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 <queue>

using namespace std;

struct locate{
  int x;
  int y;
};
 
char graph[20][20];
int cou = 0;
int H,W;

void bfs(locate z){
  queue<locate> Q;
  locate u;
  locate v;
  Q.push(z);
 

  while(!Q.empty()){

    u = Q.front();Q.pop();

    if(graph[u.y + 1][u.x] == '.' && graph[u.y + 1][u.x] != 'N'){
      graph[u.y + 1][u.x] = 'N';
      cou++;
      v.y = u.y + 1;
      v.x = u.x;
      Q.push(v);
    }

    if(graph[u.y][u.x + 1] == '.' && graph[u.y][u.x + 1] != 'N'){
      graph[u.y][u.x + 1] = 'N';
      cou++;
      v.x  = u.x + 1;
      v.y = u.y;
      Q.push(v);
    }

    if(graph[u.y - 1][u.x] == '.' && graph[u.y - 1][u.x] != 'N'){
      graph[u.y - 1][u.x] = 'N';
      cou++;
      v.x = u.x;
      v.y = u.y - 1;
      Q.push(v);
    }

    if(graph[u.y][u.x - 1] == '.' && graph[u.y][u.x - 1] != 'N'){
      graph[u.y][u.x - 1] = 'N';
      cou++;
      v.x = u.x - 1;
      v.y = u.y;
      Q.push(v);
    }
    
  }
  cout << endl;
    for(int i = 0; i< H ;i++){
      for(int j = 0; j < W; j++){
	cout << graph[i][j];
      }
      cout << endl;
      }
}

int main(){

  int a1,a2;
  locate a;
  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 >> graph[i][j];
	if(graph[i][j] == '@'){
	  a1 = i;
	  a2 = j;
	}
      }
    }

    /*
      for(int i = 0; i< H ;i++){
      for(int j = 0; j < W; j++){
      cout << graph[i][j];
      }
      cout << endl;
      }
    */

    a.y = a1;
    a.x = a2;
    bfs(a);

    cout << cou+1 << endl;

    cou = 0;
    for(int i = 0;i < H ;i++){
      for(int j = 0;j < W; j++){
	graph[i][j] = 'N';
      }
    }
    cou = 0;
  }
  return 0;
}

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];
    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;
}

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


int main(){
    std::vector<int> row;
    int prev_col, width, height, max = 0, last = 0;
    bool connect_prev_row;
    
    std::cin >> width >> height;
    row.resize(width+1);
    for( int i = 0; i <= width; i++ ) row[i] = 0;

    std::string line;
    while( true ){
        std::cin >> line;
        if( line == "00" ) {
            std::cout << max;
            break;
        }
        connect_prev_row = false;
        for(std::string::size_type i = 0; i < line.size(); i++ ){
            if( line[i] == '.' ){
                if( connect_prev_row ){
                    if( row[i] > 0 ) row[i+1] = row[i] + 1;
                    else row[i+1] = row[i] - 1;
                } else {
                    connect_prev_row = true;
                    if( row[i] <= 0 && row[i+1] <= 0 ) row[i+1] = row[i+1] + row[i] - 1;
                    else row[i+1] = std::abs(row[i+1]) + std::abs(row[i]) + 1;
                }
            } else if( line[i] == '@' ) {
                if ( connect_prev_row ) row[i+1] = - row[i] + 1;
                else {
                    connect_prev_row = true;
                    row[i+1] = - row[i+1] - row[i] + 1;
                }
            } else {
                row[i+1] = 0;
                connect_prev_row = false;
            }
            max = max > row[i+1] ? max : row[i+1];
        }
        last = 0;
        for( int i = line.size(); i > 0; i-- ) {
            if( row[i] == 0 ) last = 0;
            else if( row[i] > 0 ){
                if(  last == 0 ) last = row[i];
                else row[i] = last;
            } else {
                if(  last == 0 ) last = row[i];
                else row[i] = last;
            } 
        }
    }
    return 0;
}