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 <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<iostream>
#include<algorithm>

using namespace std;

char tyle[30][30]={'0'};

int find_tyle(int i,int j){
  if(tyle[i+1][j]=='.'){
    tyle[i+1][j]='a';
    find_tyle(i+1,j);
  }
  if(tyle[i][j+1]=='.'){
    tyle[i][j+1]='a';
    find_tyle(i,j+1);
  }
  if(tyle[i-1][j]=='.'){
    tyle[i-1][j]='a';
    find_tyle(i-1,j);
  }
  if(tyle[i][j-1]=='.'){
    tyle[i][j-1]='a';
    find_tyle(i,j-1);
  }
}

int main(){
  
  int w,h;
  
  while(cin>>w>>h,w){
    int ans=0;
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
      cin>>tyle[i][j];
      }
    }
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
	if(tyle[i][j]=='@'){
	  find_tyle(i,j);
	  tyle[i][j]='a';
	}
      }
    }
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
	if(tyle[i][j]=='a'){
	  ans++;
      }
    }
  }
    cout<<ans<<endl;

   for(int i=0;i<30;++i){
      for(int j=0;j<30;++j){
      tyle[i][j]='0';
      }
    }
    
  }
}

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

#define MAX_WH 20

typedef std::pair<int, int> P;

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

int W, H, sx, sy;
char field[MAX_WH][MAX_WH];
int solve();

int bfs();

int main(){
	while(true){
		std::cin >> W >> H;
		if(W == 0 && H == 0)
			break;

		for(int i = 0; i < H; ++i){
			for(int j = 0; j < W; ++j){
				char in;
				std::cin >> in;

				if(in == '@')
					sx = j, sy = i;

				field[j][i] = in;
			}
		}

		std::cout << solve() << std::endl;
	}
	return 0;
}

int solve(){
	return bfs();
}

int bfs(){
	std::queue<P> que;
	int ans = 0;
	bool flag[MAX_WH][MAX_WH];

	for(int i = 0; i < MAX_WH; ++i){
		for(int j = 0; j < MAX_WH; ++j)
			flag[i][j] = true;
	}

	que.push(P(sx, sy));
	flag[sx][sy] = false;

	while(!que.empty()){
		++ans;
		P p = que.front(); que.pop();

		for(int i = 0; i < 4; ++i){
			int nx = p.first + dx[i], ny = p.second + dy[i];

			if(nx >= 0 && ny >= 0 && nx < W && ny < H && field[nx][ny] != '#' && flag[nx][ny]){
				flag[nx][ny] = false;
				que.push(P(nx, ny));
			}
		}
	}

	return ans;
}

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

using namespace std;

int solve( int w, int h, vector< vector<char> > &v );
void dfs( int W, int H, int w_index, int h_index, vector< vector<char> > &V, int &ans );

int main(int argc, char const *argv[]) {
    vector< vector<char> > vec;

    int w, h;
    vector<int> sums;

    while (1) {
        cin >> w >> h;

        if( w==0 || h==0 ) break;

        vec.resize(w);
        for (int i = 0; i < w; i++) {
            vec[i].resize(h);
        }

        for (int i = 0; i < w; i++) {
            for (int j = 0; j < h; j++) {
                cin >> vec[i][j];
            }
        }

        sums.push_back( solve( w, h, vec ) );
    }

    for( auto it = sums.begin(); it != sums.end(); it ++ ){
        cout << *it << endl;
    }

    return 0;
}

int solve( int w, int h, vector< vector<char> > &v ){
    int w_index, h_index, total;
    int ans = 0;

    for (auto it = v.begin(); it != v.end(); it++ ) {
        auto iter = find(it->begin(), it->end(), '@');

        if(iter!=it->end()){
            w_index = distance( v.begin(), it );
            h_index = distance( it->begin(), iter );
        }
    }

    dfs( w, h, w_index, h_index, v, ans );

    return ans;
}

void dfs( int W, int H, int w_index, int h_index, vector< vector<char> > &V , int &ans ){
    int dx[4] = { 1, 0, -1, 0 };
    int dy[4] = { 0, 1, 0, -1 };
    V[w_index][h_index] = '#';

    for( int i = 0; i < 4; i++ ){
        int nx = w_index + dx[i];
        int ny = h_index + dy[i];

        if( 0 <= nx && nx < W && 0 <= ny && ny < H && V[nx][ny] == '.' ){
            ans++;
            dfs( W, H, nx, ny, V, ans );
        }
    }

    return;
}