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++: #define _USE_MATH_DEFINES
#define INF 100000000

#include <iostream>
#include <sstream>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <stack>
#include <limits>
#include <map>
#include <string>
#include <cstring>
#include <set>
#include <deque>
#include <bitset>
#include <list>
#include <cctype>
#include <utility>

using namespace std;

typedef long long ll;
typedef pair <int,int> P;
typedef pair <int,P> PP;

static const double EPS = 1e-8;

const int tx[] = {0,1,0,-1};
const int ty[] = {1,0,-1,0};

char stage[20][20];

int main(){
	int W,H;
	while(~scanf("%d %d",&W,&H)){
		if(W==H && H==0) break;
		for(int y=0;y<H;y++){
			char buf[32];
			scanf("%s",buf);
			for(int x=0;x<W;x++){
				stage[y][x] = buf[x];
			}
		}

		int sx=0;
		int sy=0;
		for(int y=0;y<H;y++){
			for(int x=0;x<W;x++){
				if(stage[y][x] == '@'){
					sx = x;
					sy = y;
					goto found;
				}
			}
		}
found:;
		queue<P> que;
		que.push(P(sx,sy));

		int res = 0;
		bool visited[20][20];
		memset(visited,0,sizeof(visited));

		visited[sy][sx] = true;
		while(!que.empty()){
			int sx = que.front().first;
			int sy = que.front().second;
			res++;
			que.pop();
			
			for(int i=0;i<4;i++){
				int dx = sx + tx[i];
				int dy = sy + ty[i];

				if(dx < 0 || dx >= W || dy < 0 || dy >= H) continue;
				if(stage[dy][dx]=='#') continue;
				if(visited[dy][dx]) continue;

				visited[dy][dx] = true;
				
				que.push(P(dx,dy));
			}
		}

		printf("%d\n",res);

	}
}

C++: #include<iostream>
#include<cstdio>
#include<vector>
#include<algorithm>
#include<queue>
using namespace std;
typedef long long ll;
//Global
int x, y;
const int MAX_N = 20;
char maze[MAX_N][MAX_N+1];
const int INF = 1000000;
int X, Y;//"@"???????????????
int memo[MAX_N][MAX_N];
int dx[4] = { 1,0,-1,0 };
int dy[4] = { 0,1,0,-1 };
int counter;

void bsd(int a, int b) {
	maze[b][a] = '#';
	memo[b][a] = INF;
	counter++;
	for (int i = 0; i < 4; i++) {
		int nx = a + dx[i];
		int ny = b + dy[i];
		if (memo[ny][nx] == 0 && maze[ny][nx] == '.' && 0 <= nx && nx < x && 0 <= ny && ny < y) {
			bsd(nx, ny);
		}
	}

}



//solve
//??±???????????¢?´¢??§"@"??¨???????????£?????????"."????????°????±???????
void solve()
{
	//memo????????????
	//memset(memo,0, sizeof(memo));
	for (int i = 0; i < y; i++) {
		for (int j = 0; j < x; j++) {
			memo[i][j] = 0;
		}
	}

	//"@"??¨??£??\????????????  ??????  INF??§??????  ??????  ????????????????????£?????????????????????"."??§??????
	counter = 0;//"@"??????
	bsd(X, Y);


	cout << counter << endl;

}


//main
int main()
{
	while (true) {
		cin >> x >> y;
		if (x == 0 && y == 0)break;
		//"W"??¨"."??¨"@"?????\???
		for (int i = 0; i < y; i++) {
			for (int j = 0; j < x; j++) {
				cin >> maze[i][j];
				if (maze[i][j] == '@') {
					X = j; Y = i;//@???????????????????¨????
				}
			}
		}
		solve();
	}
    return 0;
}

C++: #include <bits/stdc++.h>
#define FOR(v, a, b) for(int v = (int)(a); v < (int)(b); ++v)
#define FORE(v, a, b) for(int v = (int)(a); v <= (int)(b); ++v)
#define REP(v, n) FOR(v, 0, n)
#define REPE(v, n) FORE(v, 0, n)
#define RS resize
#define CLR clear
#define PB push_back
#define ALL(x) (x).begin(), (x).end()
#define C complex
#define CI complex< int >
#define CD complex< double >
using namespace std;

void split(string str, char chr, vector< string > *res){
	stringstream ss(str);
	string itm;
	while(getline(ss, itm, chr)){
		(*res).PB(itm);
	}
}


int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	int w, h, x, y, cnt;
	vector< vector< char > > field;
	stack< CI > st;
	
	while(cin >> w >> h , w){
		field.RS(h);
		REP(i, h){
			field[i].RS(w);
			REP(j, w){
				cin >> field[i][j];
				if(field[i][j] == '@'){
					x = j;
					y = i;
					field[i][j] = '.';
				}
			}
		}
		
		st.push(CI(x, y));
		cnt = 0;
		
		while(!st.empty()){
			complex< int > cur = st.top();
			st.pop();
			
			if(field[cur.imag()][cur.real()] != '.') continue;
			
			field[cur.imag()][cur.real()] = '#';
			++cnt;
			
			if(cur.imag() > 0){
				if(field[cur.imag() - 1][cur.real()] == '.'){
					st.push(cur + CI(0, -1));
				}
			}
			if(cur.imag() < h - 1){
				if(field[cur.imag() + 1][cur.real()] == '.'){
					st.push(cur + CI(0, 1));
				}
			}
			if(cur.real() > 0){
				if(field[cur.imag()][cur.real() - 1] == '.'){
					st.push(cur + CI(-1, 0));
				}
			}
			if(cur.real() < w - 1){
				if(field[cur.imag()][cur.real() + 1] == '.'){
					st.push(cur + CI(1, 0));
				}
			}			
		}
		
		cout << cnt << endl;
	}
	
	return 0;
}

C++: #include<iostream>
#include<utility>
#include<queue>
using namespace std;
char room[21][21];
int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};
typedef pair<int,int> p;


int main(){
	int x,y;
	queue<p> que;
	int w,h;
	cin>>w>>h;
	
	for(int a=0;a<h;a++){
		for(int b=0;b<w;b++){
			cin>>room[a][b];
			if(room[a][b]=='@'){
				x=b,y=a;
				que.push(p(b,a));
				room[a][b]='#';
			}
		}
	}
	if(room[20]=="00")return 0;
	int cnt=1;
	int nx,ny;
	while(que.size()){
		p ap=que.front();
		que.pop();
		for(int i=-1;i<=1;i++){
			nx=ap.first+dx[i];
			ny=ap.second+dy[i];
			if(0<=nx&&nx<=w&&0<=ny&&ny<=h&&room[ny][nx]!='#'){
				room[ny][nx]='#';
				que.push(p(nx,ny));
				cnt++;
			}
		}
	}
	cout<<cnt;
	return 0;
}

C++: #include<bits/stdc++.h>

using namespace std;
using ll = long long;
using ld = long double;
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define reps(i,s,n) for(int i=(int)(s);i<(int)(n);i++)

int w, h;
char t[20][20];
void dfs(int i, int j) {
	t[i][j] = '*';
	for (int di = -1; di <= 1;di++) {
			for (int dj = -1;dj <= 1;dj++) {
				int ni = i + di;
				int nj = j + dj;
				if (di!=dj && di+dj!=0 && ni >= 0 && ni < h&&nj >= 0 && nj < w&&t[ni][nj] == '.') {
					dfs(ni, nj);
				}
			}
		}
	return;
}

int main() {
	cin.sync_with_stdio(false);
	vector<int>ans;
	while (1) {

		cin >> w >> h;
		if (w == 0)break;
		rep(i, h) {
			rep(j,w) {
				cin >> t[i][j];
			}
		}
		rep(i, h) {
			rep(j, w) {
				if (t[i][j] == '@') {
					dfs(i, j);
					break;
				}
			}
		}
		int count = 0;
		rep(i, h) {
			rep(j, w) {
				if (t[i][j] == '*') {
					count++;
				}
			}
		}
		ans.push_back(count);
	}
	rep(i, ans.size()) {
		cout << ans[i] << endl;
	}

	return 0;
}