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>
#include<vector>
#include<utility>
#include<queue>
#include<algorithm>
#include<cmath>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<complex>
#define INF 2147483647
#define llINF 9223372036854775807
#define pb push_back
#define mp make_pair 
#define F first
#define S second
#define ll long long

using namespace std;
int main(){
  int w,h;
  int vx[4]={1,0,-1,0};
  int vy[4]={0,-1,0,1};
  while(cin>>w>>h,h+w){
    string str[h];
    for(int i=0;i<h;i++)cin>>str[i];
    bool Map[h][w];
    for(int i=0;i<h;i++)
      for(int j=0;j<w;j++)
	Map[i][j]=false;
    queue<pair<int,int> >que;
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	if(str[i][j]=='@'){
	  que.push(mp(i,j));
	  Map[i][j]=true;
	}
      }
    }
    while(!que.empty()){
      pair<int,int>now=que.front();
      que.pop();
      for(int i=0;i<4;i++){
	if(now.F+vy[i]>=0&&now.S+vy[i]<w&&now.F+vx[i]>=0&&now.F+vx[i]<h&&(!Map[now.F+vx[i]][now.S+vy[i]])&&str[now.F+vx[i]][now.S+vy[i]]=='.'){
	  Map[now.F+vx[i]][now.S+vy[i]]=true;
	  que.push(mp(now.F+vx[i],now.S+vy[i]));
	}
      }
    }
    ll ans=0;
    for(int i=0;i<h;i++)
      for(int j=0;j<w;j++)
	if(Map[i][j])
	  ans++;
    cout<<ans<<endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
const int inf = 1001001001;


int w, h;
int a[20][20];
int sw, sh;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
int D[20][20];
int Ans = 1;


int main()
{
    while(cin >> w, w != 0)
    {
        Ans = 1;
        cin >> h;
        
        for(int i = 0; i < 20; i++)
        {
            for(int j = 0; j < 20; j++)
            {
                D[i][j] = inf;
            }
        }
        
        for(int i = 0; i < h; i++)
        {
            for(int j = 0; j < w; j++)
            {
                char x;
                cin >> x;
                if(x == '.')
                {
                    a[j][i] = 1;
                }
                else if(x == '#')
                {
                    a[j][i] = 0;
                }
                else
                {
                    sw = j;
                    sh = i;
                    a[j][i] = 100;
                }
            }
        }
        queue< pair<int, int> > Q;
        pair<int, int> S;
        S = make_pair(sw, sh);
        Q.push(S);
        D[sw][sh] = 0;
        int k = 0;
        while(! Q.empty())
        {
            pair<int, int> cur = Q.front();
            Q.pop();
            int x = cur.first;
            int y = cur.second;
            for(int i = 0; i < 4; i++)
            {
                if(a[x+dx[i]][y+dy[i]] == 1 && 0 <= x+dx[i] && x+dx[i] < w && 0 <= y+dy[i] && y+dy[i] < h && D[x+dx[i]][y+dy[i]] == inf)
                {
                    D[x+dx[i]][y+dy[i]] = D[x][y] + 1;
                    Ans = Ans + 1;
                    pair<int, int> T;
                    T = make_pair(x+dx[i], y+dy[i]);
                    Q.push(T);
                }
            }
        }
        cout << Ans << endl;
    }
}

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

using namespace std;

/*??°?????????????????°*/
int W, H, ans;
int tile[20][20];

/*??¢??°??????????????????*/
void DPS(int x, int y);

int main() {
	int sx = 0, sy = 0;
	string str;

	while (cin >> W >> H, W, H) {
		/*??\???*/
		for (int i = 0; i < H; i++) {
			cin >> str;
			for (int j = 0; j < W; j++) {
				tile[i][j] = (str[j] == '#') ? 0 : 1;
				
				if (str[j] == '@') {
					sx = j;
					sy = i;
				}
			}
		}

		ans = 0;
		DPS(sx, sy);

		cout << ans << endl;
	}
}

void DPS(int x, int y) {
	if (tile[y][x]) {
		tile[y][x] = 0;
		ans++;
		
		if (x - 1 >= 0)	DPS(x - 1, y);
		if (x + 1 < W)	DPS(x + 1, y);
		if (y - 1 >= 0)	DPS(x, y - 1);
		if (y + 1 < H)	DPS(x, y + 1);
	}

	return;
}