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 <algorithm>
#include <cstdio>
#include <iostream>
#include <string>
#include <vector>
#include <map>
#define rep(i, n) for (int i=0; i<n; ++i)
#define FOR(i, a, b) for (int i=a; i<b; ++i)
#define pb(x) push_back(x)
#define ll long long
#define ul unsigned long long
#define dbg(x) cout  << #x << " : " << (x) << endl
#define out(x) cout << (x)
#define outl(x) cout << (x) << endl
#define int(n); int n; cin >> n;
#define long(n); long long n; cin >> n;
#define dbl(f); double f; cin >> f;
#define s(s); string s; cin >> s;
#define into(a, n) for (int i = 0; i < n; ++i) cin >> a[i]
using namespace std;

const int inf =1500000000;
char a[20][20];
int cnt; 
int w,h; 
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1}; 
void dfs(int x, int y) {
  a[x][y] = '#';
  cnt++;
  rep(i, 4) {
    int nx=x+dx[i];
    int ny=y+dy[i]; 
    if(nx<w&&ny<h&&nx>=0&&ny>=0&&a[ny][nx]=='.') dfs(nx, ny);
  }
}

int main () {
  while(1){
    cnt = 0;
    cin>>w>>h;
    if (!w && !h) break;
    int x0,y0;
    rep(i, h) {
      rep(j, w) {
        cin>>a[i][j];
        if(a[i][j]=='@' ) {
          x0=j; y0=i;
          a[i][j]='.';
        }
      } 
    } 
    dfs(x0, y0);
    outl(cnt);
  }
  return 0;
}

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

#define FOR(i,b,n) for(int i=b;i<n; i++)
#define CLR(mat) memset(mat, 0, sizeof(mat))

using namespace std;

int w, h, sx, sy;
int dx[] = {0, 0, 1, -1}, dy[] = {1, -1, 0, 0};
char field[30][30] = {0};
bool visit[30][30] = {0};

int solve();
void solve_rec(int, int);

int solve()
{
	int res = 0;
	CLR(visit);
	solve_rec(sx, sy);
	
	FOR(j,0,h)
		FOR(i,0,w)
		{
			if( visit[j][i] == true	)
			{
				res++;
			}
		}
	
	return res;
}

void solve_rec(int x, int y)
{
	if( !(x < 0 || x >= w || y < 0 || y >= h
	|| field[y][x] == '#' || visit[y][x] == true ) )
	{
		visit[y][x] = true;
		
		FOR(i, 0, 4)
		{
			solve_rec(x+dx[i], y+dy[i]);
		}
	}
	
	return;
}
	
int main()
{
	while(cin >> w >> h, (w||h))
	{
		FOR(j,0,h)
			FOR(i,0,w)
			{
				cin >> field[j][i];
				
				if( field[j][i] == '@')
				{
					sx = i; sy = j;
				}
			}
			
		cout << solve() << endl;
		
	}
	
	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
typedef long long L;
char grid[25][25];
L w,h,xi,yi,c=1,flg=1;
L X[]={0,1,0,-1},Y[]={1,0,-1,0},mark[25][25];
void dfs(L x,L y){
    mark[x][y]=1;
    for(L i=0;i<4;i++){
        if(X[i]+x<0||X[i]+x>=h || Y[i]+y<0||Y[i]+y>=w || mark[X[i]+x][Y[i]+y] )continue;
        if(grid[X[i]+x][Y[i]+y]=='.'){
            c++;
            dfs(X[i]+x,Y[i]+y);
        }
    }
}
int main(){
    while(scanf("%lld%lld",&w,&h)){flg=1;
        if(w==0&&h==0)return 0;
        for(L i=0;i<h;i++){
           scanf("%s",grid[i]);
           for(L j=0;j<w&&flg;j++){
            if(grid[i][j]=='@'){
                xi=i;yi=j;flg=0;
            }
           }

        }
        dfs(xi,yi);
        cout<<c<<"\n";
        c=1;memset(mark,0,sizeof(mark));
    }
    return 0;
}

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

using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
char fld[20][20];
int visited[20][20];

int main(){

   while(1){
      int W, H;
      cin>> W>> H;
      if(!(H||W)) break;
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
            }
         }
      }

      for(int i=0; i<H; i++)for(int j=0; j<W; j++)visited[i][j]=0;
      queue<pii> Q;
      Q.push(pii(sy, sx));
      int cnt=0;
      visited[sy][sx]=1;
      while(!Q.empty()){
         pii now=Q.front(); Q.pop();
         //visited[now.first][now.second]=1;
         for(pii dd: dxdy){
            int ni=now.first+dd.first;
            int nj=now.second+dd.second;
            if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
            if(fld[ni][nj]=='#') continue;
            if(visited[ni][nj]) continue;
            visited[ni][nj]=1;
            Q.push(pii(ni, nj));
         }
      }

      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=visited[i][j];
      }

      cout<< ans<< endl;
   }

   return 0;
}