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;

#define ll long long
#define pq priority_queue
#define debug(x) cerr << #x << ": " << x << endl;
#define line() cerr << "---------------" << endl;

int dx[] = {0, 0, -1, 1};
int dy[] = {1, -1, 0, 0};
int H, W;
char b[30][30];
int ans;

void dfs(int y, int x) {
   ans++;
   b[y][x] = '*';   
   for (int i = 0; i < 4; i++) {
      int ny = y + dy[i], nx = x + dx[i];
      if (ny>=0 && ny<H && nx>=0 && nx<W && b[ny][nx] == '.') {
         dfs(ny, nx);
      }
   }
}

int main() {
   while (cin >> W >> H, W) {
      for (int i = 0; i < H; i++) {
         cin >> b[i];
      }

      ans = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (b[i][j] == '@') {
               dfs(i, j);
            }
         }
      }
      cout << ans << endl;
   }

   return 0;
}

C++: #include <iostream>
#include <string>
#include <cstdlib>
#include <cmath>
#include <vector>
#include <map>
#include <set>
#include <algorithm>
#include <queue>
#include <stack>
#include <functional>
#include <bitset>
#include <assert.h>
using namespace std;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<char> vc;
typedef vector<string> vs;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef pair<ll,ll> P;
typedef vector<P> vpl;
typedef tuple<ll,ll,ll> tapu;
#define rep(i,n) for(ll i=0; i<(n); i++)
#define REP(i,a,b) for(int i=(a); i<(b); i++)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
const int inf = 1<<30;
const ll linf = 1LL<<62;
const int MAX = 510000;
ll dy[8] = {0,1,0,-1,1,-1,1,-1};
ll dx[8] = {1,0,-1,0,1,-1,-1,1};
const double pi = acos(-1);
const double eps = 1e-7;
template<typename T1,typename T2> inline bool chmin(T1 &a,T2 b){
	if(a>b){
		a = b; return true;
	}
	else return false;
}
template<typename T1,typename T2> inline bool chmax(T1 &a,T2 b){
	if(a<b){
		a = b; return true;
	}
	else return false;
}
template<typename T1,typename T2> inline void print2(T1 a, T2 b){cout << a << " " << b << "\n";}
template<typename T1,typename T2,typename T3> inline void print3(T1 a, T2 b, T3 c){
	cout << a << " " << b << " " << c << "\n";
}
const int mod = 1e9 + 7;
//const int mod = 998244353;

bool in(ll y, ll x, ll h, ll w){
	return 0 <= y && y < h && 0 <= x && x < w;
}

int main(){
	while(1){
		ll w,h; cin >> w >> h;
		if(!w && !h) break;
		vector<vc> a(h,vc(w));
		ll sy,sx;
		rep(i,h) rep(j,w) cin >> a[i][j];
		rep(i,h) rep(j,w) if(a[i][j]=='@') sy = i, sx = j;
		vvl d(h,vl(w,-1));
		d[sy][sx] = 1;
		queue<P> q;
		q.emplace(sy,sx);
		while(!q.empty()){
			P p = q.front(); q.pop();
			rep(i,4){
				ll ny = p.first + dy[i];
				ll nx = p.second + dx[i];
				if(!in(ny,nx,h,w) || d[ny][nx] > -1) continue;
				d[ny][nx] = a[ny][nx]=='.' ? 1 : 0;
				if(d[ny][nx]) q.emplace(ny,nx);
			}
		}
		ll ans = 0;
		rep(i,h) rep(j,w) if(d[i][j]!=-1) ans += d[i][j];
		cout << ans << endl;
	}
}

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

const int dx[] = {1,0,-1,0};
const int dy[] = {0,-1,0,1};




int main() {
	int W,H;
	int q[20][20];
	char p[20][20];
	queue<pair<int,int> > M;
	pair<int,int> X;
	while(cin >> W >> H){
		
		if (W == 0) return 0;
		int count = 0;
		for (int i = 1; i <= H; i++){
			for (int j = 1; j <= W; j++){
				cin >> p[i][j];
				if (p[i][j] == '@'){
					M.push(make_pair(i,j));
					count = 1;
					q[i][j] = 1;
				}
			}
		}
		while(!M.empty()){
			W = M.front().first;
			H = M.front().second;
			M.pop();
			for (int i = 0; i <= 3; i++){		
				if (p[W+dx[i]][H+dy[i]] == '.' && q[W+dx[i]][H+dy[i]] == 0){
					M.push(make_pair(W + dx[i], H + dy[i]));
					q[W + dx[i]][H + dy[i]] = 1;
					count += 1;
				}
			}
		}
		cout << count << endl;
	}
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <queue>
#include <utility>
using namespace std;
int xd[]={1,0,0,-1};
int yd[]={0,1,-1,0};
int main()
{
	int W,H;
	while(cin>>W>>H,W!=0){
		vector<string> board(H);
		queue<pair<int,int>> q;
		for(int i=0;i<H;i++)
			cin>>board[i];
		for(int i=0;i<H;i++){
			string::iterator
			p=find(board[i].begin(),board[i].end(),'@');
			if(p!=board[i].end()){
				q.push(pair<int,int>(i,p-board[i].begin()));
				break;
			}
		}
		int count=0;
		while(!q.empty()){
			pair<int,int> v=q.front();
			q.pop();
			if(board[v.first][v.second]=='#')continue;
			board[v.first][v.second]='#';
			count++;
			for(int i=0;i<4;i++){
				int newx=xd[i]+v.first;
				int newy=yd[i]+v.second;
				if(0<=newx&&newx<H&&0<=newy&&newy<W
						&&board[newx][newy]=='.')
					q.push(pair<int,int>(newx,newy));

			}
		}
		cout<<count<<endl;
	}
	return 0;
}