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 <queue>
#include <utility>
#include <stack>
#include <string>
using namespace std;

typedef pair<int,int> pii;

int main(void){
	int h,w;
	queue<pii> s;
	string m[21];
	while(cin>>w>>h,w||h){
		for(int i=0;i<h;i++)cin>>m[i];
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(m[i][j]=='@'){
					s.push(pii(i,j));
					i=j=999;
				}
			}
		}
		int c=0;
		while(!s.empty()){
			pii a=s.front();s.pop();
			int i=a.first;
			int j=a.second;
			if(!(0<=i&&i<h&&0<=j&&j<w))continue;
			if(m[i][j]=='#')continue;
			c++;
			m[i][j]='#';
			s.push(pii(i+1,j));
			s.push(pii(i-1,j));
			s.push(pii(i,j+1));
			s.push(pii(i,j-1));
		}
		cout<<c<<endl;
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
int w,h;
int count; 
int dx[4]={1,0,-1,0};int dy[4]={0,1,0,-1};
char board[100][100];
void dfs(int x,int y){
       count++;
       board[y][x]='#';
    for(int i=0;i<4;i++){
      int nx=x+dx[i];int ny=y+dy[i];
      if(nx>=0&&nx<w&&ny>=0&&ny<h&&board[ny][nx]=='.'){
          dfs(nx,ny);
      }
    }
}
int main(){
	while(1){
  cin>>w>>h;
  if(w==0&&h==0) break;
  count=0;
  for(int k=0;k<h;k++){
    cin>>board[k];
  }
  for(int i=0;i<h;i++){
    for(int j=0;j<w;j++){
      if(board[i][j]=='@'){
         dfs(j,i);
      }
    }
  }
  cout<<count<<endl;
}
  return 0;
}

C++: #include <iostream>
#include <vector>
#include <string>
#include <queue>
using namespace std;
int main() {
  int W, H, x, y, ans, c;
  string s;
  while (cin >> W >> H, W) {
    vector<string> M;
    ans = 0;
    M.push_back(string(W+2, '#'));
    for (int i = 0; i < H; ++i) {
      cin >> s;
      for (int j = 0; j < W; ++j) {
	if (s[j] == '@') {
	  x = i+1;
	  y = j+1;
	  s[j] = '!';
	  break;
	}
      }
      s = '#' + s + '#';
      M.push_back(s);
    }
    M.push_back(string(W+2, '#'));
    queue<vector<int>> que;
    que.push({x, y});
    while (!que.empty()) {
      ++ans;
      x = que.front()[0];
      y = que.front()[1];
      que.pop();
      if (M[x-1][y] == '.') {
	M[x-1][y] = '!';
	que.push({x-1,y});
      }
      if (M[x][y-1] == '.') {
	M[x][y-1] = '!';
	que.push({x,y-1});
      }
      if (M[x+1][y] == '.') {
	M[x+1][y] = '!';
	que.push({x+1,y});
      }
      if (M[x][y+1] == '.') {
	M[x][y+1] = '!';
	que.push({x,y+1});
      }
    }
    cout << ans << endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
char maze[21][21];
#define r(z,x) for(int z=0;z<x;z++)
int ans,a,b;
void bfs(int x,int y){
	int dx[4]={0,-1,0,1},dy[4]={1,0,-1,0};
	maze[x][y]='#',ans++;
	r(i,4)if(maze[x+dx[i]][y+dy[i]]=='.')bfs(x+dx[i],y+dy[i]);
}
int main(){
	while(cin>>a>>b,a||b){
		ans=0;
		fill(maze[0],maze[20],'#');
		r(i,b)r(j,a)cin>>maze[i][j];
		r(i,b)r(j,a)if(maze[i][j]=='@')bfs(i,j),cout<<ans<<endl;
	}
}

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

using namespace std;
using vec = vector<int>;
using mat = vector<vec>;

void init(vec & parent, vec & size)
{
	for (int i = 0; i < parent.size(); ++i)
	{
		parent[i] = i;
		size[i] = 1;
	}
}


int find(vec & parent, int x)
{
	if (parent[x] == x)
	{
		return x;
	}
	return x = find(parent,parent[x]);
}


void unite(vec & parent, vec & size, int x, int y)
{
	x = find(parent, x);
	y = find(parent, y);

	if (x == y)return;

	parent[y] = x;
	size[x] += size[y];
}


int change(int y,int x,int W)
{
	return y*W + x;
}


int main()
{
	while (true)
	{
		int W, H;
		cin >> W >> H;
		if (W == 0 && H == 0)return 0;

		mat color(H, vec(W));
		vec parent(W*H);
		vec size(W*H);

		init(parent, size);

		int start;

		for (int y = 0; y < H; ++y)
		{
			string c;
			cin >> c;

			for (int x = 0; x < W; ++x)
			{
				
				if (c[x] == '.') { 
					
					color[y][x] = 0; 
					
				}
				else if (c[x] == '#') { color[y][x] = 1; }
				else
				{
					color[y][x] = 0;
					start = change(y, x, W);
				}

				if (x != 0 && color[y][x] == color[y][x - 1])
				{
					unite(parent, size, change(y, x, W), change(y, x - 1, W));
				}
				if (y != 0 && color[y][x] == color[y - 1][x])
				{	
					unite(parent, size, change(y, x, W), change(y - 1, x, W));
				}
				
			}
		}

		cout << size[find(parent, start)] << endl;
	}
}

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

const int dy[4] = {-1,0,1,0}, dx[4] = {0,1,0,-1};
int H,W,ans;
char fld[20][20];

void dfs(int y,int x){
    int ny,nx;
    ans++; fld[y][x] = '#';
    for(int i=0;i<4;i++){
        ny = y+dy[i]; nx = x+dx[i];
        if(0<=ny && ny<H && 0<=nx && nx<W && fld[ny][nx]=='.') dfs(ny,nx);
    }
}

int main(){
    int sy,sx;
    while(cin>>W>>H,H||W){
        ans = 0;
        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;
            }
        }
        dfs(sy,sx);
        printf("%d\n",ans);
    }
}