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 <stdio.h>
#include <string.h>
#include <queue>

using namespace std;
const int dx[4] = {1,0,-1,0},dy[4] = {0,1,0,-1};

int main(void){
	queue<int> qx, qy;
	int x, y, i, j, w, h, flag[20][20], count;
	char field[20][21];
	while(1){
		scanf("%d%d%*c",&w,&h);
		if(w == 0 && h == 0)
			break;
		memset(flag,0,sizeof(flag));
		for(i = 0;i < h;i++){
			for(j = 0;j < w;j++){
				field[i][j] = getchar();
				if(field[i][j] == '@')
					y = i, x = j;
			}
			getchar();
		}
		qx.push(x);
		qy.push(y);
		while(qx.size()){
			x = qx.front();
			y = qy.front();
			qx.pop();
			qy.pop();
			flag[y][x] = 1;
			for(i = 0;i < 4;i++){
				if(x + dx[i] >= 0 && x + dx[i] < w && y + dy[i] >= 0 && y + dy[i] < h && !flag[y + dy[i]][x + dx[i]] && field[y + dy[i]][x + dx[i]] == '.')
					qx.push(x + dx[i]), qy.push(y + dy[i]);
			}
		}
		count = 0;
		for(i = 0;i < h;i++){
			for(j = 0;j < w;j++)
				if(flag[i][j]) count++;
		}
		printf("%d\n",count);
	}
	return 0;
}

C++: #include <cstdio>

int W, H;
char tile[21][21];

int search(int y, int x) {
  if (tile[y][x] == '#') return 0;
  int cnt = 1;
  tile[y][x] = '#';
  if (y > 0)   cnt += search(y-1, x);
  if (y < H-1) cnt += search(y+1, x);
  if (x > 0)   cnt += search(y, x-1);
  if (x < W-1) cnt += search(y, x+1);
  return cnt;
}

int main() {
  while (true) {
    scanf("%d %d", &W, &H);
    if (W == 0 && H == 0) break;
    for (int i=0;i<H;i++) scanf("%s", tile[i]);
    int cnt = 0;
    for (int i=0;i<H;i++) {
      for (int j=0;j<W;j++) {
        if (tile[i][j] == '@') {
          cnt = search(i, j);
          break;
        }
      }
    }
    printf("%d\n", cnt);
  }
  return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <vector>
#include <math.h>
#include <set>
#include <map>
#include <string>
#include <stack>
#include <queue>
#include <iomanip>
#include <numeric>
#include <tuple>
#define _USE_MATH_DEFINES
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> plglg;
typedef tuple<int, int, int> tiii;
typedef tuple<ll, ll, ll> tlglglg;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
double pi = 3.141592653589793;
ll mod = 1000000007;
int intmax = 2147483647;
int intmin = -2147483648;
ll llmax = 9223372036854775807;
ll llmin = -9223372036854775807;

struct edge {
    int to, cost;
};

int H, W;
bool marked[100][100];
int sum;

void BFS(pii st) {
    queue<pii> que;
    que.push(st);
    sum++;
    marked[st.first][st.second] = false;
    while (que.size() > 0) {
        pii base = que.front();
        que.pop();
        int basex = base.first;
        int basey = base.second;
        for (int i = 0; i < 4; i++) {
            int x = basex + dx[i];
            int y = basey + dy[i];
            if (x >= 0 && x < H && y >= 0 && y < W) {
                if (marked[x][y]) {
                    que.push(pii(x, y));
                    sum++;
                    marked[x][y] = false;
                }
            }
        }
    }
}

int main() {
    while (true) {
        cin >> W >> H;
        if (W == 0) {
            break;
        }
        for (int i = 0; i < H; i++) {
            fill(marked[i], marked[i] + W, true);
        }
        pii st;
        for (int i = 0; i < H; i++) {
            string s;
            cin >> s;
            for (int j = 0; j < W; j++) {
                if (s[j] == '#') {
                    marked[i][j] = false;
                } else if (s[j] == '@') {
                    st = pii(i, j);
                }
            }
        }
        sum = 0;
        BFS(st);
        cout << sum << endl;
    }
}

C++: #include<iostream>

using namespace std;

char field[21][21];
int direction_x[] = {1, -1, 0, 0};
int direction_y[] = {0, 0, 1, -1};
int w, h;
int lx = 0, ly = 0;
int cnt = 0;

void set_data(int w, int h){
    for(int i=0; i < h; i++){
        for(int j=0; j < w; j++){
            cin >> field[i][j];
            if(field[i][j] == '@'){
                ly = i;
                lx = j;
            }
        }
    }
}

void count(int x, int y){
    field[y][x] ='#';
    cnt++;
    int nx = 0, ny = 0;

    for(int i=0; i<4; i++){
        nx = x + direction_x[i];
        ny = y + direction_y[i];

        if(nx < 0 || nx >= w){
            continue;
        }
        if(ny < 0 || ny >= h){
            continue;
        }
        if(field[ny][nx] != '.'){
            continue;
        }
        count(nx,ny);
    }

}

int main(void){

    while(1){
        cnt = 0;
        cin >> w >> h;
        if(w==0 || h==0){
            break;
        }

        set_data(w,h);
        count(lx,ly);

        cout << cnt << endl;
    }
    return 0;
}