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>
using namespace std;

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

char board[21][21];
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<cstdio>
#include<cstdlib>
#include<iostream>
#include<string>
#include<map>
#include<vector>
#include<algorithm>
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)

using namespace std;
int ans = 0;

void bfs(char **tile, int w, int h){
  ans++;
  tile[w][h] = '#';

  if(tile[w][h-1] == '.')
  bfs(tile, w, h-1);

  if(tile[w][h+1] == '.')
  bfs(tile, w, h+1);

  if(tile[w-1][h] == '.')
  bfs(tile, w-1, h);

  if(tile[w+1][h] == '.')
  bfs(tile, w+1, h);

  return;
}


int main(){
  int W,H,index_w,index_h;

  while(true){
    cin >> W >> H;
    if(W == 0 && H == 0) break;

    char **tile;
    tile = new char* [H+2];
    REP(i,H+2) tile[i] = new char[W+2];

    REP(i,H+2){
      REP(j,W+2){
        if(i == 0 || i == H+1 || j == 0 || j == W+1) tile[i][j] = '#'; //sentry?
        else scanf(" %c",&tile[i][j]);

        if(tile[i][j] == '@') {
          index_h = j;
          index_w = i;
        }

      }
    }

    bfs(tile,index_w,index_h);

    REP(i,H) delete[] tile[i];
    delete[] tile;

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

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

char maze[21][21];
int dx[4] = {0, 0, -1, 1};
int dy[4] = {1, -1, 0, 0};
int ans = 0;
int h, w;

void dfs(int y, int x){
	ans += 1;
	maze[y][x] = '#';
	for(int i=0; i<4; i++){
			int nx = x + dx[i];
			int ny = y + dy[i];
			if(0 <= nx and nx < w and 0 <= ny and ny < h and maze[ny][nx] == '.'){
				dfs(ny, nx);
				//ans += 1;
			}
		}
	return;
}

int main(){
while(1){	
	ans = 0;
	int sx, sy;
	cin >> w >> h;
	if(w == 0 and h == 0) break;
	for(int i=0; i < h; i++){
		for(int j=0; j < w; j++){
			cin >> maze[i][j];
			if(maze[i][j] == '@'){
				sx = j;
				sy = i;
			}
		}
	}
	
	dfs(sy, sx);
	cout << ans << endl;
	/*
	for(int i=0; i < h; i++){
		for(int j=0; j < w; j++){
			cout << maze[i][j];
		}
		cout << endl;
	}
	*/
}//end	
	
	return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <algorithm>
#include <math.h>
#include <vector>
#include <string>
#include <sstream>

#define rep(i, j) for(int i = 0; i < j; i++)
#define all(i) i.begin(), i.end()
#define ll long long
#define bl bool
#define nn printf("\n");
#define mod 1000000007

using namespace std;

int w, h, cnt;
void srch(int x, int y, vector<vector<char>> &inp, vector < vector <bool>> &hit)
{
	if (x < 0 || w <= x || y < 0 || h <= y || inp[x][y] == '#') { return; }
	if (hit[x][y]) return;
	hit[x][y] = true;
	cnt += 1;
	srch(x + 1, y, inp, hit);
	srch(x - 1, y, inp, hit);
	srch(x, y + 1, inp, hit);
	srch(x, y - 1, inp, hit);
}
int main()
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	while (1)
	{
		cin >> w >> h;
		if (w == 0 && h == 0) { break; }
		cnt = 0;
		vector<vector<char>> inp(w, vector<char>(h, 0));
		vector<vector<bool>> hit(w, vector<bool>(h, false));
		int x = 0, y = 0;
		rep(i, h)
		{
			rep(j, w)
			{
				cin >> inp[j][i];
				if (inp[j][i] == '@'){ x = j; y = i; }
			}
		}
		srch(x, y, inp, hit);
		printf("%d\n", cnt);
	}
	return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<vector>
#include<queue>
#include<algorithm>
#include<cmath>
#include<climits>
#include<string>
#include<set>
#include<numeric>
#include<map>
#include<iostream>
using namespace std;
#define rep(i,n) for(int i = 0;i<((int)(n));i++)
#define reg(i,a,b) for(int i = ((int)(a));i<=((int)(b));i++)
#define irep(i,n) for(int i = ((int)(n)-1);i>=0;i--)
#define ireg(i,a,b) for(int i = ((int)(b));i>=((int)(a));i--)
typedef long long ll;
typedef pair<ll, ll> mp;

int W,H,f[20][20],ans=0,sx,sy;
string m[20],t,s;

void find(int x,int y){
	if(x>=W || x<0 || y>=H || y<0)return;
	t = m[x][y];
	if(t=="#")return;
	if(f[x][y]!=0)return;
	f[x][y] = 1;
	ans++;
	find(x+1,y);
	find(x-1,y);
	find(x,y+1);
	find(x,y-1);
}

int main(void){
	while(1){
		cin>>W>>H;
		if(W==0 && H==0)break;
		ans=0;
		rep(i,H){
			cin>>s;
			rep(j,W){
				f[j][i]=0;
				m[j][i]=s[j];
				t=m[j][i];
				if(t=="@"){
					sx=j;
					sy=i;
				}
			}
		}
		find(sx,sy);
		cout<<ans<<endl;
	}
	return 0;
}