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;
#define REP(i,n) for(int (i)=0;(i)<(int)(n);(i)++)
int A[30][30];
int x,y;
int dx[4]={0,0,1,-1};
int dy[4]={1,-1,0,0};
int dfs(int x,int y){
    A[x][y]=1;
    int ans=0;
    REP(i,4){
        int xx=dx[i]+x;
        int yy=dy[i]+y;
        if(A[xx][yy]==0){
           ans+=dfs(xx,yy);
        }
    }
    return ans+1;
}

int main(){
    int W,H;
    while(cin>>W>>H,W+H){
        REP(i,30){
            REP(j,30){
                A[i][j]=1;
            }
        }
        REP(i,H){
            string s;
            cin>>s;
            REP(j,W){
                if(s[j]=='.')A[i+1][j+1]=0;
                if(s[j]=='@'){
                   A[i+1][j+1]=0;
                   x=i+1;
                   y=j+1;
                }
            }
        }
        cout<<dfs(x,y)<<endl;
    }
    return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;

int h,w;
char mp[111][111];
int xx[4]={0,1,0,-1};
int yy[4]={-1,0,1,0};
int sx,sy;

int tile(int y,int x){
    if(y<h&&x<w&&y>=0&&x>=0&&mp[y][x]!='#'){
        mp[y][x]='#';
        int t=0;
        for(int i=0;i<4;i++){
            t+=tile(y+yy[i],x+xx[i]);
        }
        return t+1;
    }
return 0;
}

int main(){
    
    while(1){
    
        scanf(" %d %d",&w,&h);
        
        if(w==0&&h==0)break;
        
        for(int i=0;i<h;i++){
            scanf(" %s",mp[i]);
            for(int j=0;j<w;j++){
                if(mp[i][j]=='@'){
                    sx=j;
                    sy=i;
                }
            }
        }
        
        cout << tile(sy,sx) << endl;
        
    }
    
    return 0;
    
}

C++: #include <cstdio>
#include <algorithm>
#include <vector>
using namespace std;

struct UF {
  vector<int> p;
  UF(int N):p(N,-1) {}
  int rt(int x){return p[x]<0?x:p[x]=rt(p[x]);}
  int s(int x){return -p[rt(x)];}
  void cat(int x,int y) {
    if((x=rt(x))==(y=rt(y))) return;
    if(p[x]<p[y])swap(x,y);
    p[y]+=p[x];
    p[x]=y;
  }
};

int main() {
  for(int tci = 0; ; tci++) {
    int W, H; scanf("%d%d", &W, &H);
    if(W==0) break;
    static char tbl[30][30];
    for(int y = 0; y < H; y++) scanf(" %s", tbl[y]);
    UF uf(W*H);
    int k = 0;
    for(int y = 0; y < H; y++) {
      for(int x = 0; x < W; x++) {
        if(tbl[y][x]=='#') continue;
        if(y>0 && tbl[y-1][x]!='#') uf.cat(y*W+x,(y-1)*W+x);
        if(x>0 && tbl[y][x-1]!='#') uf.cat(y*W+x,y*W+(x-1));
        if(tbl[y][x]=='@')k=y*W+x;
      }
    }
    printf("%d\n", uf.s(k));
  }
  return 0;
}