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;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int INF = 1e9;
const ll LINF = 1e18;
template<class S,class T> ostream& operator << (ostream& out,const pair<S,T>& o){ out << "(" << o.first << "," << o.second << ")"; return out; }
template<class T> ostream& operator << (ostream& out,const vector<T> V){ for(int i = 0; i < V.size(); i++){ out << V[i]; if(i!=V.size()-1) out << " ";} return out; }
template<class T> ostream& operator << (ostream& out,const vector<vector<T> > Mat){ for(int i = 0; i < Mat.size(); i++) { if(i != 0) out << endl; out << Mat[i];} return out; }
template<class S,class T> ostream& operator << (ostream& out,const map<S,T> mp){ out << "{ "; for(auto it = mp.begin(); it != mp.end(); it++){ out << it->first << ":" << it->second; if(mp.size()-1 != distance(mp.begin(),it)) out << ", "; } out << " }"; return out; }

/*
 <url:>
 問題文============================================================
 =================================================================
 解説=============================================================
 ================================================================
 */

int solve(int w,int h){
    int res = 0;
    vector<vector<char>> masu(h+2,vector<char>(w+2,'#'));
    pii S;
    for(int i = 1; i <= h; i++){
        for(int j = 1; j <= w; j++){
            cin >> masu[i][j];
            if(masu[i][j] == '@'){
                masu[i][j] = '.';
                S = {i,j};
            }
        }
    }
    vector<vector<int>> flag(h+2,vector<int>(w+2,0));
    queue<pii> q; q.push(S);
    int dxy[5] = {1,0,-1,0,1};
    while(q.size()){
        int x,y; tie(x,y) = q.front(); q.pop();
        if(masu[x][y] == '#') continue;
        if(flag[x][y] == 1) continue;
        flag[x][y] = 1;
        for(int k = 0; k < 4;k++){
            int nx = x + dxy[k],ny = y + dxy[k+1];
            q.push({nx,ny});
        }
    }
    for(auto& vec:flag) res += accumulate(vec.begin(),vec.end(),0);
    return res;
}
int main(void) {
    cin.tie(0); ios_base::sync_with_stdio(false);
    int w,h;
    while(cin >> w >> h, w|h){
        cout << solve(w,h) << endl;
    }
    return 0;
}

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

using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
char fld[20][20];
int visited[20][20];
int main(){

   while(1){
      int W, H;
      cin>> W>> H;
      if(!(H||W)) break;
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
            }
         }
      }

      for(int i=0; i<H; i++)for(int j=0; j<W; j++)visited[i][j]=0;
      queue<pii> Q;
      Q.push(pii(sy, sx));
      while(!Q.empty()){
         pii now=Q.front(); Q.pop();
         visited[now.first][now.second]=1;
         for(pii dd: dxdy){
            int ni=now.first+dd.first;
            int nj=now.second+dd.second;
            if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
            if(fld[ni][nj]=='#') continue;
            if(visited[ni][nj]) continue;
            Q.push(pii(ni, nj));
         }
      }
      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=visited[i][j];
      }

      cout<< ans<< endl;
   }

   return 0;
}

C++: #include<cstdio>
#include<iostream>
#include<algorithm>
using namespace std;
int solve(int x,int y);
char room[30][30];
int dx[4]={0,1,0,-1};
int dy[4]={-1,0,1,0};
int w,h;
int main(void){
  int ans;
  while(1){
    scanf("%d %d",&w,&h);
    if(w==0 && h==0) break;
    ans=0;
    for(int i=0;i<h;i++) scanf("%s",room[i]);
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	if(room[i][j]=='@') ans=solve(i,j);
      }
      if(ans)break;
    }
    printf("%d\n",ans);
  }
}

int solve(int x,int y){
  int res=1;
  room[x][y]='#';
  for(int i=0;i<4;i++){
    int nx=x+dx[i];
    int ny=y+dy[i];
    if(0<=nx && nx<h && 0<=ny && ny<w){
      if(room[nx][ny]=='.') res+=solve(nx,ny);
    }
  }
  return res;
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <cmath>
#include <utility>

#define ll (long long)
#define REP(i,n) for(int i = 0;i<(int)n;i++)
#define FOR(i,a,b) for(int i = (a);i < (b);i++)
#define RFOR(i,a,b) for(int i = (a);i>(b);i--)
#define COUNTER int i= 0,j = 0,k = 0,l = 0,m = 0, n= 0;
#define RETURN cout << endl;
/*vector<vector<int>>ArrayName = vector<vector<int>>(sizeofx, vector<int>(sizeofy, 0));*/
using namespace std;



int dfs (int x,int y,vector<vector<char>>&v);

int lim_x,lim_y;
int mx[] = {-1,1,0,0};
int my[] = {0,0,-1,1,};

int main (void){
    
    int x,y;
    int sh_x = 0,sh_y = 0;
    
    while(cin >> lim_x >> lim_y,lim_x+lim_y)
        {
            vector<vector<char>>v = vector<vector<char>>(lim_y, vector<char>(lim_x, 0));
            REP(i,lim_y)
            REP(j,lim_x)
            {
                cin >> v[i][j];
                if(v[i][j] == '@')
                    {
                        sh_y = i;
                        sh_x = j;
                    }
            }
            //cout << lim_x << " " << lim_y << endl;
            //cout << sh_x  <<" "<< sh_y << endl;
            cout << dfs(sh_x,sh_y,v) << endl;
            /*v is array name(pointer)*/
      
        }
    return 0;
}

int dfs(int x,int y, vector<vector<char>>&v)
{
    int counter = 1;
    for(int i = 0; i<4;i++){
            int tx = x+mx[i],ty = y+my[i];
            if((tx >= 0 && tx<lim_x) && (ty >= 0 && ty <lim_y) && (v[ty][tx] == '.'))
            {
                v[ty][tx] = '#';
                //cout << v[ty][tx] << endl;
                counter += dfs(tx, ty, v);
                //cout << "come on !" << endl;
            }
                
        }
            
    return counter;
}