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>
using namespace std;

const int NUM = 7;

void grooping(int y, int x, int w, int h, char** room, unsigned long*** group, unsigned long* next)
{
	if (room[y][x] != '#') {
		for (int i = y - 1; i <= y + 1; i++) {
			for (int j = x - 1; j <= x + 1; j++) {
				if (i < 0 || j < 0 || i >= h || j >= w) {
					continue;
				}
				if ((i == y && j != x) || (i != y && j == x)) {
					for (int k = 0; k < NUM; k++) {
						group[y][x][k] |= group[i][j][k];
					}
				}
			}
		}
		for (int i = 0; i < NUM; i++) {
			if (group[y][x][i] == 0UL && next[i] != 0UL) {
				group[y][x][i] |= next[i];
				next[i] <<= 1;
				break;
			}
		}
	}
}

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

		unsigned long next[NUM] = { 1UL, 1UL, 1UL, 1UL, 1UL, 1UL, 1UL };
		char** room = new char*[h];
		unsigned long*** group = new unsigned long**[h];
		int x, y;
		for (int i = 0; i < h; i++) {
			room[i] = new char[w];
			group[i] = new unsigned long*[w];
			for (int j = 0; j < w; j++) {
				cin >> room[i][j];
				if (room[i][j] == '@') {
					x = j;
					y = i;
				}
				group[i][j] = new unsigned long[NUM];
				for (int k = 0; k < NUM; k++) {
					group[i][j][k] = 0UL;
				}
			}
		}

		for (int k = 0; k < 1000; k++) {
			for (int i = 0; i < h; i++) {
				for (int j = 0; j < w; j++) {
					grooping(i, j, w, h, room, group, next);
				}
			}
			for (int i = h - 1; i >= 0; i--) {
				for (int j = w - 1; j >= 0; j--) {
					grooping(i, j, w, h, room, group, next);
				}
			}
			for (int j = 0; j < w; j++) {
				for (int i = 0; i < h; i++) {
					grooping(i, j, w, h, room, group, next);
				}
			}
			for (int j = w - 1; j >= 0; j--) {
				for (int i = h - 1; i >= 0; i--) {
					grooping(i, j, w, h, room, group, next);
				}
			}
		}

		int count = 0;
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				for (int k = 0; k < NUM; k++) {
					if ((group[i][j][k] & group[y][x][k]) != 0UL) {
						count++;
						break;
					}
				}
			}
		}

		cout << count << '\n';

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

	return 0;
}

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

int main(){
  int w,h;
  while(cin>>w>>h , w+h){
    char Map[20][20]={0};
    int pos = 0; 
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin>>Map[i][j];
	if(Map[i][j]=='@') pos=i*w+j;
      }
    }
    int cnt=0;
    queue<int> q;
    q.push(pos);
    while(!q.empty()){
      pos = q.front();
      q.pop();
      cnt++;
      int x=pos%w;
      int y=pos/w;
      if(x>=1 && Map[y][x-1]=='.'){
	q.push(y*w+x-1);
	Map[y][x-1]='@';
      }
      if(y>=1 && Map[y-1][x]=='.'){
	q.push((y-1)*w+x);
	Map[y-1][x]='@';
      }
      if(x+1<w && Map[y][x+1]=='.'){
	q.push(y*w+x+1);
	Map[y][x+1]='@';
      }
      if(y+1<h && Map[y+1][x]=='.'){
	q.push((y+1)*w+x);
	Map[y+1][x]='@';
      }
      //      cout<<" x = "<<x<<" y = "<<y<<endl;
    }
    cout<<cnt<<endl;

  }
  return 0;
}

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

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

void dfs(int x, int y) {
	a[x][y] = 0;
	rep(i, 4) {
		int nx = x + dx[i], ny = y + dy[i];
		if (0 <= nx&&nx < h && 0 <= ny&&ny < w&&a[nx][ny] == '.') {
			cnt++; dfs(nx, ny);
		}
	}
}
int main() {
	while (scanf("%d%d", &w, &h), w) {
		cnt = 1;
		rep(i, h)scanf("%s", a[i]);
		rep(i, h)rep(j, w) {
			if (a[i][j] == '@') {
				dfs(i, j);
				break;
			}
		}
		printf("%d\n", cnt);
	}
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <map>
#include <stack>
#include <queue>
#include <algorithm>
#include <set>

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,j) FOR(i,0,j)
#define mp std::make_pair

typedef long long ll;
typedef unsigned long long ull;
typedef std::pair<int,int> P;
typedef std::pair<int,P> State;

const int INF = 1001001001;

// S N E W(南北東西)
const int dx[8] = {0, 0, 1, -1, 1, 1, -1, -1}, dy[8] = {1, -1, 0, 0, 1, -1, 1, -1};

int width, height;
char map[20][20];
int sx, sy;
bool used[20][20];

int dfs(int x, int y){
    used[y][x] = true;

    int res = 1;
    REP(i, 4){
        int nx = x + dx[i], ny = y + dy[i];
        if(0 <= nx && nx < width && 0 <= ny && ny < height &&
           !used[ny][nx] && map[ny][nx] == '.'){
            res += dfs(nx, ny);
        }
    }
    
    return res;
}

int main(){
    while(std::cin >> width >> height, width || height){
        REP(i, height){
            REP(j, width){
                std::cin >> map[i][j];
                if(map[i][j] == '@'){
                    sx = j; sy = i;
                    map[i][j] = '.';
                }
            }
        }

        REP(i, 20){
            REP(j, 20){
                used[i][j] = false;
            }
        }
        std::cout << dfs(sx, sy) << std::endl;
    }
}

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

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

int W, H, ans;
char maze[20][20];

bool check(int x, int y){
	return x >= 0 && x < W && y >= 0 && y < H;
}

void dfs(int x, int y){
	
	
	for(int i = 0; i < 4; ++i){
		int nx = x + dx[i], ny = y + dy[i];
		if(check(nx, ny) && maze[ny][nx] != '#'){
			maze[ny][nx] = '#';
			ans++;
			dfs(nx, ny);
		}
	}
}

int main() {
	while(cin >> W >> H && (W || H)){
		for(int i = 0; i < H; ++i)
			for(int j = 0; j < W; ++j)
				cin >> maze[i][j];
		
		ans = 0;
		for(int i = 0; i < H; ++i){
			for(int j = 0; j < W; ++j){
				if(maze[i][j] == '@'){
					dfs(j, i);
					break;
				}
			}
		}
		cout << ans << endl;
	}
	return 0;
}