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>
using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
char fld[20][20];
bool used[20][20];
int W, H;

void rec(int i, int j){
   used[i][j]=1;
   for(pii dd: dxdy){
      int ni=i+dd.first;
      int nj=j+dd.second;
      if((0<=ni&&ni<H&&0<=nj&&nj<W)&&fld[ni][nj]=='#'&&used[ni][nj]==0){
         rec(ni, nj);
      }
   }
}

int main(){

   while(1){
      cin>> W>> H;
      if(!(H||W)) break;
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
            }
         }
      }

      for(int i=0; i<20; i++)for(int j=0; j<20; j++) used[i][j]=0;
      used[sy][sx]=1;
      rec(sy, sx);
      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=used[i][j];
      }
      cout<< ans<< endl;

   }

   return 0;
}

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

void dfs(int, int);
int limx,limy,cnt=0,ni,nj;
char data[20][21];

int main(){
  int i,j;
  while(1){
    scanf("%d%d",&limx,&limy);
    if(limx==0 && limy==0)
      break;
    for(i=0;i<limy;i++)
      scanf("%s",data[i]);

    for(i=0;i<limy;i++)
      for(j=0;j<limx;j++)
	if(data[i][j]=='@') ni=i,nj=j;
    dfs(ni,nj);
    printf("%d\n",cnt);
    cnt=0;
  }
  return 0;
}

void dfs(int i, int j){
  int di[4]={0,-1,0,1};
  int dj[4]={1,0,-1,0};

  data[i][j]='#';
  cnt++;
  for(int r=0;r<4;r++){
    ni=i+di[r];
    nj=j+dj[r];
    if(data[ni][nj]=='.') dfs(ni,nj);
  }
}

C++: #include <iostream>
#include <stack>
#include <utility>

using namespace std;

using Point = pair<int,int>;

int main(){
  int W,H;
  while((cin>>W>>H)&&W>0&&H>0){
    int map[22][22]={0};
    int sy,sx;
    for(int i=1;i<=H;++i){
      for(int j=1;j<=W;++j){
        char c;
        cin >> c;
        if(c=='.'){
          map[i][j] = 1;
        }else if(c=='@'){
          map[i][j] = 1;
          sy = i ; sx = j;
        }
      }
    }

    stack<Point> s;
    s.push(make_pair(sy,sx));
    int tile = 0;
    while(!s.empty()){
      auto p = s.top();s.pop();
      int y = p.first , x = p.second;
      if(map[y][x]){
        map[y][x] = 0;
        tile++;
      }else{
        continue;
      }

      s.push(make_pair(y-1,x));
      s.push(make_pair(y+1,x));
      s.push(make_pair(y,x-1));
      s.push(make_pair(y,x+1));
    }

    cout << tile << endl;
  }
  return 0;
}

C++: //前置き
#pragma region

#include<iostream>
#include<cstdio>
#include<cmath>
#include<string>
#include<vector>
#include<list>
#include<algorithm>
#include<set>
#include<map>
#include<stack>
#include<queue>

using namespace std;

typedef  long long ll;

#define pb(a) push_back(a)
#define all(x) (x).begin(),(x).end()

#define FOR(i, a, n) for(int i = (int)(a); i < (int)(n); ++i)
#define REP(i, n) FOR(i, 0, n)
#define SORT(a) sort(all(a))
#define REVERSE(a) reverse(all(a))

#define PRINT(str) printf(#str "\n")
#define yOUT PRINT(Yes)
#define nOUT PRINT(No)
#define YOUT PRINT(YES)
#define NOUT PRINT(NO)

#pragma endregion

//関数定義
#pragma region 
//偶数 ->　true :: 奇数　-> false
bool EVEN(ll a){
    return a%2-1;
}

//最大公約数
ll gcd(ll a, ll b) {  
    if (!b)  
        return a;  
    return gcd(b, a%b);  
}  

//最小公倍数
ll lcm(ll a, ll b) {  
    return a*b/gcd(a,b);  
} 

#pragma endregion


int main(){
    while(1){
        int w, h, cnt =1;
        cin >> w >> h;
        if(w == 0 && h == 0) return 0;

        bool vis[22][22];
        char gragh[22][22];

        REP(i,22){
            REP(j,22){
                vis[i][j] = false;
                gragh[i][j] = '#';
            }
        }

        


        int a,b;
        for(int i=1;i<h+1;i++){
            for(int j=1;j<w+1;j++){
                cin >> gragh[i][j];

                if(gragh[i][j] == '@'){
                    a = i;b = j;
                }
            }   
        }

        
        stack<int> st;
        st.push(a); st.push(b);
        int dx[4] = {0,1,-1,0};
        int dy[4] = {1,0,0,-1};
 

        while (!st.empty()){
            int x,y;
            y = st.top(); st.pop();
            x = st.top(); st.pop();

            for(int i = 0;i<4;i++){
                int ix = x + dx[i];
                int iy = y + dy[i];
            
                if(gragh[ix][iy] == '.' && vis[ix][iy] == false){
                    st.push(ix); st.push(iy);
                    vis[ix][iy] = true;
                    cnt ++;
                }
            }
        }
        cout << cnt <<endl;
    }
}