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<vector>
#include<queue>
#include<utility>
using namespace std;
int w,h;
string matrix[20];

const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1};
bool valid(int x,int y)
{
    return x>=0&&x<w&&y>=0&&y<h&&matrix[x][y]!='#';
}
int main()
{
    while(true)
    {
        queue<pair<int,int> > q;
        pair<int,int> man;
        int amount=0;
        
        cin>>w>>h;
        if(w==0&&h==0)
            break;
        for(int i=0;i<h;i++)
        {
            cin>>matrix[i];
        }
        
        for(int i=0;i<h;i++)
        {
            for(int j=0;j<w;j++)
            {
                if(matrix[i][j]=='@')
                {
                    man.first=i;
                    man.second=j;
                }
            }
        }
        
        q.push(man);
        while(!q.empty())
        {
            pair<int,int> current;
            current=q.front();
            q.pop();
            
            for(int i=0;i<4;i++)
            {
                if(valid(current.first+dx[i],current.second+dy[i]))
                {
                    q.push(make_pair(current.first+dx[i],current.second+dy[i]));
                    amount++;
                }
            }
            
            matrix[current.first][current.second]='#';
        }
        
        cout<<amount<<endl;
    }
}

C++: #include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define fr first
#define sc second
typedef pair<int,int> pii;
typedef vector<int> vi;
const int INF = (1<<25);
const int dx[]={1,0,-1,0},dy[]={0,-1,0,1};

int w,h;
int sx,sy;
char room[20][21];
int res;

int dfs(int y,int x){
  room[y][x]='#';
  
  for(int i=0;i<4;i++){
    int ny=y+dy[i],nx=x+dx[i];
    if(0<=nx && nx<w && 0<=ny && ny<h && room[ny][nx]=='.'){
      res++;
      dfs(ny,nx);
    }
  }
}

int main(){
  while(cin>> w>> h,w){
    res=0;
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin>> room[i][j];
	if(room[i][j]=='@'){
	  sy=i; sx=j;
	}
      }
    }
    dfs(sy,sx);
    cout<< res+1<< endl;
  }
}

C++: #include <bits/stdc++.h>

using namespace std;
typedef long long ll;

#define LOOP(i, a, n) for (int i = (int)a; i < (int)n; ++i)
#define REP(i, n) LOOP(i, 0, n)
#define MOD 1000000007
#define INF 1000000000
#define PI 3.14159265359
#define MAX_H 20
#define MAX_W 20

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

int dfs(int x, int y) {
  if (board[y][x] == '#') {
    return 0;
  } else {
    ans++;
    board[y][x] = '#';
  }

  for (int i = 0; i < 4; i++) {
    int nextx = x + dx[i], nexty = y + dy[i];
    if (nextx >= w || nextx < 0 || nexty >= h || nexty < 0)
      continue;
    dfs(x + dx[i], y + dy[i]);
  }

  return ans;
}

int main(void) {
  vector<int> ret;
  while (true) {
    ans = 0;
    scanf("%d %d", &w, &h);
    if (w == 0 && h == 0)
      break;
    for (int i = 0; i < h; i++)
      cin >> board[i];

    int sx = 0, sy = 0;
    for (int i = 0; i < h; i++) {
      for (int j = 0; j < w; j++) {
        if (board[i][j] == '@') {
          sx = j;
          sy = i;
        }
      }
    }

    dfs(sx, sy);
    ret.push_back(ans);
  }

  for (int i = 0; i < ret.size(); i++) {
    cout << ret[i] << endl;
  }

  return 0;
}

C++: #include <iostream>
#include <fstream>
#include <algorithm>
#include <vector>
#include <string>
#include <functional>
#include <array>
#include <stack>
#include <queue>
#include <chrono>
#include <numeric>

#pragma warning(disable:4996)
#define REP(i,n) for(int i = 0 ; i < n ; ++i)
typedef long long Int;

using namespace std;

struct point{
	int x;
	int y;
	point(int _x, int _y) :x(_x), y(_y){}
};

char tile[20][20];
stack<point> st;

int main()
{
	cin.tie(0); ios::sync_with_stdio(false); 
	//fstream fs("input.txt");
	////////////ここから開始///////////

	
	int x(-1), y(-1);
	while (x != 0 && y != 0){
		REP(i, 20)REP(j, 20)tile[i][j] = '#';
		cin >> x >> y;
		//fs >> x >> y;
		REP(i, y){
			REP(j, x){
				cin >> tile[i][j];
				//fs >> tile[i][j];
			}
		}

		REP(i, y){
			REP(j, x){
				if (tile[i][j] == '@')
					st.emplace(i, j);
			}
		}

		int dx[]{0, 1, 0, -1};
		int dy[]{1, 0, -1, 0};
		int ans(1);

		while (!st.empty()){
			point now = st.top();
			st.pop();
			REP(i, 4){
				int x1 = now.x + dx[i];
				int y1 = now.y + dy[i];
				if (x1 > 19 || x1 < 0 || y1 > 19 || y1 < 0)
					continue;

				if (tile[x1][y1] == '.'){
					st.emplace(x1, y1);
					tile[x1][y1] = '#';
					++ans;
				}
			}
		}

		cout << ans << endl;
	}
	return 0;
}