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 pair<int, int> Pair;

int w;
int h;
char field[1145][1419];
bool used[1145][1419];
queue<Pair> q;

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

bool WithinRange(int y, int x) {
  if (y < 0 || h <= y) return false;
  if (x < 0 || w <= x) return false;
  return true;
}

int main() {
  while (1) {
    scanf("%d%d", &w, &h);
    if (w == 0 && h == 0) return 0;

    for (int i=0; i<h; i++) {
      for (int j=0; j<w; j++) {
        scanf(" %c", &field[i][j]);
        if (field[i][j] == '@') q.push(Pair(i, j));
        used[i][j] = false;
      }
    }

    int ans = 0;
    while (!q.empty()) {
      Pair p = q.front(); q.pop();
      int y = p.first;
      int x = p.second;
  
      if (used[y][x]) continue;    
      used[y][x] = true;
      ans++;

      for (int i=0; i<4; i++) {
        int newy = y + dy[i];
        int newx = x + dx[i];
        if (!WithinRange(newy, newx)) continue;
        if (field[newy][newx] == '#') continue;
        if (used[newy][newx]) continue;
        
        q.push(Pair(newy, newx));
      }
    }

    printf("%d\n", ans);
  }
}

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 i=0;i<w;i++){
      for(int j=0;j<h;j++){
	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 "bits/stdc++.h"
using namespace std;

int COUNT=0;

void dfs(vector<string> map,int state[20][20],int x,int y,int W,int H){
  int dx[4]={-1,0,1,0};
  int dy[4]={0,-1,0,1};
  if(x<0 || x==H || y<0 || y==W)return; //????????????
  if(state[x][y]!=0 || map[x][y]=='#')return; //?????§????????£??????????????????????????????
  ++COUNT;
  state[x][y]=1;
  for(int i=0;i<4;++i)dfs(map,state,x+dx[i],y+dy[i],W,H); //??????????????¨??¢?´¢
}

int main(){
  int W,H,x,y;
  vector<string> map;
  string a;
  while(true){
    COUNT=0;
    cin >> W >>H;
  if(!W && !H)break;
  int state[20][20]={0}; //?????§????????£???????????????
  for(int i=0;i<H;++i){ //???????????\???
    cin >> a;
    map.push_back(a);
  }
  for(int i=0;i<H;++i){//????????????????????¢?´¢
    if(0<map[i].find('@') && map[i].find('@')<W){
      y=map[i].find('@');
      x=i;
      break;
    }
  }
    dfs(map,state,x,y,W,H);
    cout << COUNT << endl;
    map.clear();
  }
}

C++: //C++14 (Clang 3.8.0)

#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <string>
#define rp(i,a,b) for(int (i)=(int)(a);i<(int)(b);++i)

typedef long long ll;

int mx[]={-1,1,0,0};
int my[]={0,0,-1,1};
using namespace std;
int n=0;
void dfs(int startx,int starty,vector<vector<bool> > &map){
    for(int i=0;i<4;i++){
        int x=startx+mx[i],y=starty+my[i];
        if(x>=0 and y>=0 and y<map.size() and x<map[0].size() and map[y][x]==true){
        map[y][x]=false;
        n++;
        dfs(x,y,map);}
        
    }
    
}

int main(){
    while(1){
        int x,y,startx,starty;
        vector<vector<bool> > map;
        cin>>x>>y;

        if(x==0 and y==0) return 0;
        for(int i=0;i<y;i++){
            vector<bool> tmp;
            string s;
            cin>>s;
            for(int j=0;j<x;j++){

                if(s[j]=='@') {startx=j;starty=i;tmp.push_back(true);}
                else if(s[j]=='.') tmp.push_back(true);
                else tmp.push_back(false);    
            }
            map.push_back((tmp));
        }
        n=0;
        dfs(startx,starty,map);
        cout<<n<<endl;
        
    }

    return 0;
};

C++: #include <bits/stdc++.h>
using namespace	std;
static int dx[4]={-1,0,1,0},dy[4]={0,1,0,-1};
int h,w,r;
string s[20];
void dfs(int y,int x){
  r++;
  s[y][x]='#';
  for(int i=0;i<4;i++)
      if(0<=y+dy[i]&&y+dy[i]<h&&0<=x+dx[i]&&x+dx[i]<w)
	if(s[y+dy[i]][x+dx[i]]=='.') dfs(y+dy[i],x+dx[i]);
}
int main(){

  int y,x;
  for(;cin>>w>>h;){
  r=0;
  for(int i=0;i<h;i++) cin>>s[i];
  for(int i=0;i<h;i++) for(int j=0;j<w;j++) if(s[i][j]=='@') y=i,x=j;
  dfs(y,x);
  cout<<r<<endl;
  }
  return 0;
}