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>
using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
char fld[20][20];
bool used[20][20];
int W, H;
 
void rec(int i, int j){
   used[i][j]=1;
   for(pii dd: dxdy){
      int ni=i+dd.first;
      int nj=j+dd.second;
      if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
      if(used[ni][nj]) continue;
      if(fld[ni][nj]=='#') continue;
      rec(ni, nj);
   }
}
 
int main(){
 
   while(1){
      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<20; i++)for(int j=0; j<20; j++) used[i][j]=0;
      used[sy][sx]=1;
      rec(sy, sx);
      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=used[i][j];
      }
      cout<< ans<< endl;
 
   }
 
   return 0;
}

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

int w, h;
char tile[21][21];

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

int func(int x, int y){
	if(x < 0 || x >= w || y < 0 || y >= h || tile[y][x] == '#'){
		return 0;
	}
	int res = 1;
	tile[y][x] = '#';
	for(int i = 0; i < 4; ++i){
		for(int j = 0; j < 4; ++j){
			res += func(x + mx[i], y + my[i]);
		}
	}
	return res;
}

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	while(cin >> w >> h && w != 0 && h != 0){
	
		int sx = 0, sy = 0;
		for(int i = 0; i < h; ++i){
			for(int j = 0; j < w; ++j){
				cin >> tile[i][j];
				if(tile[i][j] == '@'){
					sy = i;
					sx = j;
				}
			}
		}
		
		cout << func(sx, sy) << endl;
	}
	
	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include<vector>
#include<stdlib.h>
#include<set>
#include<string>
#include<queue>
#include<map>
#include<stack>
#include <algorithm>

using namespace std;

int main(){
	int w, h;
	int sx, sy;
	int dx[4] = { 1, 0, -1, 0 }, dy[4] = {0 , 1 , 0 , -1};
	char str[20];

	cin >> w >> h;
	do{
		int masu[20][20] = { 0 };
		int check_masu[20][20] = { 0 };

		for (int i = 0; i < h; i++){
			cin >> str;
			for (int j = 0; j < w; j++){
				masu[j][i] = str[j];
				if (masu[j][i] == '@'){
					sx = j; sy = i;
				}
			}
		}

		int x, y;
		int count = 0; //count looked check
		stack< pair<int, int> > s;
		s.push(make_pair(sx, sy));	//set start place 
		while (s.size()){
			x = s.top().first;
			y = s.top().second;
			s.pop();

			if (check_masu[x][y] == 'c' || masu[x][y] == '#')
				continue;

			check_masu[x][y] = 'c';	//looked
			count++;

			for (int i = 0; i < 4; i++){
				if ((0 <= x + dx[i] && x + dx[i] < w)
					&& (0 <= y + dy[i] && y + dy[i] < h)){

					s.push(make_pair(x + dx[i], y + dy[i]));
				}
			}
		}

		cout << count << endl;

		cin >> w >> h;
	} while (w != 0 && h != 0);

	return 0;
}