=0;i<ans.size();i++) cout<<ans[i]<<endl;
            return 0;
        }
        //入力と初期位置の決定
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                cin>>maze[i][j];
                if(maze[i][j]=='@'){
                    sx=j;
                    sy=i;
                }
            }
        }
        //cout<<sx<<" "<<sy<<endl;
        bfs();
    }
    return 0;
}

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

using namespace std;

#define WH 22

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

int main(int argc, char const *argv[])
{
	int w,h;
	char f[WH][WH];

	char c;
	int cnt;

	int x,y;

	while(1){
		cin>>w>>h;
		if(w+h==0) break;

		for(int i1=0;i1<WH;i1++){
			for(int i2=0;i2<WH;i2++){
				f[i1][i2]='#';
			}
		}
		queue<pair<int,int> > q;
		cnt=1;

		for(int i1=1;i1<=h;i1++){
			for(int i2=1;i2<=w;i2++){
				cin>>f[i1][i2];
				if(f[i1][i2]=='@'){
					x=i2;
					y=i1;
				}
			}
		}

		q.push(make_pair(x,y));
		while(!q.empty()){
			x=q.front().first;
			y=q.front().second;
			q.pop();

			for(int i1=0;i1<4;i1++){
				if(f[y+dy[i1]][x+dx[i1]]=='.') {
					cnt++;
					q.push(make_pair(x+dx[i1],y+dy[i1]));
					f[y+dy[i1]][x+dx[i1]]='@';
				}
			}
		}

		cout<<cnt<<endl;


	}


	return 0;
}

C++: #include <bits/stdc++.h>
#define MOD 1000000007
typedef long long ll;
using namespace std;

int H,W;
char S[22][22];

bool used[22][22];
int dx[]={0,1,0,-1};
int dy[]={1,0,-1,0};

int dfs(int x,int y){
  used[x][y]=true;
  int res=1;
  for(int i=0;i<4;i++){
    int nx=x+dx[i];
    int ny=y+dy[i];
    if(nx<0||ny<0||nx>=H||ny>=W) continue;
    if(used[nx][ny]) continue;
    if(S[nx][ny]!='.') continue;
    res+=dfs(nx,ny);
  }
  return res;
}

void solve(){
  for(int i=0;i<H;i++) for(int j=0;j<W;j++) cin>>S[i][j];
  for(int i=0;i<H;i++) for(int j=0;j<W;j++) used[i][j]=false;

  int stx,sty;
  for(int i=0;i<H;i++){
    for(int j=0;j<W;j++){
      if(S[i][j]=='@'){
        stx=i;
        sty=j;
      }
    }
  }

  cout<<dfs(stx,sty)<<endl;

}

int main(){
  while(1){
    cin>>W>>H;
    if(W==0) break;
    solve();
  }

  return 0;
}