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 <string>
#define MAX 22
using namespace std;
char map[ MAX ][ MAX ];
int n;
void visit( int i, int j ){
  if( map[ i ][ j ] == '#' ){ return; }
  n++;
  map[ i ][ j ] = '#';
  visit( i - 1, j );
  visit( i, j - 1 );
  visit( i + 1, j );
  visit( i, j + 1 );
}
int main(){
  while( 1 ){
    int w, h;
    int pi, pj;
    n = 0;
    cin >> w >> h;
    if( !w || !h ){ return 0; }
    for( int i = 0; i <= h + 1; i++ ){
      if( i == 0 || i == h + 1 ){
        for( int j = 0; j <= w + 1; j++ ){
          map[ i ][ j ] = '#';
        }
        continue;
      }
      string line;
      cin >> line;
      for( int j = 0; j <= w + 1; j++ ){
        if( j == 0 || j == w + 1 ){
          map[ i ][ j ] = '#';
          continue;
        }
        map[ i ][ j ] = line[ j - 1 ];
        if( map[ i ][ j ] == '@' ){
          pi = i;
          pj = j;
        }
      }
    }
    visit( pi, pj );
    cout << n << endl;
  }
}

C++: #include<cstdio>
#include<cstdlib>
#include<cmath>
#include<climits>
#include<iostream>
#include<algorithm>
#include<stack>
#include<queue>
#include<vector>
#include<list>
#include<map>
#include<set>
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
#define pb push_back
int dy[]={0, 0, 1, -1, 0};
int dx[]={1, -1, 0, 0, 0};
  
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
char room[100][100];
bool fin[100][100];
int w,h;

int dfs(int i,int j){
    int sum = 1;
 
    fin[i][j] = false;
 
    for(int k = 0;k < 4;k++){
        int ii = i + dy[k];int jj = j + dx[k];
        if(ii >= 0 && ii < h && jj >= 0 && jj < w && fin[ii][jj] && room[ii][jj] == '.')
            sum += dfs(ii,jj);
    }
    return sum;
}
 
int main(){
    while(1){
        scanf("%d%d",&w,&h);
 
        if(w == 0 && h == 0)
            break;
 
        for(int i = 0;i < h;i++)
            scanf("%s",&room[i][0]);
 
        for(int i = 0;i < h;i++)
            for(int j = 0;j < w;j++)
                fin[i][j] = true;
 
        for(int i = 0;i < h;i++){
            for(int j = 0;j < w;j++){
                if(room[i][j] == '@')
                    printf("%d\n",dfs(i,j));
            }
        }
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
using namespace std;

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

void dfs(vector< vector<char> > &plane, int x, int y, int &ans){  
  plane[x][y] = '#';
  ans++;
  for(int i=0; i<4; i++){
    if(0 <= x+dx[i] && x+dx[i] < w && 0 <= y+dy[i] && y+dy[i] < h)
      if(plane[x + dx[i]][y + dy[i]] != '#')
        dfs(plane, x + dx[i], y + dy[i], ans);
  }
  
  return;
}

int main(){
  while(true){
    cin >> w >> h;
    if(w == 0)
      break;

    int sx, sy;
    vector< vector<char> > plane(w, vector<char>(h));
    for(int i=0; i<h; i++){
      for(int j=0; j<w; j++){
        cin >> plane[j][i];
        if(plane[j][i] == '@'){
          sx = j;
          sy = i;
        }
      }
    }

    int ans = 0;
    dfs(plane, sx, sy, ans);
    cout << ans << endl;
  }
  
  return 0;
}

C++: #include<iostream>
#include<map>
#include<vector>
#include<algorithm>
#include<cmath>
#include<climits>
#include<ctime>
#include<cstring>
#include<numeric>

#define ALL(v) (v).begin(),(v).end()
#define REP(i,p,n) for(int i=p;i<(int)(n);++i)
#define rep(i,n) REP(i,0,n)
#define dump(a) (cerr << #a << "=" << (a) << endl)
#define DUMP(list) cout << "{ "; for(auto nth : list){ cout << nth << " "; } cout << "}" << endl;

using namespace std;

int W, H;
int sx, sy;
int ans;
int dx[4] = {1,0,-1,0};
int dy[4] = {0,1,0,-1};
vector<vector<char>>  field;

bool is_out_of_range(int x, int y) {
    return x < 0 || x >= W || y < 0 || y >= H;
}

void dfs(int x, int y) {
    if (is_out_of_range(x, y)) return;
    if (field[y][x] == '#') return;

    field[y][x] = '#';
    ans++;

    for (int i = 0; i < 4; ++i) {
        dfs(x + dx[i], y + dy[i]);
    }
    return;
}

int main() {
	while(cin >> W >> H, W) {
        ans = 0;
        field.assign(H, vector<char>(W));
        for (int y = 0; y < H; ++y) {
            for (int x = 0; x < W; ++x) {
                cin >> field[y][x];
                if (field[y][x] == '@') {
                    sx = x; sy = y;
                }
            }
        }
    
        dfs(sx, sy);
        cout << ans << endl; 
	}
	return 0;
}