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[21][21];
int used[20][20];
int W, H;
 
int rec(int i, int j){
 
   if(fld[i][j]=='#') return 0;
   if(used[i][j]) return 0;
   used[i][j]=1;
   int ret=0;
   for(pii dd: dxdy){
      int ni=i+dd.first;
      int nj=j+dd.second;
      if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
      ret+=rec(ni, nj);
   }
 
   return ret+1;
}
 
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;
               fld[i][j]='.';
            }
         }
      }
 
      for(int i=0; i<20; i++)for(int j=0; j<20; j++) used[i][j]=0;
      cout<< rec(sy, sx)<< endl;
   }
 
   return 0;
}

C++: #include <stdio.h>
#include <queue>
#include <algorithm>
using namespace std;

typedef struct point {
  int x, y;
} POINT;

int W, H;
char str[20][21];
queue<POINT> que;

bool input() {
  int i;

  scanf("%d %d", &W, &H);
  if (!W && !H) return false;

  for (i = 0; i < H; i++) {
    scanf("%s", str[i]);
  }

  return true;
}

int main() {
  int i, j, cnt;
  POINT p, q;

  while (input()) {

    while (!que.empty()) que.pop();
    for (i = 0; i < H; i++) {
      for (j = 0; j < W; j++) {
        if (str[i][j] == '@') {
          p.x = j;
          p.y = i;
          que.push(p);
          i = H;
          j = W;
        }
      }
    }

    cnt = 0;
    while (!que.empty()) {
      p = que.front();
      que.pop();
      if (p.x < 0 || W <= p.x || p.y < 0 || H <= p.y) continue;
      if (str[p.y][p.x] == '#') continue;
      cnt++;
      str[p.y][p.x] = '#';
      q.x = p.x;
      q.y = p.y+1;
      que.push(q);
      q.x = p.x+1;
      q.y = p.y;
      que.push(q);
      q.x = p.x-1;
      q.y = p.y;
      que.push(q);
      q.x = p.x;
      q.y = p.y-1;
      que.push(q);
    }

    printf("%d\n", cnt);

  }

  return 0;
}

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

using namespace std;

int n,m;
int cnt;
int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
char t[20][20];

void fanc(int x,int y){
	cnt++;
	int ny,nx;
	t[y][x]=',';
	for(int i=0;i<4;i++){
		ny=y+dy[i];
		nx=x+dx[i];
		if(ny>=0&&nx>=0&&ny<m&&nx<n&&t[ny][nx]=='.'){
			fanc(nx,ny);
		}
	}
}

int main(){
	
	while(1){
		
		cin >> n >> m;
		
		if(n==0) break;
		
		int nx,ny;
		for(int i=0;i<m;i++){
			for(int j=0;j<n;j++){
				cin >> t[i][j];
				if(t[i][j]=='@'){
					ny=i;nx=j;
				}
			}
		}
		cnt=0;
		fanc(nx,ny);
		cout << cnt << endl;
	}
	return 0;
}

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

struct dsu {
  const int n;
  vector<int> p, sz;
  dsu(int _n) : n(_n), p(n, -1), sz(n, 1) {}
  int root(int v) { return p[v] == -1 ? v : v = root(p[v]); }
  void unite(int u, int v) {
    u = root(u), v = root(v);
    if (u == v) return;
    if (sz[u] < sz[v]) swap(u, v);
    p[v] = u;
    sz[u] += sz[v];
  }
  int size(int v) { return sz[root(v)]; }
};

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  int h, w;
  while (cin >> w >> h, w) {
    vector<string> s(h);
    for (auto&& e : s) {
      cin >> e;
    }
    dsu d(h * w);
    for (int i = 0; i < h; ++i) {
      for (int j = 0; j < w; ++j) {
        if (s[i][j] != '#') {
          if (i + 1 < h and s[i + 1][j] != '#') {
            d.unite(i * w + j, (i + 1) * w + j);
          }
          if (j + 1 < w and s[i][j + 1] != '#') {
            d.unite(i * w + j, i * w + (j + 1));
          }
        }
      }
    }
    for (int i = 0; i < h; ++i) {
      for (int j = 0; j < w; ++j) {
        if (s[i][j] == '@') {
          cout << d.size(i * w + j) << '\n';
        }
      }
    }
  }
}