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<queue>
using namespace std;
int main(){
  while(1){
    int w,h,s,dx[4]={0,1,0,-1},dy[4]={1,0,-1,0},an=1;
    string mp[20];
    queue<int> t;
    cin>>w>>h;
    if(w==0)break;
    for(int i=0;i<h;i++){
      cin>>mp[i];
      for(int j=0;j<w;j++)if(mp[i][j]=='@')s=w*i+j;
    }
    t.push(s);
    while(!t.empty()){
      s=t.front();
      int x=s%w,y=s/w;
      t.pop();
      for(int i=0;i<4;i++){
	int x1=x+dx[i],y1=y+dy[i];
	if(0<=x1&&x1<w&&0<=y1&&y1<h){
	  if(mp[y1][x1]=='.'){
	    t.push(w*y1+x1);
	    mp[y1][x1]='#';
	    an++;
	  }
	}
      }
    }
    cout<<an<<endl;
  }
  return 0;
}

C++: #include <iostream>
#include <fstream>
#include <algorithm>
#include <vector>
#include <string>
#include <functional>
#include <array>
#include <stack>
#include <queue>
#include <chrono>
#include <numeric>

#pragma warning(disable:4996)
#define REP(i,n) for(int i = 0 ; i < n ; ++i)
using Int = long long;

using namespace std;

struct point{
	int x;
	int y;
	point(int _x, int _y) :x(_x), y(_y){}
};

char tile[20][20];
stack<point> st;

int main()
{
	cin.tie(0); ios::sync_with_stdio(false); 
	fstream fs("input.txt");
	////////////ここから開始///////////

	
	int x(-1), y(-1);
	while (x != 0 && y != 0){
		REP(i, 20)REP(j, 20)tile[i][j] = '#';
		cin >> x >> y;
		//fs >> x >> y;
		REP(i, y){
			REP(j, x){
				cin >> tile[i][j];
				//fs >> tile[i][j];
			}
		}

		REP(i, y){
			REP(j, x){
				if (tile[i][j] == '@')
					st.emplace(i, j);
			}
		}

		int dx[]{0, 1, 0, -1};
		int dy[]{1, 0, -1, 0};
		int ans(1);

		while (!st.empty()){
			point now = st.top();
			st.pop();
			REP(i, 4){
				int x1 = now.x + dx[i];
				int y1 = now.y + dy[i];
				if (x1 > 19 || x1 < 0 || y1 > 19 || y1 < 0)
					continue;

				if (tile[x1][y1] == '.'){
					st.emplace(x1, y1);
					tile[x1][y1] = '#';
					++ans;
				}
			}
		}

		cout << ans << endl;
	}
	return 0;
}

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

#define repl(i, a, b) for(int i = ((int) a); i < ((int) b); i++)
#define rep(i, n) repl(i, 0, n)

int w, h;
char tile[20][20];
bool vis[20][20];
int dd[] = {-1, 0, 1, 0, -1};

bool range(int y, int x) {
  return 0 <= y && y < h && 0 <= x && x < w;
}

int dfs(int y, int x) {
  if (tile[y][x] == '#') return 0;
  // cout << y << " " << x << endl;
  vis[y][x] = true;
  int cnt = 1;
  rep(i, 4) {
    int ni = y + dd[i], nj = x + dd[i + 1];
    if(range(ni, nj) && tile[ni][nj] == '.' && !vis[ni][nj]) {
      cnt += dfs(ni, nj);
    }
  }
  return cnt;
}


int main(int argc, char const *argv[])
{
  while(cin >> w >> h, w | h) {
    int x, y;
    memset(vis, 0, sizeof(vis));
    rep(i, h)rep(j, w) {
      cin >> tile[i][j];
      if(tile[i][j] == '@') x = j, y = i;
    }

    cout << dfs(y, x) <<endl;

  }
  return 0;
}

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

char mas[20][20];
int dx[4]={0,1,0,-1};
int dy[4]={1,0,-1,0};
int sx,sy;
int w,h,cnt;

void dfs(int x,int y);

int main(){
  while(1){
    cin>>w>>h;
    if(w==0&&h==0)break;
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin>>mas[i][j];
	if(mas[i][j]=='@'){
	  sx=j;
	  sy=i;
	}
      }
    }
    cnt=0;
    dfs(sx,sy);
    cout<<cnt<<endl;
  }
}

void dfs(int x,int y){
  mas[y][x]='#';
  cnt++;
  for(int i=0;i<4;i++){
    int nextx=x+dx[i];
    int nexty=y+dy[i];
    if(0<=nextx && nextx<w && 0<=nexty && nexty<h && mas[nexty][nextx]=='.'){
      dfs(nextx,nexty);
    }
  }
  return;
}

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

void checkboard(void);
void startmove(void);
int checked(int i,int j);

char board[20][20];
int H,W,now[2];

int main(void){
	while(1){
		cin >> H >> W ;
		if (!H && !W) break;
		// count=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				cin >> board[i][j];
				if(board[i][j]=='@'){
					now[0]=i;
					now[1]=j;
				}
			}
		}
		startmove();
		checkboard();
	}

}


void startmove(void){
	int n;
	do{
		n=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				if(board[i][j]=='@')
					n+=checked(i,j);
			}
		}

	}while(n);
}

int checked(int i,int j){
	int a=0;
	if(i>=0 && board[i-1][j]=='.'){
		board[i-1][j]='@';
		a=1;
	}
	if(i<W && board[i+1][j]=='.'){
		board[i+1][j]='@';
		a=1;
	}
	if(j>=0 && board[i][j-1]=='.'){
		board[i][j-1]='@';
		a=1;
	}
	if(j<H && board[i][j+1]=='.'){
		board[i][j+1]='@';
		a=1;
	}
	// checkboard();
	return a;
}
void checkboard(void){
		int count=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				if(board[i][j]=='@') count++; 
				// cout << board[i][j];
			}
		}
			cout << count << endl;

}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;

int cnt;
int w, h;
int dx[] = {0, 1, 0, -1};
int dy[] = {1, 0, -1, 0};
char tile[20][20];

void dfs(int x, int y);

int main()
{
	int x, y;

	while (true){
		scanf("%d %d", &w, &h);

		if (w == 0){
			break;
		}

		for (int i = 0; i < h; i++){
			scanf("%s", tile[i]);

			for (int j = 0; j < w; j++){
				if (tile[i][j] == '@'){
					y = i;
					x = j;
				}
			}
		}

		cnt = 0;
		dfs(x, y);

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

	return (0);
}

void dfs(int x, int y)
{
	tile[y][x] = '#';
	cnt++;

	for (int i = 0; i < 4; i++){
		int nx = x + dx[i];
		int ny = y + dy[i];

		if (nx >= 0 && nx < w && ny >= 0 && ny < h && tile[ny][nx] == '.'){
			dfs(nx, ny);
		}
	}

}