Go: package main

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

var strByColor = map[Color]string{
	ColorBlack: "black",
	ColorRed:   "red",
}

func (c Color) String() string {
	return strByColor[c]
}

type Node struct {
	ID      int
	X, Y    int
	Color   Color
	NextIDs map[int]struct{}
}

func (n *Node) String() string {
	d, _ := json.Marshal(n)
	return string(d)
}

type Map struct {
	Width, Height int
	Nodes         map[int]*Node
}

func (m *Map) GetByID(id int) *Node {
	return m.Nodes[id]
}

func (m *Map) Get(x, y int) *Node {
	return m.Nodes[m.getID(x, y)]
}

func (m *Map) SetColor(x, y int, c Color) {
	m.Nodes[m.getID(x, y)].Color = c
}

func (m *Map) getID(x, y int) int {
	return x + y*m.Width
}

func (m *Map) init() {
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			id := m.getID(x, y)
			m.Nodes[id] = &Node{ID: id, X: x, Y: y}
		}
	}
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			nextIDs := map[int]struct{}{}
			if x > 1 {
				nextIDs[m.getID(x-1, y)] = struct{}{}
			}
			if x < m.Width {
				nextIDs[m.getID(x+1, y)] = struct{}{}
			}
			if y > 1 {
				nextIDs[m.getID(x, y-1)] = struct{}{}
			}
			if y < m.Height {
				nextIDs[m.getID(x, y+1)] = struct{}{}
			}
			m.Get(x, y).NextIDs = nextIDs
		}
	}
}

func NewMap(w, h int) *Map {
	m := &Map{Width: w, Height: h, Nodes: map[int]*Node{}}
	m.init()
	return m
}

type Searcher struct {
	Map        *Map
	queue      []*Node
	touchedIDs map[int]struct{}
	visitedIDs map[int]struct{}
}

func NewSearcher(m *Map) *Searcher {
	return &Searcher{
		Map:        m,
		queue:      []*Node{},
		touchedIDs: map[int]struct{}{},
		visitedIDs: map[int]struct{}{},
	}
}

func (s *Searcher) Search(start *Node) {
	s.enqueue(start)

	for {
		if len(s.queue) == 0 {
			break
		}
		for nextID := range s.dequeue().NextIDs {
			n := s.Map.GetByID(nextID)
			if n.Color == ColorRed || s.hasTouched(n) {
				continue
			}
			s.enqueue(n)
		}
	}
}

func (s *Searcher) VisitedNodeCount() int {
	return len(s.visitedIDs)
}

func (s *Searcher) hasTouched(n *Node) bool {
	_, ok := s.touchedIDs[n.ID]
	return ok
}

func (s *Searcher) enqueue(n *Node) {
	s.queue = append(s.queue, n)
	s.touchedIDs[n.ID] = struct{}{}
}

func (s *Searcher) dequeue() *Node {
	n := s.queue[0]
	s.queue = s.queue[1:]
	s.visitedIDs[n.ID] = struct{}{}
	return n
}

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])

		m := NewMap(w, h)

		// Set colors
		var start *Node
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					m.SetColor(i+1, y, ColorBlack)
					if c == '@' {
						start = m.Get(i+1, y)
					}
				case '#':
					m.SetColor(i+1, y, ColorRed)
				}
			}
		}

		// Search
		s := NewSearcher(m)
		s.Search(start)
		fmt.Println(s.VisitedNodeCount())
	}
}

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

#define rep(idx, max)	for(int idx = 0, idx##Max = max; idx < idx##Max; idx ++)
#define rrep(idx, min, max)	for(int idx = min, idx##Max = max; idx <= idx##Max; idx ++)
#define MAX_W	32
#define MAX_H	32

typedef std::vector<int>	VEC;

int	g_map[MAX_W * MAX_H];
int	g_step;

typedef struct _POINT
{
	int x, y;
	_POINT() {}
	_POINT(int _x, int _y) : x(_x), y(_y) {}
} POINT;

enum
{
	RED_BLOCK = -1,
	BLACK_BLOCK = 0,
};

void PrintMap()
{
	rep(y, MAX_H)
	{
		rep(x, MAX_W)
		{
			::printf("%2d|", g_map[MAX_W * y + x]);
		}
		::putchar('\n');
	}
	::putchar('\n');
}

void Solve(POINT &pt, int step)
{
	static int dx[] = { +1, 0, -1, 0};
	static int dy[] = { 0, +1, 0, -1};

	//PrintMap();

	g_map[MAX_W * pt.y + pt.x] = step + 1;
	g_step++;

	rep(i, 4)
	{
		POINT ptn(pt.x + dx[i], pt.y + dy[i]);
		if (g_map[MAX_W * ptn.y + ptn.x] == BLACK_BLOCK)
		{
			Solve(ptn, step + 1);
		}
	}
}

int main(int nArgs, char **lplpszArgs)
{
	int	width, height;
	int	map[MAX_W * MAX_H];

	while (std::cin >> width >> height, width)
	{
		POINT start;

		// Init map info
		rep(y, MAX_H)
		{
			rep(x, MAX_W)
			{
				map[MAX_W * y + x] = RED_BLOCK;
			}
		}

		// Input map info
		rrep(y, 1, height)
		{
			char szLine[0x100];
			::scanf("%s", szLine + 1);
			rrep(x, 1, width)
			{
				switch(szLine[x])
				{
				case '.':	map[MAX_W * y + x] = BLACK_BLOCK;	break;
				case '#':	map[MAX_W * y + x] = RED_BLOCK;	break;
				case '@':
					map[MAX_W * y + x] = BLACK_BLOCK; 
					start = POINT(x, y);
					break;
				}
			}
		}
		//::printf("Start: (%d, %d)\n", start.x, start.y);
		g_step = 0;
		::memcpy(g_map, map, sizeof(map));
		::Solve(start, 0);
		std::cout << g_step << std::endl;
	}
}

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

using namespace std;

int w,h;
int ans=1;
char tile[21][21];

void saiki(int x,int y){
	if(x<0 | x>h | y<0 | y>w)return;
	tile[x][y]='#';
	if(tile[x-1][y]=='.')ans++,saiki(x-1,y);
	if(tile[x+1][y]=='.')ans++,saiki(x+1,y);
	if(tile[x][y-1]=='.')ans++,saiki(x,y-1);
	if(tile[x][y+1]=='.')ans++,saiki(x,y+1);
}

int main(void){
	int a,b; 
	while(1){
		for(a=0;a<21;a++){
			for(b=0;b<21;b++){
				tile[a][b]='\0';
			}
		}
		scanf("%d %d",&w,&h);
		if(w==0 && h==0)break;
		ans=1;
		for(a=0;a<h;a++){
			scanf("%s",tile[a]);
		}
		for(a=0;a<h;a++){
			for(b=0;b<w;b++){
				if(tile[a][b]=='@')saiki(a,b);
			}
		}
		printf("%d\n",ans);
	}
	return 0;
}

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;
}