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<queue>
using namespace std;

int point(int x,int y,int w,int h){
  int s;
  s= y*w+x;
  return s;
}

int point_x(int x,int w){
  int s;
  s=x%w;
  return s;
}

int point_y(int x,int w){
  int s;
  s=x/w;
  return s;
}

int main(){
  int w,h;
  cin>>w>>h;
  while(w!=0){
    int a[21][21]={};
    int sx,sy;
    for(int j=0;j<h;j++){
      for(int i=0;i<w;i++){
	char s;
	cin>>s;
	if(s=='.'){
	  a[i][j]=1;
	}else if(s=='#'){
	  a[i][j]=-1;
	}else{
	  a[i][j]=2;
	  sx=i;
	  sy=j;
	}
      }
    }

    int d[21][21];
    for(int i=0;i<21;i++){
      for(int j=0;j<21;j++){
	d[i][j]=-1;
      }
    }
    queue<int>q;
    d[sx][sy]=0;
    int str;
    str=point(sx,sy,w,h);
    q.push(str);
    while(! q.empty()){
      int cur =q.front();
      q.pop();
      int x,y;
      x=point_x(cur,w);
      y=point_y(cur,w);
      if(x>=1){
	if(a[x-1][y]==1&&d[x-1][y]==-1){
	  d[x-1][y]=0;
	  q.push(point(x-1,y,w,h));
	}
      }
      if(x<=w-2){
	if(a[x+1][y]==1&&d[x+1][y]==-1){
	  d[x+1][y]=0;
	  q.push(point(x+1,y,w,h));
	}
      }
      if(y>=1){
	if(a[x][y-1]==1&&d[x][y-1]==-1){
	  d[x][y-1]=0;
	  q.push(point(x,y-1,w,h));
	}
      }
      if(y<=h-2){
	if(a[x][y+1]==1&&d[x][y+1]==-1){
	  d[x][y+1]=0;
	  q.push(point(x,y+1,w,h));
	}
      }
    }

    int t=0;
    for(int i=0;i<w;i++){
      for(int j=0;j<h;j++){
	if(d[i][j]==0){
	  t+=1;
	}
      }
    }
    cout <<t<< endl;
    cin>>w>>h;
  }
}

C++: #include <iostream>
#include <queue>
#include <map>
#include <utility>
#include <cstring>

#define BLACK 1
#define RED 0

using namespace std;

int main() {
  while(true) {
    int h, w, ans = 0;
    cin >> w >> h;
    if(w == 0) break;
    map<pair<int, int>, bool> visits;
    map<pair<int, int>, int> table;
    queue<pair<int, int> > q;
    char t;
    pair<int, int> now;
    for(int i = 0; i < h; i++) {
      for(int j = 0; j < w; j++) {
	cin >> t;
	pair<int, int> index = make_pair(i, j);
	if(t == '.') {
	  visits[index] = false;
	  table[index] = BLACK;
	}
	else if(t == '#') {
	  visits[index] = false;
	  table[index] = RED;
	}
	else {
	  visits[index] = true;
	  table[index] = BLACK;
	  now = index;
	}
      }
    }
    q.push(now);
    ans++;
    while(!q.empty()){
      now = q.front();
      q.pop();
      pair<int, int> s = now;
      if(s.first > 0) {
	s.first--;
	if(table[s] == BLACK && visits[s] == false) {
	  ans++;
	  q.push(s);
	  visits[s] = true;
	}
	s = now;
      }
      if(s.first < h-1) {
	s.first++;
	if(table[s] == BLACK && visits[s] == false) {
	  ans++;
	  q.push(s);
	  visits[s] = true;
	}
	s = now;
      }
      if(s.second > 0) {
	s.second--;
	if(table[s] == BLACK && visits[s] == false) {
	  ans++;
	  q.push(s);
	  visits[s] = true;
	}
	s = now;
      }
      if(s.second < w-1) {
	s.second++;
	if(table[s] == BLACK && visits[s] == false) {
	  ans++;
	  q.push(s);
	  visits[s] = true;
	}
	s = now;
      }
    }
    cout << ans << endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
static const int MAX= 20;
char map[MAX][MAX];

int main() {
    int W, H, sx, sy, nx, ny, sum;
    int dx[4] = {0, 0, -1, 1};
    int dy[4] = {-1, 1, 0, 0};
    for (; ;) {
        std::cin >> W >> H;
        if (H == 0 && W == 0) break;
        for (int i = 0; i < H; i++) {
            for (int j = 0; j < W; j++) {
                std::cin >> map[i][j];
                if (map[i][j] == '@') sx = i, sy =j;
            }
        }
        std::queue<std::pair<int, int> > q;
        q.push({sx, sy});
        map[sx][sy] = '*';
        while ( !q.empty() ) {
            std::pair<int, int> p = q.front(); q.pop();
            for (int i = 0; i < 4; i++) {
                nx = p.first + dx[i], ny = p.second + dy[i];
                if (0 <= nx && nx < H && 0 <= ny && ny < W && map[nx][ny] == '.') {
                    map[nx][ny] = '*';
                    q.push({nx, ny});
                }
            }
        }
        sum = 0;
        for (int i = 0; i < H; i++) {
            for (int j = 0; j < W; j++) if (map[i][j] == '*') sum++;
        }
        std::cout << sum << std::endl;
    }


    return  0;
}

C++: #include<bits/stdc++.h>
#include<queue>
using namespace std;
int W,H;
char c[101][101];
bool visited [100][100];

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

void compute(){
  memset(visited,false,sizeof visited);


  queue<int> que;
int i,j;
 for(i=0;i<H;++i){
   for(j=0;j<W;++j){
     if(c[i][j]=='@'){
       que.push(j+i*W);
       visited[i][j]=true;
       break;}
       
	 }


   }
 int answer=1;
 
 while(!que.empty()){
   int cur = que.front();
   que.pop();
   int x = cur % W,y=cur/W;

   for(i=0;i<4;i++)
     {int nx = x + dx[i],ny= y+dy[i];
       if(!(0<=nx && nx< W && 0<=ny && ny < H))continue;
       if(c[ny][nx]=='#'||visited[ny][nx])continue;
       
       que.push(nx+ny*W);
       visited[ny][nx]=true;
       ++answer;
       
     }
 }
 cout<<answer<<endl;
 
}





int main(){
  while(cin>>W>>H,W|H) {
    for(int i=0;i<H;++i) {
      cin>>c[i];	    
    }
    compute();
  }
  return 0;
}

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

string v[20];
int dx[4]={0,1,0,-1};
int dy[4]={-1,0,1,0};
int W,H,x,y;
int cnt;

void search(int y, int x)
{
  int i;
  if(v[y][x]=='#') return;
  cnt++;
  v[y][x] = '#';
  for(i=0;i<4;i++){
    if(x+dx[i]<0||W<=x+dx[i]) continue;
    if(y+dy[i]<0||H<=y+dy[i]) continue;
    if(v[y+dy[i]][x+dx[i]] != '.') continue;
    search(y+dy[i],x+dx[i]);
  }
  return;
}

int main()
{
  int i;
  while(cin>>W>>H,W||H){
    for(i=0;i<H;i++){
      cin>>v[i];
      if(v[i].find("@")!=string::npos){
	x=i;
	y=v[i].find("@");
      }
    }
    cnt=0;
    search(x,y);
    cout<<cnt<<endl;
  }
}