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>
using namespace std;
char tile[21][21];

int dx[]={1,-1,0,0};
int dy[]={0,0,1,-1};

int sum=0;

void check(int x,int y)
{
  if(tile[x][y]=='#')
    return;
  sum++;
  tile[x][y]='#';
  for(int i=0;i<4;i++)
    {
      if(x+dx[i]>=0 &&x+dx[i]<21 &&y+dy[i]>=0 &&y+dy[i]<21)
	{
	  check(x+dx[i],y+dy[i]);
	}
    }
}

int main()
{
  int w,h;
  for(;cin>>w>>h,w||h;)
    {
      for(int i=0;i<21;i++)
	  for(int j=0;j<21;j++)
	      tile[i][j]='#';
      
      int x,y;
      for(int i=0;i<h;i++)
	for(int j=0;j<w;j++)
	  {
	    cin>>tile[j][i];
	    if(tile[j][i]=='@')
	      {
		x=j,y=i;
	      }
	  }
      sum=0;
      check(x,y);
      cout<<sum<<endl;
      /*
      for(int i=0;i<h;i++,cout<<endl)
	for(int j=0;j<w;j++)
	  {
	    cout<<tile[j][i];
	  }
      */
    }
}

C++: #include <bits/stdc++.h>
#define INF 1e18
#define int long long
#define Rep(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n) Rep(i, 0, n)
#define all(a) (a).begin(), (a).end()
using namespace std;
typedef pair<int, int> P;
typedef pair<int, P> PP;
const int mod = 1000000007;
//const int mod = 998244353;

int h, w;
int dy[] = {0, 1, 0, -1}, dx[] = {1, 0, -1, 0};

bool in(int y, int x) { return 0 <= y && y < h && 0 <= x && x < w; }

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    while (1)
    {
        cin >> w >> h;
        if (w == 0 && h == 0)
            break;
        vector<string> s(h);
        rep(i, h) cin >> s[i];
        queue<P> que;
        int vis[30][30] = {};
        rep(i, h) rep(j, w) if (s[i][j] == '@')
        {
            que.push({i, j});
            vis[i][j] = 1;
        }
        int ans = 1;
        while (!que.empty())
        {
            P p = que.front();
            que.pop();
            int y = p.first, x = p.second;
            rep(i, 4)
            {
                int ny = y + dy[i], nx = x + dx[i];
                if (!in(ny, nx) || vis[ny][nx] || s[ny][nx] == '#')
                    continue;
                vis[ny][nx] = 1;
                ans++;
                que.push({ny, nx});
            }
        }
        cout << ans << endl;
    }
}

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

using namespace std;

int dxdy[4][2] = { {-1,0}, {1,0}, {0,1}, {0,-1} };

int fill(vector<vector<char> >& v, int mi, int mj){
    int sum = 0;
    for(int i=0; i < 4; ++i){
        if(v[ mi+dxdy[i][0] ][ mj+dxdy[i][1] ] == '.'){
            sum++;
            v[mi+dxdy[i][0]][mj+dxdy[i][1]] = ':';
            sum += fill(v, mi + dxdy[i][0], mj + dxdy[i][1]);
        }
    }
    return sum;
}

int main(){
    int w, h;
    int mi, mj;
    char c;
    while(true){
        cin >> w;
        cin >> h;
        if(w == 0 && h == 0) break;
        vector<vector<char> > v(w+2, vector<char>(h+2));
        for(int j = 0; j <= h+1; ++j){
            for(int i = 0; i <= w+1; ++i){
                if(i == 0 || j == 0 || i == w+1 || j == h+1 ){
                    v[i][j] = '#';
                } else {
                    cin >> c;
                    if(c == '@') {
                        mi = i, mj = j;
                        c = ':';
                    }
                    v[i][j] = c;
                }
            }
            if( j >= 1 && j <= h ) cin.ignore();
        }

        cout << fill(v, mi, mj) + 1 << endl;
    }
}

C++: #include<cstdio>
#include<cstring>
#include<vector>
#include<queue>
#include<algorithm>
#include<cmath>
#include<climits>
#include<string>
#include<set>
#include<numeric>
#include<map>
#include<iostream>
using namespace std;
#define rep(i,n) for(int i = 0;i<((int)(n));i++)
#define reg(i,a,b) for(int i = ((int)(a));i<=((int)(b));i++)
#define irep(i,n) for(int i = ((int)(n)-1);i>=0;i--)
#define ireg(i,a,b) for(int i = ((int)(b));i>=((int)(a));i--)
typedef long long ll;
typedef pair<ll, ll> mp;

int W,H,f[20][20],ans[50],cnt=0,sx,sy;
string m[20],t,s;

void find(int x,int y){
	if(x>=W || x<0 || y>=H || y<0)return;
	t = m[x][y];
	if(t=="#")return;
	if(f[x][y]!=0)return;
	f[x][y] = 1;
	ans[cnt]++;
	find(x+1,y);
	find(x-1,y);
	find(x,y+1);
	find(x,y-1);
}

int main(void){
	while(1){
		cin>>W>>H;
		if(W==0 && H==0)break;
		ans[cnt]=0;
		rep(i,H){
			cin>>s;
			rep(j,W){
				f[j][i]=0;
				m[j][i]=s[j];
				t=m[j][i];
				if(t=="@"){
					sx=j;
					sy=i;
				}
			}
		}
		find(sx,sy);
		cnt++;
	}
	rep(i,cnt)cout<<ans[i]<<endl;
	return 0;
}