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;

int W, H;
char board[30][30];
bool visited[30][30];

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

void dfs(int x, int y) {
   for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= H || ny < 0 || ny >= W) continue;
      if (board[nx][ny] == '#') continue;
      if (visited[nx][ny]) continue;
      visited[nx][ny] = true;
      dfs(nx, ny);
   }
}

int main() {
   while (cin >> W >> H, W || H) {
      memset(visited, false, sizeof(visited));
      int sx = 0, sy = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            cin >> board[i][j];
            if (board[i][j] == '@') {
               sx = i, sy = j;
            }
         }
      }

      visited[sx][sy] = true;
      dfs(sx, sy);

      int cnt = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (visited[i][j]) cnt++;
         }
      }

      cout << cnt << endl;
   }

   return 0;
}

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

const int dx[] = {1, -1, 0, 0};
const int dy[] = {0, 0, 1, -1};
int W, H;
char board[30][30];

int dfs(int y, int x) {
   board[y][x] = '#';
   int ret = 1;
   for (int i = 0; i < 4; i++) {
      int ny = y + dy[i], nx = x + dx[i];
      if (ny >= 0 && ny < H && nx >= 0 && nx < W && board[ny][nx] == '.') {
         ret += dfs(y, x);
      }
   }
   return ret;
}

int main() {
   while (cin >> W >> H, W) {
      for (int i = 0; i < H; i++) {
         cin >> board[i];
      }

      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (board[i][j] == '@') {
               cout << dfs(i, j) << endl;
            }
         }
      }
   }

   return 0;
}

C++: #include <iostream>
#include <climits>
#include <set>
#include <string>
#include <algorithm>
#include <vector>
#include <queue>
#define MAX 1000000007


using namespace std;
#define ll long long
#define dbg if(0)
#define ISRANGE(val,lo,hi) ((lo<=val)&&(val<hi))



int main(){
    ll ans;
    char t[21][21];//w,h
    ll dx[4]={-1,1,0,0};
    ll dy[4]={0,0,-1,1};
    ll dbg_cnt=0;
    while(true){
        ans=0;
        ll w,h,px,py;
        cin >>w>>h;
        if(w==0&&h==0)break;
        for(int y=0;y<h;y++){
            for(int x=0;x<w;x++){
                cin >>t[x][y];
                if(t[x][y]=='@'){
                    px=x,py=y;
                    t[x][y]='.';
                }
            }
        }

        queue< pair<ll,ll> > kouho;//x,y
        kouho.push(make_pair(px,py));
        t[px][py]='#';
        dbg{
            cout <<"kokomadeiketa "<<dbg_cnt<<endl;
            dbg_cnt++;
            if(dbg_cnt>5)break;
        }
        ans++;
        while(!kouho.empty()){
            pair<ll,ll> now=kouho.front();//x,y
            kouho.pop();
            if(t[now.first][now.second]=='.'){
                t[now.first][now.second]='#';
                ans++;
            }
            for(int i=0;i<4;i++){
                ll qx=now.first+dx[i],qy=now.second+dy[i];
                if(ISRANGE(qx,0,w) && ISRANGE(qy,0,h)){
                    if(t[qx][qy]=='.'){
                        kouho.push(make_pair(qx,qy));
                    }
                }
            }
            dbg{
                for(int y=0;y<h;y++){
                    for(int x=0;x<w;x++){
                        cout << t[x][y];
                    }
                    cout <<endl;
                }
            }
        }

        cout <<ans<<endl;
    }

    return 0;

}

C++: #include<iostream>
#include<string>
using namespace std;
int w,h;
string map[40];
int vec[4][2]={{0,1},{0,-1},{-1,0},{1,0}};
int move(int x,int y){
	int ans=1;
	map[x][y]='#';
	for(int i=0;i<4;i++){
		if((x+vec[i][0]>=0)&&(y+vec[i][1]>=0)&&(x+vec[i][0]<h)&&(y+vec[i][1]<w)){
			if(map[x+vec[i][0]][y+vec[i][1]]=='.'){
				ans+=move(x+vec[i][0],y+vec[i][1]);
			}
		}
	}
	return ans;
}

int main(){
	while(cin>>w>>h,w){
		int fx,fy;
		for(int i=0;i<h;i++){
			cin>>map[i];
		}
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(map[i][j]=='@'){
					fx=i;
					fy=j;
					break;
				}
			}
		}
		int ans=move(fx,fy);
		cout<<ans<<endl;
		
	}
}