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 <queue>
using namespace std;
int dx[] = { -1, 0, 1, 0 };
int dy[] = { 0, -1, 0, 1 };

int main(){
	int W, H;
	while( cin >> W >> H, W, H ){
		char room[20][20];
		queue< pair<int, int> > xy;
		for( int i = 0; i < H; i++ ){
			for( int j = 0; j < W; j++ ){
				cin >> room[i][j];
				if( room[i][j] == '@' ){
					xy.push(pair<int, int>(i, j));
					room[i][j] = '#';
				}
			}
		}
		//cout << xy.front().first << ' ' << xy.front().second << endl;
		int sum = 1;
		while( !xy.empty() ){
			for( int i = 0; i < 4; i++ ){
				int x = xy.front().second + dx[i];
				int y = xy.front().first + dy[i];
				//cout << room[y][x];
				if( room[y][x] == '.' ){
					room[y][x] = '#';
					xy.push(pair<int, int>(y, x));
					sum++;
				}
			}
			//cout << endl;
			xy.pop();
		}
		cout << sum << endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
using ll = long long;
using P = pair<ll, ll>;
const long double PI = acos(-1.0L);
ll GCD(ll a, ll b) { return b?GCD(b, a%b):a; }
ll LCM(ll a, ll b) { return a/GCD(a, b)*b; }

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

int w, h;
struct xydata {
    int x, y;
};

int main() {
    while(1) {
        cin >> w >> h;
        if(w == 0 && h == 0) break;
        int sx, sy;
        vector< vector<char> > tile(h, vector<char>(w, '.'));
        for(int i = 0; i < h; ++i) {
            for(int j = 0; j < w; ++j) {
                cin >> tile[i][j];
                if(tile[i][j] == '@') {
                    sx = j; sy = i;
                }
            }
        }
        vector< vector<char> > ntile = tile;
        queue<xydata> que;
        xydata first; first.x = sx; first.y = sy;
        que.push(first);
        ntile[sy][sx] = '#';
        while(!que.empty()) {
            xydata now = que.front();
            que.pop();
            for(int i = 0; i < 4; ++i) {
                int nx = now.x+dx[i], ny = now.y+dy[i];
                if(nx >= 0 && nx < w && ny >= 0 && ny < h) {
                    if(ntile[ny][nx] == '.') {
                        xydata next;
                        next.x = nx, next.y = ny;
                        que.push(next);
                        ntile[ny][nx] = '#';
                    }
                }
            }
        }

        int cnt = 0;
        for(int i = 0; i < h; ++i) {
            for(int j = 0; j < w; ++j) {
                if(tile[i][j] == '.' || tile[i][j] == '@') {
                    if(ntile[i][j] == '#') cnt++;
                }
            }
        }
        cout << cnt << endl;
    }
}

C++: #include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <stack>
#include <queue>
#include <set>
#include <map>
//#include <functional>
using namespace std;
#define MOD 1000000007
#define ADD(X,Y) ((X) = ((X) + (Y)%MOD) % MOD)
typedef long long i64; typedef vector<int> ivec; typedef vector<string> svec; typedef pair<int, int> pi;
int dd[] = { 0, 1, 0, -1 };


int main()
{
	while (true)
	{
		int W, H;
		cin >> W >> H;
		if (W == 0 && H == 0) break;

		char C[30][30];
		int B[30][30];
		for (int i = 0; i < H; i++)
		{
			scanf("%s", C[i]);
		}
		int sx = -1; int sy = -1;
		for (int i = 0; i < H; i++)
		{
			for (int j = 0; j < W; j++)
			{
				B[i][j] = 0;
				if (C[i][j] == '@')
				{
					sy = i;
					sx = j;
				}
			}
		}

		queue<pi> Q;
		Q.push(pi(sy, sx));
		B[sy][sx] = 1;
		int ans = 1;
		while (!Q.empty())
		{
			int ty = Q.front().first; int tx = Q.front().second;
			Q.pop();

			for (int r = 0; r < 4; r++)
			{
				int y = ty + dd[r];
				int x = tx + dd[r + 1];
				if (y < 0 || y >= H || x < 0 || x >= W)continue;
				if (C[y][x] == '#')continue;
				if (B[y][x])continue;
				ans++;
				B[y][x] = 1;
				Q.push(pi(y, x));
			}
		}

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

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

int dx[4]={0,-1,0,1};
int dy[4]={1,0,-1,0};

char ma[30][30];

int W,H;
int dfs(int nx,int ny){
	if(ma[ny][nx]=='#')return 0;
	ma[ny][nx]='#';
	int re=1;
	for(int i=0;i<4;i++){
		int nex=nx+dx[i],ney=ny+dy[i];
		if(nex>=0 && ney>=0 && nex<W && ney<H)re+=dfs(nex,ney);
	}
	return re;
}


int main(){
	int sx,sy;
	while(cin>>W>>H && W+H){
		for(int i=0;i<H;i++)for(int j=0;j<W;j++){
			cin>>ma[i][j];
			if(ma[i][j]=='@'){sx=j,sy=i;}
		}
		cout<<dfs(sx,sy)<<endl;
	}
	
}