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 <fstream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <string>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <algorithm>
#include <functional>
using namespace std;

#define REP(i,n) for(int i=0; i<n; ++i)
#define FOR(i,a,b) for(int i=a; i<=b; ++i)
#define FORR(i,a,b) for (int i=a; i>=b; --i)

typedef long long ll;

int w,h;
int a[20][20];
int dx[4] = {1,-1,0,0} , dy[4] = {0,0,1,-1};
int ans;

void dfs(int i , int j){
    a[i][j]=0;
    ans++;
    REP(k,4){
        int ii=i+dx[k] , jj=j+dy[k];
        if (ii>=0 && ii<h && jj>=0 && jj<w && a[ii][jj]){
            dfs(ii,jj);
        }
    }
}

int main(void) {
    while (cin>>w>>h && w){
        int x , y;
        REP(i,h){
            REP(j,w){
                char c;
                cin >> c;
                if (c=='.')
                    a[i][j]=1;
                else if (c=='#')
                    a[i][j]=0;
                else{
                    a[i][j]=1;
                    x=i;
                    y=j;
                }
            }
        }
        ans = 0;
        dfs(x,y);
        cout << ans << endl;
    }
    
    return 0;
}

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

int W, H;
char visit[21][21];
int sum;
int  dfs(int w, int h){
    int sum = 0;
    if(0<=w && w<W && 0<=h && h<H){
    if(visit[w][h]=='.'){
        visit[w][h] = '!';
        return 1;
    }
    sum += dfs(w+1, h);
    sum +=dfs(w-1, h);
    sum +=dfs(w, h+1);
    sum +=dfs(w, h-1);
    return sum;
    }
    return sum;
}

int main(void){
    string str;
    while(cin >> W >> H){
        if(W == 0 && H == 0)    break;
        for(int i=0; i<H; i++){
            cin >> str;
            for(int j=0; j<W; j++){
                visit[i][j] = str[j];
                if(visit[i][j] == '@')
                    dfs(i, j);
            }
        }
        cout << sum <<endl;
    }
    
    return 0;
}

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


using namespace std;

#define MAX_W 50
#define MAX_H 50

int main(){

  int w,h;
  char tile[MAX_H][MAX_W];

  cin>>w>>h;
  while(w!=0 && h!=0){
    pair<int,int> s;
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin>>tile[i][j];
	if(tile[i][j] == '@'){
	  s = make_pair(i,j);
	}
      }
    }

    int ans = 0;
    queue<pair<int,int> > que;
    que.push(s);
    int next[4][2] = {{0,1},{0,-1},{1,0},{-1,0}};

    while(!que.empty()){
      pair<int,int> now = que.front();
      que.pop();

      if(tile[now.first][now.second] != '#'){
	ans++;
	tile[now.first][now.second] = '#';

	for(int i=0;i<4;i++){
	  int ny = now.first + next[i][0];
	  int nx = now.second + next[i][1];
	  if(ny>=0 && ny<h && nx>=0 && nx<w && tile[ny][nx]!='#'){
	    que.push(make_pair(ny,nx));
	  }
	}
      }
    }

    cout<<ans<<endl;
    cin>>w>>h;
  }

  return 0;
}

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

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




int main() {
	int W,H;
	int q[20][20];
	char p[20][20];
	queue<pair<int,int> > M;
	pair<int,int> X;
	while(cin >> W >> H){
		for (int i=1; i <= 21; i++){
			for (int j=1; j <= 21; j++){
				q[i][j] = 0;
				p[i][j] = ' ';
			}
		}
		if (W == 0) return 0;
		int count = 0;
		for (int i = 1; i <= H; i++){
			for (int j = 1; j <= W; j++){
				cin >> p[i][j];
				if (p[i][j] == '@'){
					M.push(make_pair(i,j));
					count = 1;
					q[i][j] = 1;
				}
			}
		}
		while(!M.empty()){
			W = M.front().first;
			H = M.front().second;
			M.pop();
			for (int i = 0; i <= 3; i++){		
				if (p[W+dx[i]][H+dy[i]] == '.' && q[W+dx[i]][H+dy[i]] == 0){
					M.push(make_pair(W + dx[i], H + dy[i]));
					q[W + dx[i]][H + dy[i]] = 1;
					count += 1;
				}
			}
		}
		cout << count << endl;
	}
}

C++: //#define _GRIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
#define int long long

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

signed main()
{
    int h, w;
    while(cin>>w>>h, w) {
        char field[h+1][w+1];
        int sx, sy;
        for(int i=0; i<h; ++i) {
            for(int j=0; j<w; ++j) {
                char c;
                cin >> c;
                if(c=='@') {
                    sx = i;
                    sy = j;
                }
                field[i][j] = c;
            }
        }

        queue<pair<int, int> > q;
        q.push(make_pair(sx, sy));
        int ans = 0;
        while(!q.empty()) {
            ans++;
            pair<int, int> p = q.front();
            q.pop();
            int x = p.first;
            int y = p.second;
            for(int i=0; i<4; ++i) {
                int nx = x + dx[i];
                int ny = y + dy[i];
                if(nx>=0&&nx<=h&&ny>=0&&ny<=w&&field[nx][ny]=='.') {
                    q.push(make_pair(nx, ny));
                    field[nx][ny] = '#';
                }
            }
        }
        cout << ans << endl;
    }
    return 0;
}

C++: #include<stdio.h>
char a[21][21];
int c;
void dfs(int y,int x){
  c++;
  a[y][x]=0;
if(y!=0)
  if(a[y-1][x]=='.')
	dfs(y-1,x);
if(y!=20)
  if(a[y+1][x]=='.')
	dfs(y+1,x);
if(x!=0)
  if(a[y][x-1]=='.'&&x!=0)
	dfs(y,x-1);
if(x!=20)
  if(a[y][x+1]=='.'&&x!=20)
	dfs(y,x+1);
  return;
}
int main(){
  int w,h,i,j;
  while(1){
    scanf("%d%d",&w,&h);
    if(w==0&&h==0)break;
    for(i=0;i<21;i++){
      for(j=0;j<21;j++){
	a[i][j]=0;
      }
    }
    for(i=0;i<h;i++){
      for(j=0;j<=w;j++){
	scanf("%c",&a[i][j]);
      }
    }
    c=0;
    for(i=0;i<h;i++){
      for(j=0;j<w;j++){
	if(a[i][j]=='@')dfs(i,j);
      }
    }
    printf("%d\n",c);

   
  }
  return 0;
}