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>

using namespace std;
typedef long long int ll;
typedef vector<int> VI;

#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()

#define EACH(i, c) \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define SORT(c) sort((c).begin(), (c).end())

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)
const double PI = acos(-1.0);
#define CLR(a) memset((a), 0, sizeof(a))

char field[20][20];

int p, q;

void ta(int i, int j) {
  if (i < 0 || j < 0 || i >= q || j >= p) return;
  if (field[i][j] == '.' or field[i][j] == '@') {
    field[i][j] = 'x';
    ta(i + 1, j);
    ta(i, j + 1);
    ta(i - 1, j);
    ta(i, j - 1);
  }
}

void solve(int p, int q) {
  REP(i, q) {  // i:y
    REP(j, p) { cin >> field[i][j]; }
  }
  REP(i, q) {  // i:y
    REP(j, p) {
      if (field[i][j] == '@') ta(i, j);
    }
  }
  int ans = 0;
  REP(i, q) {  // i:y
    REP(j, p) {
      if (field[i][j] == 'x') ans++;
    }
  }
  cout << ans << "\n";
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  // int p,q;
  while (cin >> p >> q && p && q) {
    solve(p, q);
  }
  return 0;
}

C++: #include<iostream>
#include<queue>

using namespace std;
typedef pair<int, int> P;
#define rep(i,a,b) for(int i=int(a);i<int(b);++i)

int W, H;
int sx, sy;
int dx[] = { 0,1,0,-1 };
int dy[] = { 1,0,-1,0 };

int main() {
	while (true) {
		int ans = 0;
		char map[20][20];
		bool d[20][20];
		cin >> W >> H;

		if (W == 0)return 0;

		rep(i, 0, H)rep(j, 0, W) {
			cin >> map[i][j];
			if (map[i][j] == '@') { sy = i; sx = j; }
		}

		queue<P> que;
		rep(i, 0, 20)rep(j, 0, 20)d[i][j] = false;
		que.push(P(sy, sx));
		d[sy][sx] = true;

		while (!que.empty()) {
			P p = que.front();
			que.pop();
			rep(i, 0, 4) {
				int ny = p.first + dy[i];
				int nx = p.second + dx[i];
				if (!(0 <= nx && 0 <= ny && ny < H && nx < W))continue;
				if (map[ny][nx] != '#' && d[ny][nx] != true) {
					d[ny][nx] = true;
					que.push(P(ny, nx));
				}
			}
		}

		rep(i, 0, 20)rep(j, 0, 20) {
			if (d[i][j] == true)ans++;
		}
		cout << ans << endl;
	}
}

//BFSでやってみる

C++: 
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<math.h>
#include<stack>
#include<queue>
#include<vector>
#include<algorithm>
#include<string>
#include<iostream>
#include<set>
#include<map>

using namespace std;
typedef long long ll;
int mod(ll a){
    int c=a%1000000007;
    if(c>=0)return c;
    else return c+1000000007;
}
typedef pair<int,int> i_i;
typedef pair<ll,ll> l_l;
#define inf 100000000/*10^8*/

/////////////////////////////////

int w,h;
int si,sj;

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

int room[25][25];
int ans=0;
int dfs(int x,int y){
    room[x][y]=2;ans++;
    for(int i=0;i<=3;i++){
        int xx=x+dx[i],yy=y+dy[i];
        if(0<=xx&&xx<=h-1&&0<=yy&&yy<=w-1&&room[xx][yy]==0)dfs(xx,yy);
    }
    return ans;
}


int main(){
    while(1){
        cin>>w>>h;
        if(w==0)break;
        ans=0;
        for(int i=0;i<=h-1;i++){
            char s[w+1];cin>>s;
            for(int j=0;j<=w-1;j++){
                if(s[j]=='.')room[i][j]=0;
                else if(s[j]=='#')room[i][j]=1;
                else if(s[j]=='@')room[i][j]=2,si=i,sj=j;
            }
        }
        cout<<dfs(si,sj)<<endl;
    }
    return 0;
}

C++: #include <iostream>

using namespace std;

void dfs(char **data, int x, int y, int &count){
  if (data[y][x] == '.' || data[y][x] == '@'){
    data[y][x] = '#';
    ++count;
    //cout << count << endl;
    dfs(data, (x-1), y, count);
    dfs(data, x, (y-1), count);
    dfs(data, (x+1), y, count);
    dfs(data, x, (y+1), count);  
  }
  return;
}

int main(int argc, char* argv[]){
  int w, h;
  
  while (cin >> w >> h && w!=0 && h!=0){
    char **data = new char*[h+2];
    for (int i=0; i<(h+2); ++i){
      data[i] = new char[w+2];
    }
    
    int x,y;
    
    for (int i=0; i<(h+2); ++i){
      for (int j=0; j<(w+2); ++j){
	if ( i==0 || j==0 || i==(h+1) || j==(w+1)){
	  data[i][j] = '#';
	}
	else{
	  cin >> data[i][j];
	  //black: .   red: #   person: @
	  if (data[i][j] == '@'){
	    x = j;
	    y = i;
	  }
	}      
      }
    }
    /*
    cout << endl << endl;
    for (int i=0; i<(h+2); ++i){
      for (int j=0; j<(w+2); ++j){
	cout << data[i][j];
      }
      cout << endl;
    }
    */
    int count = 0;
    dfs(data, x, y, count);
    cout << count << endl;
  }
  return 0;
}