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;
#define rep(i,n) for (int (i)=(0);(i)<(int)(n);++(i))
using ll = long long;
using P = pair<int, int>;
using namespace std;

template<class T> void vin(vector<T>& v, int n) {
    v.resize(n);
    for (int i = 0; i < n; ++i) {
        cin >> v[i];
    }
}

int w, h;
char ch[50][50];
int dx[] = { 0, 1, 0, -1 };
int dy[] = { 1, 0, -1, 0 };

int bfs(int y, int x) {
    int res = 0;
    rep(i, 4) {
        int ny = y + dy[i], nx = x + dx[i];
        if (0 > ny or 0 > nx or ny >= h or nx >= w) continue;
        if (ch[ny][nx] == '.') {
            ch[ny][nx] = 'a';
            res += bfs(ny, nx) + 1;
        }
    }
    return res;
}

int main() {
    while (cin >> w >> h and (w or h)) {
        int py = 0, px = 0;
        rep(i, h) rep(j, w) {
            cin >> ch[i][j];
            if (ch[i][j] == '@') {
                py = i, px = j;
                ch[i][j] = '.';
            }
        }
        cout << bfs(py, px) << endl;
    }
}

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

using namespace std;

struct DataSet{
    int x, y;
    int ax, ay;
    string str;
};

int search(int xSize, int ySize, int x, int y, string& s){
    //cout << '[' << x << ',' << y << ']' << endl;
    int ret = 0;
    int nowPos = x + y * xSize;
    s[nowPos] = '#';
    ++ret;
    
    if(s[nowPos - 1] == '.' && x != 0){
        ret += search(xSize, ySize, x-1, y, s);
    }
    if(s[nowPos + 1] == '.' && x + 1 != xSize){
        ret += search(xSize, ySize, x+1, y, s);
    }
    if(s[nowPos - xSize] == '.' && y != 0){
        ret += search(xSize, ySize,x, y-1, s);
    }
    if(s[nowPos + xSize] == '.' && y + 1 != ySize){
        ret += search(xSize, ySize, x, y+1, s);
    }
    return ret;
}

int main()
{
    vector<DataSet> dataSets;
    char word;
    
    while(cin){
        DataSet dataSet;
        cin >> dataSet.x >> dataSet.y;
        for(int i = 0; i < dataSet.y * dataSet.x; i++){
            cin >> word;
            if(word == '@'){
                dataSet.ax = i % dataSet.x;
                dataSet.ay = i / dataSet.x;
            }
            //cout << line << endl;
            dataSet.str += word;
        }
        dataSets.push_back(dataSet);
    }
    dataSets.pop_back();
    
    //cout << dataSets.back().x << ' ' << dataSets.back().y << endl;
    //cout << dataSets.back().ax << ' ' << dataSets.back().ay << endl;
    //cout << dataSets.back().str;
    
    for(auto dataSet : dataSets){
        if(dataSet.x == 0) continue;
        cout << search(dataSet.x, dataSet.y, dataSet.ax, dataSet.ay, dataSet.str) << endl;
    }
    
    return 0;
}

C++: #include <iostream>
using namespace std;
int ch[22][22],cnt;
int dfs(int,int);
int main(){
  int w,h,i,j,k,l,result;
  string in[20];
  while(1){
    cnt=0;
    cin >> w >> h;
    if(w==0&&h==0) break;
    for(i=0;i<h;i++){
      cin >> in[i];
    }
    for(i=0;i<h+2;i++){
      for(j=0;j<w+2;j++){
	ch[i][j]=1;
      }
    }
    for(i=0,k=1;k<h+1;i++,k++){
      for(j=0,l=1;l<w+1;l++,j++){
	if(in[i][j]=='.') ch[k][l]=0;
	else if(in[i][j]=='#') ch[k][l]=1;
	else ch[k][l]=2;
      }
    }
    for(i=1;i<h;i++){
      for(j=1;j<w;j++){
	if(ch[i][j]==2) result=dfs(i,j);
      }
    }
    cout << cnt << endl;
  }
  return 0;
}
int dfs(int i,int j){
  if(ch[i][j]==1) return 0;
  ch[i][j]=1;
  cnt++;
  dfs(i+1,j);
  dfs(i-1,j);
  dfs(i,j-1);
  dfs(i,j+1);
  return cnt;
}

C++: #include <bits/stdc++.h>
using namespace std;

#define FOR( i, m, n )  for( int (i) = (m); (i) < (n); (i)++ )
#define REP( i, n )     FOR( i, 0, n )
#define ALL( a )        (a).begin(), (a).end()             

#define F				first
#define S				second

int w, h, cnt = 0;

void solve( vector<vector<char>>& f, pair<int, int> mp ) {

	if( mp.F - 1 >= 0 && f[mp.S][mp.F - 1] == '.' ) {
		cnt++;
		f[mp.S][mp.F - 1] = '#';
		solve( f, make_pair( mp.F - 1, mp.S ) );
	}
	if( mp.F + 1 < w && f[mp.S][mp.F + 1] == '.' ) {
		f[mp.S][mp.F + 1] = '#';
		cnt++;
		solve( f, make_pair( mp.F + 1, mp.S ) );
	}
	if( mp.S - 1 >= 0 && f[mp.S - 1][mp.F] == '.' ) {
		f[mp.S - 1][mp.F] = '#';
		cnt++;
		solve( f, make_pair( mp.F, mp.S - 1 ) );
	}
	if( mp.S + 1 < h && f[mp.S + 1][mp.F] == '.' ) {
		f[mp.S + 1][mp.F] = '#';
		cnt++;
		solve( f, make_pair( mp.F, mp.S + 1 ) );
	}
}

int main() {

	while( 1 ) {
		cin >> w >> h; if( w == 0 && h == 0 ) break;
		
		pair<int, int> mp;
		vector<vector<char>> f;
		vector<char> tmp( w );
		REP( i, h ) {
			REP( j, w ) {
				cin >> tmp[j];
				if( tmp[j] == '@' ) { 
					mp.first = j;
					mp.second = i; 
					tmp[j] = '#';
					cnt++;
				}
			}
			f.push_back( tmp );
		}
		solve( f, mp );
		cout << cnt << endl;
		cnt = 0;
	}
}