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>

#define REP(i, n) for(int (i)=0; (i)<(n); (i)++)
#define ALL(v) (v).begin(), (v).end()
#define INF 1e9

using namespace std;
typedef pair<int, int> pii;

int w, h;
int m[20][20];
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

void dfs(int nw, int nh){
  REP(i, 4){
    if(nw + dx[i] < 0 or nw + dx[i] >= w or nh + dy[i] < 0 or nh + dy[i] >= h) continue;
    if(m[nw+dx[i]][nh+dy[i]] == 1) { m[nw+dx[i]][nh+dy[i]] = 2; dfs(nw+dx[i], nh+dy[i]); }
  }
  return;
}

int main(){
  while(cin >> w >> h and w + h){
    int sw, sh;
    REP(i, h){
      string str;
      cin >> str;
      REP(j, w) { m[j][i] = (str[j] == '#' ? -1 : 1); if(str[j] == '@') { sw = j; sh = i; m[j][i] = 2; } }
    }
    dfs(sw, sh);
    int res = 0;
    REP(i, h) REP(j, w) if(m[j][i] == 2) res++;
    cout << res << endl;
  }
  return 0;
}

C++: #define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstring>
#include <cmath>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<ll, int> ll_i;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int u, v; ll w; };

ll MOD = 1000000007;
ll _MOD = 1000000009;
double EPS = 1e-10;

int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, -1, 0, 1};

bool hoge(vector<string>& s) {
	int H = s.size(), W = s[0].size();
	bool flag = false;
	for (int y = 0; y < H; y++)
		for (int x = 0; x < W; x++)
			if (s[y][x] == '.')
				for (int k = 0; k < 4; k++) {
					int _x = x + dx[k], _y = y + dy[k];
					if (_x >= 0 && _y >= 0 && _x < W && _y < H && s[_y][_x] == '@') {
						s[y][x] = '@';
						flag = true;
					}
				}
	return flag;
}

int main() {
	for (;;) {
		int W, H; cin >> W >> H;
		if (W == 0 && H == 0) break;
		vector<string> s(H);
		for (int i = 0; i < H; i++) cin >> s[i];
		while (hoge(s));
		int cnt = 0;
		for (int y = 0; y < H; y++)
			for (int x = 0; x < W; x++)
				if (s[y][x] == '@') cnt++;
		cout << cnt << endl;
	}
}

C++: #include <iostream>

using namespace std;
const int dx[] = {1,0,-1,0};
const int dy[] = {0,1,0,-1};

int w,h;
char fie[21][21];

void dfs(int x, int y){
    for(int i=0; i<4; i++){
        int xx = x + dx[i];
        int yy = y + dy[i];
        if(0 <= xx && xx < w && 0 <= yy && yy < h && fie[yy][xx] == '.'){
            fie[yy][xx] = '@';
            dfs(xx,yy);
        }
    }
}

int main(){
    while(cin >> w >> h , w||h){
        int sx,sy;
        for(int i=0; i<h; i++){
            cin >> fie[i];
            for(int j=0; j<w; j++){
                if(fie[i][j] == '@'){
                    sx = j; sy = i;
                }
            }
        }
        dfs(sx,sy);
        int ans = 0;
        for(int i=0; i<h; i++){
            for(int j=0; j<w; j++){
                if(fie[i][j] == '@') ans++;
            }
        }
        cout << ans << endl;
    }
}

C++: #include <bits/stdc++.h>
using namespace std;
 
#define REP(i, s, n) for(int i = s; i < n; ++i)
#define rep(i, n)		 REP(i, 0, n)
#define SORT(c)			 sort((c).begin(), (c).end())
#define IINF					INT_MAX
#define LLINF					LLONG_MAX
#define DEBUG					false
 
typedef long long				ll;
typedef pair <int, int> ii;

int dx[] = {1, 0, -1, 0};
int dy[] = {0, 1, 0, -1};

int main() {
	
	int w, h;
	
	while(cin >> w >> h, w, h){
		vector<string> mp(h);
		//vector<vector<bool> > used(h, vector<int>(w));
		rep(i, h) cin >> mp[i];
		
		int sx, sy;
		rep(i, h){
			rep(j, w) if(mp[i][j] == '@'){
				sx = j;
				sy = i;
			}
		}
		
		queue<ii> qu;
		qu.push(make_pair(sx, sy));
		
		int ans = 0;
		while(!qu.empty()){
			ii tmp = qu.front();
			qu.pop();
			ans++;
			
			
			rep(i, 4){
				int nx = tmp.first + dx[i];
				int ny = tmp.second + dy[i];
			
				if(nx < 0 || w <= nx) continue;
				if(ny < 0 || h <= ny) continue;
				
				if(mp[ny][nx] == '.'){
					qu.push(make_pair(nx, ny));
					mp[ny][nx] = '#';	
				}
				
			}
			
			
		}
		
		cout << ans << endl;
		
		
	}
	
	
	
	return 0;
}