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;

struct UnionFind {
  vector< int > data;
  UnionFind(int sz) {
    data.assign(sz, -1);
  }
  int Size(int x) {
    return(-data[x]);
  }
  int Find(int k) {
    return(k < 0 ? k : data[k] = Find(data[k]));
  }
  void Unite(int a, int b) {
    a = Find(a), b = Find(b);
    if(a == b) return;
    data[a] += data[b];
    data[b] = a;
  }
};

int main() {
  int W, H;
  while(cin >> W >> H, W) {
    string mas[50];
    for(int i = 0; i < H; i++) cin >> mas[i];
    UnionFind uf(W * H);
    for(int i = 0; i < H; i++) {
      for(int j = 1; j < W; j++) {
        if(mas[i][j - 1] != '#' && mas[i][j] != '#') uf.Unite(i * W + j - 1, i * W + j);
      }
    }
    for(int i = 1; i < H; i++) {
      for(int j = 0; j < W; j++) {
        if(mas[i - 1][j] != '#' && mas[i][j] != '#') uf.Unite(i * W - W + j, i * W + j);
      }
    }
    for(int i = 0; i < H; i++) {
      for(int j = 0; j < W; j++) {
        if(mas[i][j] == '@') cout << uf.Size(i * W + j);
      }
    }
  }
}

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

#define rep(i, n) for (int i = 0; i < (int)n; ++i)
#define REP(i, a, b) for (int i = (int)a; i < (int)b; ++i)
#define rer(i, a, b) for (int i = (int)a; i <= (int)b; ++i)
#define each(i,c) for(__typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define all(v) v.begin(), v.end()
#define mset(a, n) memset(a, n, sizeof(a))

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<vector<int> > vvi;
typedef vector<pair<int, int> > vpii;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;

const int inf = 1000000000;
const int mod = 1000000007;
const double eps = 1e-9;
const int dx[] = { -1, 0, 1, 0};
const int dy[] = { 0, -1, 0, 1};

string field[];

void dfs(int y, int x) {
	if (x < 0 || y < 0 || x == w || y == h) return;
	if (field[y][x] == '#') return;
	ans++;
	field[y][x] = '#';
	rep(i, 4) dfs(y + dy[i], x + dx[i]);
}

int main() {
	int W, H;
	while (cin >> W >> H, W) {
		int ans = 0;
		rep(i, H) cin >> s[i];
		rep(i, H) rep(i, W) {
			if (s[i][j] == '@') solve(i, j);
		}
	}
	return 0;
}

C++: //
//AOJ volume11 1130
//http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=1130&lang=jp
//20170605
//640009E
//Yang Bo
//

#include <iostream>
#include <vector>

using namespace std;

int w, h;
int starty, startx;
vector< vector<char> > a(20, vector<char>(20, 'a')); //??£??\??????

int recursion(char iny, char inx)
{
  a[iny][inx] = '#';
  int number = 0;
  if (inx < w - 1)
    if (a[iny][inx + 1] == '.')
      number += recursion(iny, inx + 1);
  if (iny < h - 1)
    if (a[iny + 1][inx] == '.')
      number += recursion(iny + 1, inx);
  if (inx > 0)
    if (a[iny][inx - 1] == '.')
      number += recursion(iny, inx - 1);
  if (iny > 0)
    if (a[iny - 1][inx] == '.')
      number += recursion(iny - 1, inx);
  number += 1;
  return number;
  //cout << " recursion(" << iny << ", "<< inx << ") = " << number << endl; 
}

int main()
{
  while (cin >> w >> h) {
    if (w == 0)
      break;
    else {
      for (int i = 0; i < h; i++) {
	for (int j = 0; j < w; j++) {
	  cin >> a[i][j]; //??\???
	  if (a[i][j] == '@') {
	    starty = i;
	    startx = j;
	    //cout << "i = " << i << ", j = " << j << endl;
	  }
	}
      }
      int ans = recursion(starty, startx);
      cout << ans << endl;
    }
  }
}

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

#define rep(i,n) rep2(i,0,n)
#define rep2(i,m,n) for(int i=m;i<n;i++)

int w,h,ans,dy[]={-1,0,1,0},dx[]={0,-1,0,1};
char c[22][22];

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

int main(){
	while(cin>>w>>h&&w){
		ans=0;
		int y,x;
		rep(i,h)rep(j,w){
			cin>>c[i][j];
			if(c[i][j]=='@')y=i,x=j;
		}
		dfs(y,x);
		cout<<ans<<endl;
	}
}