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 <string>

using namespace std;

int N = 0;
int map[21][21] = { 0 }; //map[w][h]
int w, h;

int check(int cw, int ch){
//	cout << cw << "cheching" << ch << endl;
	if (  (cw >= 1) && (cw <= w) && (1 <= ch) && (ch <= h) ){
		if (map[cw][ch] == 0){
			map[cw][ch] = 2;
			N++;
			check(cw + 1, ch);
			check(cw, ch + 1);
			check(cw - 1, ch);
			check(cw, ch - 1);
		}
	}
	return(0);
}


int main(void){
	int i, j;
	int sw, sh;

	string input;
	cin >> w >> h;

	while (w != 0 && h != 0){
	
		for (i = 0; i < h; i++){
			cin >> input;
			for (j = 0; j < w; j++){
				if (input[j] == '.') map[j + 1][i + 1] = 0;
				else{	if(input[j] == '#') map[j + 1][i + 1] = 1;
						else{
							map[j + 1][i + 1] = 2;
							sw = j + 1; sh = i + 1;
						}
				}
			}
		}



		map[sw][sh] = 0;

/*		for (i = 0; i < h; i++){
			for (j = 0; j < w; j++){
				cout << map[j + 1][i + 1];
			}
			putchar('\n');
		}
		*/
		N = 0;
		check(sw,sh);

/*		for (i = 0; i < h; i++){
		for (j = 0; j < w; j++){
		cout << map[j + 1][i + 1];
		}
		putchar('\n');
		}
*/
		cout << N << endl;
		cin >> w >> h;
	}



	return(0);
}

C++: #if 0
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???????????????????????°????????????????±?legal??¢??°?????????????????¨?????????????????????
x<h???y<w???????????????????????????
#endif

#include <iostream>
#include <queue>
using namespace std;
int w,h;
int g[21][21];
bool visited[21][21];
int moves[4][2]={{-1,0},{1,0},{0,-1},{0,1}};
int count;
bool legal(int x, int y)
{
  if(visited[x][y]!=true&&0<=x&&x<h&&0<=y&&y<w&&g[x][y]==0)
    return true;
  else
    return false;
}

void dfs(int x,int y)
{
  visited[x][y]=true;
  count++;
  for(int i=0;i<4;i++)
  {
    int newx=x+moves[i][0];
    int newy=y+moves[i][1];
    if(legal(newx,newy))
    {
      dfs(newx,newy);
    }
  }
}

int main()
{
  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++)
      {
        g[i][j]=-1;
        visited[i][j]=false;
      }
    int psx,psy;
    for(int i=0;i<h;i++)
      for(int j=0;j<w;j++)
      {
        char s;
        cin>>s;
        switch(s)
        {
          case '.':
            g[i][j]=0;break;
          case '#':
            g[i][j]=1;break;
          case '@':
            g[i][j]=2;psx=i;psy=j;break;
        }
      }
    count=0;
    dfs(psx,psy);
/*    int count=0;
    for(int i=0;i<h;i++)
      for(int j=0;j<w;j++)
      {
        count+=visited[i][j];
      }
*/  cout<<count<<endl;

  }
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
//bool c[22][22] = {};
int cnt,w, h, p[22][22] = {};

int main() {
	int j, f,cx,cy;
	char in[21];
	while (1) {
		for (int i = 0; i < 484; i++) {
			p[i / 22][i % 22] = 0;
		}
		cnt = 0;
		f = 1;
		scanf("%d %d", &w, &h);
		if (w ==0&& h == 0)
			break;
		scanf("\n", &j);
		for (int i = 0; i < h; i++) {
			scanf("%s", &in);
			for (j = 0; j < w; j++) {
				switch (in[j]) {
				case '#':
//					c[j+1][i+1] = 0;
					p[j+1][i+1] = -1;
					break;
				case '@':
					cx = j;
					cy = i;
					p[j+1][i+1] = 1;
//				case '.':
//					c[j+1][i+1] = 1;
				}
			}
		}

		/*for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++)
				printf("%3d", c[j][i]);
			printf("\n");
		}*/

		while (f == 1) {
			f = 0;
		/*	for (int i = 0; i < h; i++) {
				for (int j = 0; j < w; j++)
					printf("%3d", p[j+1][i+1]);
				printf("\n");
			}
			printf("\n");
*/			//if (w != 1 && h != 1) {
				for (int i = 0; i < w*h; i++) {
					if (p[i / h+1][i%h+1] == 0 && (p[i / h + 2][i % h+1] == 1 || p[i / h][i % h+1] == 1 || p[i / h+1][i % h + 2] == 1 || p[i / h+1][i % h] == 1)) {
						p[i / h+1][i%h+1] = 1;
						f = 1;
					}
				}
			//}
			/*if (w == 1 && h != 1) {
				for (int i = 0; i < h; i++) {
					if (p[1][i+1] == 0 && (p[1][i + 2] == 1 || p[0][i] == 1)) {
						p[1][i+1] = 1;
						f = 1;
						printf("a");
					}
				}
			}

			if (w != 1 && h == 1) {
				for (int i = 0; i < w; i++) {
					if (p[i+1][1] == 0 && (p[i + 2][1] == 1 || p[i][1] == 1)) {
						p[i+1][1] = 1;
						f = 1;
					}
				}
			}*/
		}

		for (int i = 0; i < w*h; i++) {
			if (p[i / h+1][i%h+1] == 1)
				cnt++;
		}
		printf("%d\n", cnt);
	}
	return 0;
}