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>
#include <array>
#include <vector>
#include <math.h>
#include <algorithm>
#include <set>
#include <deque>
#include <utility>
#include <map>

#define ll long long int
#define MAX_W 20
#define MAX_H 20

using namespace std;

char field[MAX_W][MAX_H];
bool reach[MAX_W][MAX_H];
int h, w;
int cou = 0;


void search(int x, int y)
{
	if (x < 0 || w <= x || y < 0 || h <= y || field[x][y] == '#') {
		return;
	}

	if (reach[x][y] == true) {
		return;
	}

	reach[x][y] = true;


	cou++;



	search(x - 1, y);
	search(x + 1, y);
	search(x, y + 1);
	search(x, y - 1);

}

int main(void)
{
	vector <string> s;
	vector <int> ans;
	int i, j;

	int dx, dy; //start

	string temp;


	while (1) {
		cin >> w >> h;
		if (w == 0 && h == 0) {
			break;
		}

		for (i = 0; i < h; i++) {
			cin >> temp;
			s.push_back(temp);
		}

		for (i = 0; i < h; i++) {
			for (j = 0; j < w; j++) {
				field[j][i] = s[i][j];
				if (s[i][j] == '@') {
					dx = j;
					dy = i;
				}
			}
		}




		search(dx, dy);

		ans.push_back(cou);
		cou = 0;

		s.clear();

		for (i = 0; i < MAX_W; i++) {
			
			for (j = 0; j < MAX_H; j++) {


				field[i][j] = '#';
				reach[i][j] = false;
			}
		}


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

	return (0);
}

C++: #include <bits/stdc++.h>
using namespace std;
vector<string>v;
vector<int>ansl;
string s;
int ary[100005] = {0};
char mp[21][21];
int cnt = 0;
int d_x[] = {0,1,0,-1};
int d_y[] = {1,0,-1,0};
int W,H;

void dfs(int x,int y){
  mp[y][x] = '#';
  cnt++;
  for(int d = 0;d<4;d++){
    int nx = x+d_x[d],ny = y+d_y[d];
      if((nx>=0)&&(ny>=0)&&(ny<H)&&(nx<W)&&(mp[ny][nx] == '.')){
        dfs(nx,ny);
      }
  }
}
int main(){
  cin >>W>>H;

  while((W!=0)&&(H!=0)){
    char mp[21][21] = {'0'};
    cnt = 0;
    for(int i = 0;i<H;i++){
      for(int j = 0;j<W;j++){
        cin >>mp[i][j];
      }
    }
    for (int y = 0; y < H; y++) {
            for (int x = 0; x < W; x++) {
                if (mp[y][x] == '@') {
                    dfs(x, y);
                }
            }
        }
    cout <<cnt<<endl;
    cin >>W>>H;
  }
  return 0;
}

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

int tile[22][22];
int sum;

void search(int w,int h)
{
  if(tile[w][h]){
    tile[w][h]=0; sum++;
    search(w,h-1);
    search(w-1,h);
    search(w+1,h);
    search(w,h+1);
  }
}

int main()
{
  int w,h,i,j,sx,sy;
  char c;
  
  for(;;){
    cin>>w>>h;
    if(!(w|h)) break;

    sum=0;
    for(i=0;i<22;i++)
      for(j=0;j<22;j++)
	tile[i][j]=0;
    for(i=1;i<h+1;i++){
      for(j=1;j<w+1;j++){
	cin>>c;
	if(c=='#') tile[j][i]=0;
	else if(c=='.') tile[j][i]=1;
	else if(c=='@'){
	  tile[j][i]=1; sx=j; sy=i;
	}
      }
    }
    search(sx,sy);
    cout<<sum<<endl;
  }
}

C++: #include <bits/stdc++.h>
using namespace std;
const int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};

void solve(int W,int H){
    vector<string> S(H);
    for (int i=0;i<H;++i) cin >> S[i];
    vector<vector<bool>> seen(H,vector<bool>(W,false));
    int ans=0;
    auto dfs=[&](auto self,int x,int y)->void{
        seen[x][y]=true; ++ans;
        for (int i=0;i<4;++i){
            int nx=x+dx[i],ny=y+dy[i];
            if (nx<0||H<=nx||ny<0||W<=ny) continue;
            if (S[nx][ny]=='#') continue;
            if (seen[nx][ny]) continue;
            self(self,nx,ny);
        }
    };
    for (int i=0;i<H;++i){
        for (int j=0;j<W;++j){
            if (S[i][j]=='@'){
                dfs(dfs,i,j);
            }
        }
    }
    cout << ans << '\n';
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int W,H;
    while(cin >> W >> H,W){
        solve(W,H);
    }
}

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

string map[20];
int count,W,H;
int walk(int ,int);

int main(){
	int i,j;
	while(cin >> W >> H,(W || H)){
		count=0;
		for(i=0;i<H;i++) cin >> map[i];
		for(i=0;i<H;i++){
			for(j=0;j<W;j++){
				if(map[i][j]=='@') goto LOOP;
			}
		}
	LOOP:
		walk(i,j);
	cout << count << endl;
	}

}

int walk(int a,int b){
	map[a][b] = '#';
	count++;
	if(a+1<H && map[a+1][b]  == '.') walk(a+1,b);
	if(b+1<W && map[a][b+1]  == '.') walk(a,b+1);
	if(a-1>=0 && map[a-1][b]  == '.') walk(a-1,b);
	if(b-1>=0 && map[a][b-1]  == '.') walk(a,b-1);
	return 0;
}