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 G[410];
int R[410][410];
int D[410];
int bfs(int src,int num);

int main() {
	int W,H,root;
	char c;


	while (true) {
		for(int i=0;i<400;i++) {
			for(int j=0;j<400;j++) {
				R[i][j] = 0;
			}
		}
		for(int i=0;i<400;i++) {
			G[i] = 0;
		}
		for(int i=0;i<400;i++) {
			D[i] = 100;
		}
		cin >> W >> H;
		if((W == 0) && (H == 0)) break;
		for(int i=0;i<W*H;i++) {
			cin >> c;
			if(c == '.') G[i] = 1;
			if(c == '#') G[i] = 0;
			if(c == '@') {
				G[i] = 1;
				root = i;
			}
		}
		for(int i=0;i<W*H;i++) {
			if(G[i] == 1) {
				if(i%W != 0) { //????????????
					if(G[i-1] == 1) {//??????????????? 
						R[i][i-1] = 1;
					}
				}
				if(i%W != W-1) { //????????????
					if(G[i+1] == 1) { //???????????????
						R[i][i+1] = 1;
					}			

				}
				if(i > W-1) { //???????????????
					if(G[i-W] == 1) { //???????????????
						R[i][i-W] = 1;
					}	

				}
				if(i < W*H-W) {//???????????????
					if(G[i+W] == 1){
						R[i][i+W] = 1;
					}	

				}
			}
		}
		cout << bfs(root,W*H) << endl;
	}
	/*
	for(int i=0;i<99;i++) {
		cout << i << " " << G[i] << endl;
	}
	cout << G[98] << endl;
	cout << R[87][98] << endl;
	*/
	
}





int bfs(int src,int num) {
	int cur,dst;
	int count = 1;
	//cerr << "bfs root = " << src << endl;
	queue<int> Q; // ??´??°????????????????????\????????????
	Q.push(src);
	D[src] = 0; // ?????????
	while (!Q.empty()) {
		
		int cur = Q.front(); // ??????????´????????????????
		Q.pop();
// ???????¢??????¨??¨???
		//cerr << cur << " " << D[cur] << endl;
		for (int dst=0;dst<num;dst++) { // ???????????? dst ????????????
			if ((R[cur][dst] == 1) && (D[dst] == 100)) { // cur ?????? dst ??????????????????dst ???????¨??????????
				D[dst] = D[cur]+1; //
				Q.push(dst); // dst ????¨???????????????????
				count++;
			}
		}
	}
	return count;
}

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

void calcCanReach( char c[20][20], int x, int y, int a, int b )
{
    c[b][a] = '@';
    if( b - 1 >= 0 && c[b-1][a] == '.' )
        calcCanReach( c, x, y, a, b - 1 );
    if( b + 1 < y && c[b+1][a] == '.' )
        calcCanReach( c, x, y, a, b + 1 );
    if( a - 1 >= 0 && c[b][a-1] == '.' )
        calcCanReach( c, x, y, a - 1, b );
    if( a + 1 < x && c[b][a+1] == '.' )
        calcCanReach( c, x, y, a + 1, b );
}

int main()
{
    char c[20][20];
    int x, y, a, b;

    while( cin >> x >> y )
    {
        if( x == 0 && y == 0 )
            break;

        for( int i = 0; i < y; i++ )
            for( int j = 0; j < x; j++ )
            {
                cin >> c[i][j];
                if( c[i][j] == '@' )
                {
                    a = j;
                    b = i;
                }
            }
        
        calcCanReach( c, x, y, a, b );

        int cnt = 0;
        for( int i = 0; i < y; i++ )
            for( int j = 0; j < x; j++ )
                if( c[i][j] == '@' )
                    cnt++;
        cout << cnt << endl;
    }
    return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<utility>
#include<queue>
#define loop(i,a,b) for(int i=a;i<b;i++)
#define rep(i,a) loop(i,0,a)
#define P pair<int, int>

using namespace std;
char field[20][20];
int W, H,sx,sy,nx,ny;
int dx[4] = { 1, 0, -1, 0 };
int dy[4] = { 0, 1, 0, -1 };
int cnt = 1;


void bfs(){
	queue<P> que;
	que.push(P(sx, sy));

	while (1){

		if (que.empty()){
			break;
		}
		P p = que.front();
		que.pop();
		
		rep(i, 4){
			nx = p.first + dx[i];
			ny = p.second + dy[i];
			if (nx >= 0 && nx < W && 0 <= ny && ny < H && field[ny][nx] == '.'){
				field[ny][nx] = '#';
				que.push(P(nx, ny));
				cnt++;
			}

		}

		/*

		rep(i, H){
			rep(j, W){
				cout << field[i][j];
			}
			cout << endl;
		}

		*/
	}



}





int main(){
	while (1){
		cin >> W >> H;
		if (W == 0 && H == 0){
			break;
		}
		rep(i, H){
			rep(j, W){
				cin >> field[i][j];
				if (field[i][j] == '@'){
					sx = j;
					sy = i;
				}
			}
		}
		bfs();
		cout << cnt << endl;
		cnt = 1;
	}
}