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
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <queue>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

//math
//-------------------------------------------
template<class T> inline T sqr(T x) {return x*x;}

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1000000007;

//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

int ans;
char c[40][40];
int a,b;


void bfs(int sx, int sy) {
	queue<pair<int,int>> q;
	queue<PII> qp;
	q.push(MP(sx,sy));
	c[sx][sy] = 'x';
	for (int i = 1; i < 1000; i++) {
		while (!q.empty()) {
			PII v = q.front(); //get value
			q.pop();
			int x = v.first, y = v.second;
			if (c[x+1][y] == '.'){ qp.push(MP(x+1,y)); c[x+1][y] = 'x'; ans++; }
			if (c[x-1][y] == '.'){ qp.push(MP(x-1,y)); c[x-1][y] = 'x'; ans++; }
			if (c[x][y+1] == '.'){ qp.push(MP(x,y+1)); c[x][y+1] = 'x'; ans++; }
			if (c[x][y-1] == '.'){ qp.push(MP(x,y-1)); c[x][y-1] = 'x'; ans++; }

		}
		while (!qp.empty()) {
			q.push(qp.front());
			qp.pop();
		}
//		for (int h = 0; h < b+2; h++) {
//			for (int w = 0; w < a+2; w++) {
//				cout << c[w][h];
//			}
//			cout << endl;
//		}
//		cout << endl;

	}
}

int main(int argc, char const* argv[])
{
	while (true) {
		int sx,sy;
		cin >> a >> b;
		if (a == 0 && b == 0) {
			break;
		}
		for (int h = 0; h < b+2; h++) {
			for (int w = 0; w < a+2; w++) {
				if (h == 0 || w == 0 || h == b+1 || w == a+1) {
					c[w][h] = '#';
				}
				else {
					cin >> c[w][h];
					if (c[w][h] == '@') {
						sx = w;
						sy = h;
					}
				}
			}
		}

		ans = 1;
		bfs(sx,sy);
		cout << ans << endl;
	}
	return 0;

}

C++: #include "bits/stdc++.h"
using std::cin;
using std::cout;

int cnt = 0;
char map[21][21];

void dfs(int x, int y, int mx, int my){
    if((x < 0) || (x >= mx)){
        return;
    }
    if((y < 0) || (y >= my)){
        return;
    }
    if(map[x][y] == '#'){
        return;
    }
    map[x][y] = '#';
    cnt++;
    dfs(x+1, y, mx, my);
    dfs(x-1, y, mx, my);
    dfs(x, y+1, mx, my);
    dfs(x, y-1, mx, my);
}

int main(){
    int W, H, sx, sy;
    cin >> W >> H;

    while(W != 0 && H != 0){
        for(int i = 0; i < H; i++){
            scanf("%20s", map[i]);
            if(std::string(map[i]).find('@') != std::string::npos){
                sx = i;
                sy = std::string(map[i]).find('@');
            }
        }
        dfs(sx, sy, H, W);
        cout << cnt << std::endl;

        cin >> W >> H;
        cnt = 0;
        
    }
}

C++: #include <bits/stdc++.h>
using namespace std;
int h, w;
int sx, sy;
int ct;
const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};

void dfs(int x, int y, vector<vector<char> > &v) {
  v[x][y] = '#';
  ++ct;
  for(int i = 0; i < 4; ++i) {
    int nx = dx[i] + x;
    int ny = dy[i] + y;
    if(nx >= 0 && ny >= 0 && ny < h && nx < w && v[nx][ny] == '.') {
      dfs(nx, ny, v);
    }
  }
  return;
}
int main() {
  for(;;) {
    scanf("%d %d", &w, &h);
    if(w == 0 && h == 0) break;
    vector<vector<char> > v(w, vector<char>(h));
    for(int i = 0; i < h; ++i) {
      for(int j = 0; j < w; ++j) {
        scanf(" %c", &v[j][i]);
      }
    }

    for(int i = 0; i < h; ++i) {
      for(int j = 0; j < w; ++j) {
        if(v[j][i] == '@') {
          sx = j, sy = i;
          break;
        }
      }
    }
    dfs(sx, sy, v);
    printf("%d\n", ct);
    h = 0, w = 0, sx = 0, sy = 0, ct = 0;
  }
  return 0;
}