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<string>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
using namespace std;
typedef long long ll;
const ll MOD = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
typedef pair<int, int> P;
typedef unsigned int ui;
int main(){
	int w, h;
	string s;
	int i, j;
	while (cin >> w >> h) {
		if (w == 0)break;
		char tile[22][22]; int used[22][22];
		for (i = 0; i < 22; i++) {
			for (j = 0; j < 22; j++) {
				used[i][j] = 0; tile[i][j] = '#';
			}
		}
		vector<int> q1,q2;
		for (j = 1; j <= h; j++) {
			cin >> s;
			for (i = 1; i <= w; i++) {
				tile[i][j] = s[i - 1];
				if (s[i - 1] == '@') {
					q1.push_back(i); q2.push_back(j);
				}
			}
		}
		while (!q1.empty()) {
			int x = q1[0]; int y = q2[0]; used[x][y]=1;
			q1.erase(q1.begin() + 0); q2.erase(q2.begin() + 0);
			if (used[x - 1][y] == 0 && tile[x - 1][y]=='.') {
				q1.push_back(x - 1); q2.push_back(y);
			}
			if (used[x + 1][y] == 0 && tile[x + 1][y] == '.') {
				q1.push_back(x + 1); q2.push_back(y);
			}
			if (used[x][y-1] == 0 && tile[x][y-1] == '.') {
				q1.push_back(x); q2.push_back(y-1);
			}
			if (used[x][y+1] == 0 && tile[x][y+1] == '.') {
				q1.push_back(x); q2.push_back(y+1);
			}
		}
		int count = 0;
		for (i = 0; i < 22; i++) {
			for (j = 0; j < 22; j++) {
				if (used[i][j] == 1)count++;
			}
		}
		cout << count << endl;
	}
	return 0;
}

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

using namespace std;

typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> VI;
typedef vector<VI> VVI;
const double PI = 3.14159265358979323846;
const double EPS = 1e-12;
const int INF = numeric_limits<int>::max() / 2;
const int NEG_INF = numeric_limits<int>::min() / 2;

char tiles[21][21];
int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
int ans=0;
int w,h;

void dfs(int x,int y){
    tiles[x][y]='#';
    ans++;

    for(int i=0;i<4;i++){
        int nx=x+dx[i],ny=y+dy[i];

        if(0<=nx && nx<w && 0<=ny && ny<h &&
          tiles[nx][ny] == '.') dfs(nx,ny);
    }
}

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

  while(true){
    for(int i=0;i<21;i++){
      for(int j=0;j<21;j++){
        tiles[i][j]='#';
      }
    }
    ans=0;
    cin >> w >> h;
    if(w==0) break;
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
        cin >> tiles[j][i];
      }
    }
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
        if(tiles[j][i]=='@') dfs(j,i);
      }
    }
    cout << ans << endl;
  }
}

C++: #include<stdio.h>

int m,n,ans;
char ice[22][22];

void saiki(int w,int h){
	ice[w+1][h+1]='*';
	
	if(ice[w+2][h+1]=='.')saiki(w+1,h);
	if(ice[w+1][h+2]=='.')saiki(w,h+1);
	if(ice[w][h+1]=='.')saiki(w-1,h);
	if(ice[w+1][h]=='.')saiki(w,h-1);
}

int main(void){

	for(;;){
		scanf("%d %d",&m,&n);
		if(!m||!n)break;
		
		ans=0;
		
		for(int i=0;i<n;i++){
			scanf("%s",&ice[i+1]);
		}
		
		for(int i=0;i<n;i++){
			for(int j=0;j<m;j++){
				if(ice[i+1][j+1]=='@')saiki(i,j);
			}
		}
		for(int i=0;i<n;i++){
			for(int j=0;j<m;j++){
				if(ice[i+1][j]=='*')ans++;
			}
		}
		printf("%d\n",ans);
	}
	return 0;
}

C++: #define _USE_MATH_DEFINES
#define INF 100000000

#include <iostream>
#include <sstream>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <stack>
#include <limits>
#include <map>
#include <string>
#include <cstring>
#include <set>
#include <deque>
#include <bitset>
#include <list>
#include <cctype>
#include <utility>

using namespace std;

typedef long long ll;
typedef pair <int,int> P;
typedef pair <int,P> PP;

static const double EPS = 1e-8;

const int tx[] = {0,1,0,-1};
const int ty[] = {1,0,-1,0};

char stage[20][20];

int main(){
	int W,H;
	while(~scanf("%d %d",&W,&H)){
		if(W==H && H==0) break;
		for(int y=0;y<H;y++){
			char buf[32];
			scanf("%s",buf);
			for(int x=0;x<W;x++){
				stage[y][x] = buf[x];
			}
		}

		int sx=0;
		int sy=0;
		for(int y=0;y<H;y++){
			for(int x=0;x<W;x++){
				if(stage[y][x] == '@'){
					sx = x;
					sy = y;
					goto found;
				}
			}
		}
found:;
		queue<P> que;
		que.push(P(sx,sy));

		int res = 0;
		bool visited[20][20];
		memset(visited,0,sizeof(visited));

		visited[sy][sx] = true;
		while(!que.empty()){
			int sx = que.front().first;
			int sy = que.front().second;
			res++;
			que.pop();
			
			for(int i=0;i<4;i++){
				int dx = sx + tx[i];
				int dy = sy + ty[i];

				if(dx < 0 || dx >= W || dy < 0 || dy >= H) continue;
				if(stage[dy][dx]=='#') continue;
				if(visited[dy][dx]) continue;

				visited[dy][dx] = true;
				
				que.push(P(dx,dy));
			}
		}

		printf("%d\n",res);

	}
}