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 <bits/stdc++.h>
using namespace std;

#define ll long long
#define vvi vector<vector<int>>
#define vec vector
#define pq priority_queue
#define all(v) (v).begin(), (v).end()
#define rep(i, n) for (int (i) = (0); (i) < (n); ++(i))
#define repp(i, m, n) for (int (i) = (m); (i) < (n); ++(i))
#define debug(x) cerr << #x << ": " << x << endl;
#define debug2(x, y) cerr<<"("<<#x<<", "<<#y<<") = "<<"("<<x<<", "<<y<<")"<<endl;
#define debug3(x, y, z) cerr<<"("<<#x<<", "<<#y<<", "<<#z<<") = "<<"("<<x<<", "<<y<<", "<<z<<")"<<endl;
#define debugB(x, y) cerr<<#x<<": "<<bitset<y>(x) << endl;
#define line() cerr << "---------------" << endl;

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

template<typename T>
void printA(vector<T> &printArray, char between = ' ') {
   int paSize = printArray.size();
   for (int i = 0; i < paSize; i++) { cerr << printArray[i] << between;}
   if (between != '\n') {cerr << endl;}
}

// ------------------------------------------------------------------------------------------

int W, H;
char B[30][30];

int dfs(int y, int x) {
   int ret = 1;
   B[y][x] = '#';
   rep (i, 4) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= W || ny < 0 || ny >= H || B[ny][nx] == '#') continue;
      ret += dfs(ny, nx);
   }
   return ret;
}

void solve() {
   rep (i, H) {
      cin >> B[i];
   }
   rep (i, H) {
      rep (j, W) {
         if (B[i][j] == '@') {
            cout << dfs(i, j) << endl;
            return;
         }
      }
   }
}

int main() {
   while (1) {
      cin >> W >> H;
      if (H*W == 0) break;
      solve();
   }

   return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <cstring>
#include <sstream>
#include <cassert>
#ifndef M_PI
const double M_PI=acos(-1);
#endif
using namespace std;
static const double EPS = 1e-8;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> PI;
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define ALL(c) (c).begin(), (c).end()
#define mp(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define SZ(a) (int(a.size()))
#define F first
#define S second
int dx[]={0,1,0,-1},dy[]={1,0,-1,0};

bool vis[20][20];
string in[20];

main(){
  int w,h;
  while(cin>>w>>h,w|h){
    memset(vis,0,sizeof(vis));
    rep(i,h)cin>>in[i];
    int ans=0;
    rep(i,h){
      rep(j,w){
        if(in[i][j]!='@')continue;
        queue<PI> q;
        q.push(mp(i,j));
        while(!q.empty()){
          int x=q.front().F,y=q.front().S;q.pop();
          if(vis[x][y])continue;
          vis[x][y]=true;
          ++ans;
          rep(k,4){
            int nx=x+dx[k],ny=y+dy[k];
            if(nx<0 || h<=nx ||
               ny<0 || w<=ny ||
               vis[nx][ny] || in[nx][ny]=='#')continue;
            q.push(mp(nx,ny));
          }
        }
      }
    }
    cout<<ans<<endl;
  }
}

C++: #include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <vector>
#include <string>
#include <queue>
#include <deque>
#include <list>
#include <stack>
#include <set>
#include <map>
#include <algorithm>

#define int long long
#define MOD7 1000000007
#define MOD9 1000000009

#define rep(i, n) for (int i = 0; i < (n); i++)
#define all(a) (a).begin(), (a).end()

using namespace std;

int nextInt() {int a; cin >> a; return a;}
char nextChar() {char a; cin >> a; return a;}
double nextDouble() {double a; cin >> a; return a;}
string nextString() {string a; cin >> a; return a;}

void inputVector(vector<int> &v, int &n) {rep(i,n){v.push_back(nextInt());}}
void inputVector(vector<double> &v, int &n) {rep(i,n){v.push_back(nextDouble());}}
void inputVector(vector<string> &v, int &n) {rep(i,n){v.push_back(nextString());}}

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

signed main() {
	while (true) {
		int W, H;
		cin >> W >> H;

		if (W == 0) break;

		int sx, sy;

		vector<string> board;
		rep(y, H) {
			board.push_back(nextString());
			rep(x, board[y].size()) {
				if (board[y][x] == '@') {
					sx = x;
					sy = y;
				}
			}
		}

		auto visit = vector< vector<bool> >(H, vector<bool>(W, false));
		visit[sy][sx] = true;

		int cnt = 0;
		queue<int> q;
		q.push(sy * 100 + sx);
		while (!q.empty()) {
			int n = q.front();
			q.pop();

			int x = n % 100;
			int y = n / 100;

			cnt++;

			rep(dir, 4) {
				int nx = x + dx[dir];
				int ny = y + dy[dir];
				if (nx < 0 || nx >= W || ny < 0 || ny >= H) continue;
				if (visit[ny][nx]) continue;
				if (board[ny][nx] != '.') continue;

				q.push(ny * 100 + nx);
				visit[ny][nx] = true;
			}
		}

		cout << cnt << endl;
	}

    return 0;
}

C++: //??¨??¢?´¢:??±???????????¢?´¢??????????????¢?´¢ 

//??????????????¢?????¬?????????
 
//?????°??¢??°???????????????(?????????)????????\???(?????????)
#include <stdio.h>
#include <queue>
#include <string.h>
using namespace std;	
int W,H;
char c[101][101];
bool visited[100][100];
 
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
 	
void compute();//????????????????????£?¨? 
	
int main(){
	
	while(scanf("%d %d",&W,&H),W |H){
	    int i;
		
		for(i=0;i<H;i++){
			scanf(" %s",&c[i][0]);
		}
	}
	return 0;
}

void compute(){
	
	memset(visited,false,sizeof visited);  //{????????????????????????1bit??????????????????1bit????????§?????????} 
	
	queue<int> que;
	int i,j;
	for(j=0;j<W;j++){ 
	 for(i=0;i<H;i++){
	  if(c[i][j]=='@'){
	  	que.push(j+i*H);
	  	visited[i][j]=true;
	  	 break;
	  }
	 }
	}
  int answer=1;
  while(!que.empty()){
	int cur=que.front();
	        que.pop();
	        int x=cur%H;
	        int y=cur/H;
    

  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*H);
  visited[ny][nx]=true;

    }
   }
 
 }