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;
int a=0;
void kazoeruyo(int p,int P,int box[20][20]){
  a++;
  if(box[p][P+1]==1) {
    box[p][P+1]=0;
    P=P+1;
    kazoeruyo(p,P,box);
  }
  if(box[p+1][P]==1){
    box[p+1][P]=0;
    p=p+1;
    kazoeruyo(p,P,box);
  }
  if(box[p][P-1]==1){
    box[p][P-1]=0;
    P=P-1;
    kazoeruyo(p,P,box);
  }
  if(box[p-1][P]==1){
    box[p-1][P]=0;
    p=p-1;
    kazoeruyo(p,P,box);
  }
}
int main(){
  int i,I,H,W,p,P;
  int  box[20][20];
  char v;
  while(1){
  cin>>W>>H;
  if(W==0&&H==0) break;
  for(i=0;i<H;i++){
    for(I=0;I<W;I++){
      cin>>v;
      if(v=='.') box[i][I]=1;
      if(v=='#') box[i][I]=2;
      if(v=='@') {
	box[i][I]=0;
	p=i;
	P=I;
      }
    }
  }
  kazoeruyo(p,P,box);
  for(i=0;i<H;i++){
    for(I=0;I<W;I++){
      cout<<box[i][I];
    }
    cout<<endl;
  }

  cout<<a<<endl;
 
  a=0;
  }
  return 0;
}

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

int w,h,count;
char room[20][20];

void rec(int y, int x){
  if(y<0||x<0||y>=h||x>=w){
      return;
    }
    if(room[y][x] == '#'){
      return;
    }
    count++;
    room[y][x] = '#';
    rec(y-1,x);
    rec(y,x+1);
    rec(y+1,x);
    rec(y,x-1);
    }
  
  int main(){
    
    int i,j,x,y;
    while(1){
      count=0;
      cin >> w >> h;
      if( w==0 && h==0){
	break;
      }
      for(j = 0;j < h;j++){
	for(i = 0;i < w;i++){ 
	  cin >> room[j][i] ;
	  if(room[j][i] =='@'){
	    x = i;
	    y = j;
	  }
	}
      }

      rec(y,x);
      cout << count << endl;
    }
  }

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 && room[y-1][x]) dfs(x, y-1);
    if (y+1 < h && room[y-1][x]) dfs(x, y+1);
    if (x-1 > -1 && room[y-1][x]) dfs(x-1, y);
    if (x+1 < w && room[y-1][x]) 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<iostream>
#include<cstring>

#define rep(i,n) for(int i=0; i<n; i++)

int main(){
	int w,h;
	char cfield[20][21];
	int ifield[20][20];
	int already[20][20];
	char point;

	while(std::cin >> w >> h && (w || h)){
		rep(i,20){
			memset(cfield[i],'#',20);
			rep(j,20){
				ifield[i][j] = 0;
				already[i][j] = 0;
			}
		}
		int x=-1,y=-1;
		int count = 1;
		int sum;
		rep(i,h){
			std::cin >> cfield[i];
		}
		rep(i,20){
			rep(j,20){
				if(cfield[i][j] == '@'){
					x=i,y=j;
					already[x][y] = 1;
					break;
				}
			}
			if (cfield[x][y] == '@') {
				break;
			}
		}
		while(1){
			rep(i,3) {
				point = cfield[x - 1 + i][y];
				if (point == '@' || point == '.' || point == '#') {
					if (point == '@') {
						already[x - 1 + i][y] = 1;
						ifield[x - 1 + i][y] = 0;
					}
					if (point == '.') {
						if (already[x - 1 + i][y] != 1) {
							ifield[x - 1 + i][y] = 1;
						}else{
							ifield[x - 1 + i][y] = 0;
						}
					}
					if(point == '#'){
						ifield[x - 1 + i][y] = 0;
					}
				}
			}
			rep(j,3) {
				point = cfield[x][y - 1 + j];
				if (point == '@' || point == '.' || point == '#') {
					if (point == '@') {
						already[x][y - 1 + j] = 1;
					}
					if (point == '.') {
						if (already[x][y - 1 + j] != 1) {
							ifield[x][y - 1 + j] = 1;
						}else{
							ifield[x][y - 1 + j] = 0;
						}
					}
					if (point == '#') {
						ifield[x][y - 1 + j] = 0;
					}
				}
			}

			sum = 0;
			rep(i,20){
				rep(j,20){
					sum += ifield[i][j];
					if(ifield[i][j] == 1){
						cfield[i][j] = '@';
						cfield[x][y] = '.';
						x=i,y=j;
					}
				}
			}
			if(sum == 0)break;
			count++;
		}
		std::cout << count << std::endl;
	}
	return 0;
}

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

using namespace std;

static int W, H;
static vector<string> field;

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

int dfs(int x, int y) {
    if (x < 0 || x >= W || y < 0 || y >= H || field[y][x] == '#')
        return 0;

    field[y][x] = '#';
    int sum = 0;
    for (int k = 0; k < 4; k++) {
        sum += dfs(x + dx[k], y + dy[k]);
    }

    return sum + 1;
}

int main() {
    for (;;) {
        cin >> W >> H;
        if (W == 0) break;

        field.resize(H);
        int sx, sy;
        for (int y = 0; y < H; y++) {
            cin >> field[y];
            for (int x = 0; x < W; x++) {
                if (field[y][x] == '@') {
                    sx = x;
                    sy = y;
                    field[y][x] = '.';
                }
            }
        }

        cout << dfs(sx, sy) << endl;
    }
}

C++: #include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;

void dfs(int, int);
int limx,limy,cnt=0,ni,nj;
char data[21][21];

int main(){
  int i,j;
  while(1){
    scanf("%d%d",&limx,&limy);
    if(limx==0 && limy==0)
      break;
    for(i=0;i<limy;i++)
      scanf("%s",data[i]);

    for(i=0;i<limy;i++)
      for(j=0;j<limx;j++)
	if(data[i][j]=='@') ni=i,nj=j;
    dfs(ni,nj);
    printf("%d\n",cnt);
    cnt=0;
  }
  return 0;
}

void dfs(int i, int j){
  int di[4]={0,-1,0,1};
  int dj[4]={1,0,-1,0};

  data[i][j]='#';
  cnt++;
  for(int r=0;r<4;r++){
    ni=i+di[r];
    nj=j+dj[r];
    if(data[ni][nj]=='.' && ni >= 0 && ni < limy && nj >= 0 && nj < limx) 
      dfs(ni,nj);
  }
}