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++: //C++14 (Clang 3.8.0)

#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <string>
#define rp(i,a,b) for(int (i)=(int)(a);i<(int)(b);++i)

typedef long long ll;

int mx[]={-1,1,0,0};
int my[]={0,0,-1,1};
using namespace std;
int n=0;
void dfs(int startx,int starty,vector<vector<bool> > &map){
    for(int i=0;i<4;i++){
        int x=startx+mx[i],y=starty+my[i];
        if(x>=0 and y>=0 and y<map.size() and x<map[0].size() and map[y][x]==true){
        map[y][x]=false;
        n++;
        dfs(x,y,map);}
    }
    
}

int main(){
    int x,y;
    while(cin>>x>>y,x|y){
        int startx,starty;
        vector<vector<bool> > map;

        if(x==0 and y==0) return 0;
        for(int i=0;i<y;i++){
            vector<bool> tmp;
            string s;
            cin>>s;
            for(int j=0;j<x;j++){

                if(s[j]=='@') {startx=j;starty=i;tmp.push_back(true);}
                else if(s[j]=='.') tmp.push_back(true);
                else tmp.push_back(false);    
            }
            map.push_back((tmp));
        }
        n=0;
        dfs(startx,starty,map);
        cout<<n<<endl;
        
    }

    return 0;
};

C++: #include<iostream>
#include <functional>
#include<algorithm>
#include <cmath>
#include <string>
#include <map>
using namespace std;

int W,H;
char a[30][30];
void se(int x,int y,int (*re)[30],int *u){
	
	if(x<0||W<=x||y<0||H<=y||a[y][x]=='#')
		return;
	if(re[y][x]==1)
		return;
	
	re[y][x]=1;
		*u+=1;
	
	se(x+1,y,re,u);
	se(x-1,y,re,u);
	se(x,y+1,re,u);
	se(x,y-1,re,u);
}

int main(){
	int x,y;
	int x1,y1;
	while(cin>>W>>H){
		if(W!=0&&H!=0){
			int re[30][30]={0};
			int u=0;
		for(int i=0;i<H;i++){
			for(int j=0;j<W;j++){
			cin>>a[i][j];
				if(a[i][j]=='@'){
				x1=j;
				y1=i;}
			}
		}
			se(x1,y1,re,&u);
				cout<<u<<endl;
		}
		else
			break;
	}

	
}

C++: #include<iostream>
using namespace std;
int w,h,x,y,i,j,k,masu[22][22],checked[22][22],*g,cont;
void check(int xx,int yy){
cont++;
checked[xx][yy]=1;
int m,n;m=1;n=0;
if(checked[xx+m][yy+n]==0 && 
masu[xx+m][yy+n]==0)
check(xx+m,yy+n);
m=0;n=1;
if(checked[xx+m][yy+n]==0 && 
masu[xx+m][yy+n]==0)
check(xx+m,yy+n);
m=-1;n=0;
if(checked[xx+m][yy+n]==0 && 
masu[xx+m][yy+n]==0)
check(xx+m,yy+n);
m=0;n=-1;
if(checked[xx+m][yy+n]==0 && 
masu[xx+m][yy+n]==0)
check(xx+m,yy+n);
}
int main(void){
char a[22];
while(1){
scanf("%d%d",&w,&h);
if(w==0 && h==0)
break;
cont=0;
for(j=0;j<22;j++)
for(i=0;i<22;i++){
masu[i][j]=1;
checked[i][j]=0;
}
for(j=1;j<=h;j++){
scanf("%s",a);
for(i=1;i<=w;i++)
switch(a[i-1]){
case '@':x=i;y=j;
case '.':masu[i][j]=0;break;
}
}
check(x,y);
printf("%d\n",cont);
}
}

C++: #include <bits/stdc++.h>

#define ll long long

using namespace std;


int W,H;

char r[1000][1000];

int sh,sw;
int bnum=0;

int solve(int i,int j){
    int ans=0;

    if(i <0 || H <= i || j < 0 || W <= j)
        return 0;
    if(r[i][j] == '#'){
        return 0;
    }
    ans = 1;
    r[i][j] = '#';
    ans += solve(i-1,j);
    ans += solve(i+1,j);
    ans += solve(i,j-1);
    ans += solve(i,j+1);

    return ans;
}


int main(){

    while(1){
        scanf("%d %d",&W,&H);
        if(W == 0 && H == 0) break;

        for(int i=0;i<H;i++){
            for(int j=0;j<W;j++){
                cin >> r[i][j];
                if(r[i][j] == '@'){
                    sh = i;
                    sw = j;
                    r[i][j] = '.';
                }
            }
        }
        cout << solve(sh,sw) << endl;
    }

}

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

#define BLACK '.'
#define RED '#'
#define ST '@'

int w,h, ans=0;
char t[100][100];

int rec(int x, int y){
  if( x<0 || x>=h ) return 0;
  if( y<0 || y>=w ) return 0;

  if( t[x][y] == RED ) return 0;
  t[x][y] = RED;

  int dx[4] = {0,0,1,-1};
  int dy[4] = {1,-1,0,0};
  int r=1;
  for( int d=0;d<4;d++ )
     r+= rec(x+dx[d], y+dy[d]);
  return r;
}
int main(){
  while( cin>>w >> h, w){
    int si, sj;
    for( int i=0;i<h;i++ )
      for( int j=0;j<w;j++) {
	cin >> t[i][j];
	if( t[i][j] == ST ){
	  si=i; sj=j;
	}
      }
    cout << rec(si,sj) << endl;
  }
}