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 <bits/stdc++.h>
using namespace std;

const int dx[] = {0, 0, -1, 1};
const int dy[] = {1, -1, 0, 0};
int W, H;
char board[30][30];

int dfs(int y, int x) {
   board[y][x] = '#';
   int ret = 1;
   for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx >= 0 && nx < W && ny >= 0 && ny < H && board[ny][nx] == '.') {
         ret += dfs(ny, nx);
      }
   }
   return ret;
}

int main() {
   while (cin >> W >> H, W) {
      for (int i = 0; i < H; i++) {
         cin >> board[i];
      }
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (board[i][j] == '@') {
               cout << dfs(i, j) << endl;
            }
         }
      }
   }

   return 0;
}

C++: #include <iostream>

using namespace std;


void search(int h, int w, int tairu[22][22], int check[22][22]){
    if(check[h][w]==1)return;
    if(tairu[h][w]==0)return;
    
    check[h][w]=1;
    search(h+1,w,tairu,check);
    search(h-1,w,tairu,check);
    search(h,w+1,tairu,check);
    search(h,w-1,tairu,check);
}






int main()
{

    int H,W;
    while(cin>>W>>H){
        int tairu[22][22]={0};
        int start_w, start_h;
        for(int i=1; i<=H; i++){
            for(int j=1; j<=W; j++){
                char input;
                cin>>input;
                if(input=='.')tairu[i][j]=1;
                if(input=='@'){
                    tairu[i][j]=1;
                    start_w=j;
                    start_h=i;
                }
            }
        }
        //
        
        int check[22][22]={0};
        search(start_h, start_w, tairu, check);
        int ans=0;
        for(int i=1; i<=H; i++){
            for(int j=1; j<=W; j++){
                ans+=check[i][j];
            }
        }
        cout<<ans<<endl;
        
        
        
    }
}

C++: #include <map>
#include <set>
#include <list>
#include <cstdio>
#include <cmath>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <cctype>
#include <string>
#include <vector>
#include <cstdlib>
#include <cstring>
#include <iomanip>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <functional>

using namespace std;

#define fr first
#define sc second
#define mp make_pair

typedef long long int64;
typedef pair< int, int > iP;
typedef pair< iP, int > iiP;

const int INF = 2 << 28;
const double EPS = 1e-10;

int H, W;
char mas[21][21];

int dfs(int x, int y) {
    static int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
    int ret = 1;
    mas[y][x] = '#';
    for(int i = 0; i < 4; i++) {
        int nx = x + dx[i], ny = y + dy[i];
        if(0 <= nx && nx < W && 0 <= ny && ny < H && mas[ny][nx] == '.') {
            ret += dfs(nx, ny);
        }
    }
    return ret;
}

int main() {

    while(cin >> W >> H, W) {

        int sx, sy;
        for(int i = 0; i < H; i++) {
            for(int j = 0; j < W; j++) {
                cin >> mas[i][j];
                if(mas[i][j] == '@') sx = j, sy = i;
            }
        }
        cout << dfs(sx, sy) << endl;
    }
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <cctype>
#include <algorithm>
#include <string>
#include <queue>
#include <cstring>
using namespace std;

char map[21][21];
int w, h;
int tile = 0;

void dfs(int x, int y){
    if (!(0 <= x && x < w && 0 <= y < h)) return;
    tile++;
    map[y][x] = '#';
    if (map[y-1][x] == '.') dfs(x, y-1);
    if (map[y+1][x] == '.') dfs(x, y+1);
    if (map[y][x-1] == '.') dfs(x-1, y);
    if (map[y][x+1] == '.') dfs(x+1, y);
}

int main(){
    int sx, sy;
    while (1){
	scanf("%d %d", &w, &h);
	if (w == 0 && h == 0) break;
	memset(map, '#', sizeof(map));
	tile = 0;
	for (int i = 0; i < h; i++){
	    cin >> map[i];
	    for (int p = 0; p < w; p++){
		if (map[i][p] == '@'){
		    sx = p;
		    sy = i;
		}
	    }
	}
	dfs(sx, sy);
	printf("%d\n", tile);
    }
    
    return 0;
}

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

using namespace std;

char a[21][21];
int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};
int count;

int W, H;

void dfs(int x, int y)
{
    a[x][y] = 0;
    for (int i = 0; i < 4; i++)
    {
        int nx = x + dx[i];
        int ny = y + dy[i];
        if (0 <= nx && nx < H && 0 <= ny && ny < W && a[nx][ny] == '.')
        {
            count++;
            dfs(nx, ny);
        }
    }
}

int main()
{
    while (cin >> W >> H, W && H)
    {
        count = 1;
        for (int i = 0; i < H; i++)
        {
            cin >> a[i];
        }
        for (int i = 0; i < H; i++)
        {
            for (int j = 0; j < W; j++)
            {
                if (a[i][j] == '@')
                {
                    dfs(i, j);
                    break;
                }
            }
        }
        cout << count << endl;
    }
}