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 <cmath>
#include <string>
#include <iomanip>
#define GROUND 0
#define WALL 900
#define INITIAL -1

int count();
void init();
void map();
void mark_n(int,int,int,int,int);
void mark_n_all(int);
bool isValidPoint(int,int);

using namespace std;
const int MAX_W = 20;
const int MAX_H = 20;
char floor_[MAX_W][MAX_H];
//floorツつセツづ算ツ術ツ甘鳴青板づ債ャツつエツづゥ
int step[MAX_W][MAX_H];
int width,height;

int main(){
  //  char floor[MAX_W][MAX_H];
  //int step[MAX_W][MAX_H];
  
  while(cin >> width >> height && width > 0){
    string line;
    int ix,iy;
    //cout << width << " " << height << endl;
    init();
    for(int i = 0; i < height; ++i){
      cin >> line;
      for(int j=0; j < width ; ++j){
	floor_[j][i] = line[j];
	if(line[j]=='@'){
	  step[j][i]=GROUND;
	  ix = j; iy = i;
	}else if(line[j]=='#'){
	  step[j][i]=WALL;
	}
	//cout << step[j][i];
      }
      //cout << endl;
    }
    //cout << count('.') << count('#') << count('@') << endl;
    for(int k = 0; k < 400; k++){
      //map();
      mark_n_all(k);
      //mark_n(k+1,ix,iy,1,0);
      //cout << endl;
      //map();
      
      //cout << endl;
    }
    cout << count() << endl;
  }
  
}

int count(){
  int result =0;
  for(int i=0; i<height;++i){
    for(int j=0; j< width; ++j){
      if(step[j][i] >= 0 && step[j][i] < WALL){
	result++;
      }
    }
  }
  return result;
}

void init(){
  for(int i=0; i<height;++i){
    for(int j=0; j< width; ++j){
      step[j][i]=INITIAL;
    }   
  }
}

void map(){
  for(int i = 0; i < height; ++i){
    for(int j=0; j < width ; ++j){
      cout << setw(3)<< step[j][i] << " ";
    }
    cout << endl;
  }
}

void mark_n(int n,int x,int y,int dx,int dy){
  int temp;
  for(int i =0; i< 4;i++){
    temp = dx;
    dx = dy;
    dy = -temp;
    if(isValidPoint(x+dx,y+dy) && step[x+dx][y+dy]==INITIAL){
      step[x+dx][y+dy] = n;
    }
  }
}


void mark_n_all(int n){
  for(int i = 0; i < height; i++){
    for(int j = 0; j < width ; j++){
      if(step[j][i]==n){
	//	cout << j <<" " << i << "jツづツづ個値"<< endl;
	mark_n(n+1,j,i,1,0);
      }
    }
  }
}



bool isValidPoint(int x,int y){
  if(x < 0 || x >  width || y < 0 || y > height){
    return false;
  }else{
    return true;
  }
}

C++:   1 #include <bits/stdc++.h>
  2
  3 using namespace std;
  4
  5 int w, h, cnt;
  6 int vx[] = {0, 1, 0, -1};
  7 int vy[] = {1, 0, -1, 0};
  8 char field[20][20];
  9
 10 void solve(int x, int y){
 11
 12     ++cnt;
 13     field[y][x] = '#';
 14
 15     for(int i = 0; i < 4; ++i){
 16
 17         int nx = x + vx[i];
 18         int ny = y + vy[i];
 19
 20         if(nx >= 0 && nx < w && ny >= 0 && ny < h && field[ny][nx] == '.')
 21             solve(nx, ny);
 22
 23     }
 24
 25 }
 26
 27 int main(){
 28
 29     int sx, sy;
 30     while(cin >> w >> h){
 31
 32         if(!(w || h)) break;
 33
 34         for(int i = 0; i < h; ++i){
 35             for(int j = 0; j < w; ++j){
 36                 cin >> field[i][j];
 37                 if(field[i][j] == '@'){
 38                     sx = j;
 39                     sy = i;
 40                 }
 41             }
 42         }
 43
 44         cnt = 0;
 45
 46         solve(sx, sy);
 47
 48         cout << cnt << endl;
 49
 50     }
 51
 52 }
~

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

int tansaku(char tile[20][20],int w1,int h1){
    if(tile[w1][h1]!='.'&&tile[w1][h1]!='@')return 0;
    tile[w1][h1]='t';
    return 1+tansaku(tile,w1+1,h1)+tansaku(tile,w1-1,h1)+tansaku(tile,w1,h1+1)+tansaku(tile,w1,h1-1);
}

int main(){
    int w,h;
    int w1,h1; //スタート地点
   // 標準入力
   while(1){
    std::cin >> w;
    std::cin >> h;
    if(w==0&&h==0)break;
    //配列の初期化
    char tile[20][20]={};
    // 配列を取得.
    for(int i=0; i<h; i++){
        for(int j=0; j<w; j++){
            std::cin >> tile[j][i];
            if(tile[j][i]=='@'){
                w1=j; h1=i;
            }
        }
    }
    std::cout << tansaku(tile,w1,h1) << std::endl;
   }

    return 0;
}