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 <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#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>
#include <fstream>
#include <queue>

#pragma warning( disable: 4996 )

using namespace std;

typedef long long ll;

#define INF 100000000
#define EPS 1e-9

#define MAX_N 50
#define MAX_R 50

#define MAX_W 20
#define MAX_H 20

int W, H;
int ans[100000];
int C = 0;
char maze[MAX_W][MAX_H];

void dfs(int sx, int sy){
	
	maze[sy][sx] = '#';

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

	for (int i = 0; i < 4; i++){
		int nx = sx + dx[i];
		int ny = sy + dy[i];
		if (nx >= 0 && nx < W && ny >= 0 && ny < H && maze[ny][nx] != '#'){
			ans[C]++;
			dfs(nx, ny);
		}
	}

}

int main(){

	while (true){
		cin >> W >> H;
		if (W == 0 && H == 0){
			break;
		}
		int sx;
		int sy;
		for (int i = 0; i < H; i++){
			for (int j = 0; j < W; j++){
				cin >> maze[i][j];
				if (maze[i][j] == '@'){
					sx = j;
					sy = i;
				}
			}
		}

		ans[C] = 1;
		dfs(sx,sy);
		C++;

	}

	for (int i = 0; i < C; i++){
		cout << ans[i] << endl;
	}


	return 0;

}

C++: #include <algorithm>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>

#define pb push_back
#define debug(x) cout<< #x <<": "<<x<<endl
#define debug2(x,y) cout<< #x <<": "<< x <<", "<< #y <<": "<< y <<endl

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<i_i, int> p_i;
typedef vector<int> vi;
typedef vector<vector<int> > vvi;
template<class T> using vv=vector<vector< T > >;
typedef deque<int> di;
typedef deque<deque<int> > ddi;

int main() {
  while (true) {
    int w, h;
    cin >> w >> h;
    if (h == 0) { break; }
    vvi tiles(h+2, vi(w+2, -1)); // -1 : red, 0 : black, 1 : reached
    di current(2); // w(x), h(y)
    for (int ih = 1; ih <= h; ++ih) {
      for (int iw = 1; iw <= w; ++iw) {
        char c_tmp;
        cin >> c_tmp;
        if (c_tmp != '#') {
          tiles[ih][iw] = 0;
        }
        if (c_tmp == '@') {
          current = {ih, iw};
          tiles[ih][iw] = 1;
        }
      }
    }

    ddi s;
    s.pb(current);
    int reachable = 1;
    while ( !s.empty() ) {
      current = s.front();
      vvi search = {{1,0},{0,1},{-1,0},{0,-1}};
      for (auto v : search) {
        int next_h = current[0] + v[0];
        int next_w = current[1] + v[1];
        if (tiles[next_h][next_w] == 0) {
          tiles[next_h][next_w] = 1;
          reachable += 1;
          s.pb({next_h, next_w});
        }
      }
      s.pop_front();
    }
    printf("%d\n", reachable);
  }

  return 0;
}

C++: // 縦型探索
#include <iostream>
#include <string>
#include <vector>

using namespace std;

int w, h;
int mx[] = {0, 0, -1, 1};
int my[] = {-1, 1, 0, 0};

int dfs(int x, int y, vector<string> &v) {
    int cnt = 1;
    for ( int i = 0; i < 4; i++ ) {
        int nx = x + mx[i], ny = y + my[i];
        if ( 0 <= nx && nx < w && 0 <= ny && ny < h && v[ny][nx] == '.' ) {
            v[ny][nx] = '#';
            cnt += dfs(nx, ny, v);
        }
    }
    return cnt;
}

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

        vector<string> v(h);

        for ( int i = 0; i < h; i++ ) {
            cin >> v[i];

            for ( int j = 0; j < v[i].size(); j++ ) {
                if ( v[i][j] == '@' ) {
                    y = i;
                    x = j;
                }
            }
        }

        cout << dfs(x, y, v) << endl;
    }
    return 0;
}

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

char map[21][21];
int tx[4] = {-1,0,1,0};
int ty[4] = {0,1,0,-1};
int w,h,c;

int search(int y,int x){
  int i,nx,ny;
  c++;
  for(i=0;i<4;i++){
    nx=x+tx[i];
    ny=y+ty[i];
    if(0<=nx && nx<w && 0<=ny && ny<h && map[ny][nx]!='#'){
      map[ny][nx]='#';
      search(ny,nx);
    }
  }
  
  return c;
  
}

int main(){

  int i,j,nx,ny;
  int a;

  while(1){
    c=0;
    cin>>w>>h;
  
    if(w==0&&h==0){
      break;
    }	    
 
    
    for(i=0; i<h ; i++){
      for(j=0; j<w ; j++){
	cin>>map[i][j];
	if(map[i][j]=='@'){
	  nx = j;
	  ny = i;
	}
      } 
    }
    
    map[ny][nx]='#';
    a=search(ny,nx);
    cout<<c<<endl;
    
  }
  return 0;
}