Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

type Node struct {
	ID    int
	X, Y  int
	Color Color
	Nexts []*Node
}

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])

		// Initialize map
		m := map[int]map[int]*Node{}
		for x := 1; x <= w; x++ {
			m[x] = map[int]*Node{}
			for y := 1; y <= h; y++ {
				m[x][y] = &Node{ID: x + y*w, X: x, Y: y}
			}
		}
		for x := 1; x <= w; x++ {
			for y := 1; y <= h; y++ {
				nexts := []*Node{}
				if n, ok := m[x-1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y-1]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x+1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y+1]; ok {
					nexts = append(nexts, n)
				}
				m[x][y].Nexts = nexts
			}
		}

		// Set colors
		queue := []*Node{}
		touchedIDs := map[int]struct{}{}
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					n := m[i+1][y]
					n.Color = ColorBlack
					if c == '@' {
						queue = append(queue, n)
						touchedIDs[n.ID] = struct{}{}
					}
				case '#':
					m[i+1][y].Color = ColorRed
				}
			}
		}

		// Search
		visitedIDs := map[int]struct{}{}
		for {
			if len(queue) == 0 {
				break
			}
			n := queue[0]
			queue = queue[1:]
			visitedIDs[n.ID] = struct{}{}
			for _, next := range n.Nexts {
				if next.Color == ColorRed {
					continue
				}
				if _, ok := touchedIDs[next.ID]; !ok {
					queue = append(queue, next)
					touchedIDs[n.ID] = struct{}{}
				}
			}
		}
		fmt.Println(len(visitedIDs))
	}
}

C++: #include <iostream>
#include <vector>
#include <string>
using namespace std;
vector<int> x;
vector<int> y;
vector<int> nextx;
vector<int> nexty;
int w,h;
int count;
bool check(int a,int b){
  for (int i=0;i<nextx.size();i++){
    if (nextx[i]==a&&nexty[i]==b)
      return false;
  }
  return true;
}
int search(int **field){
  x.clear();
  y.clear();
  for (int i=0;i<nextx.size();i++){
    x.push_back(nextx[i]);
    y.push_back(nexty[i]);
  }
  nextx.clear();
  nexty.clear();
  for (int i=0;i<x.size();i++){
    field[x[i]][y[i]]=0;
    count++;
  }
  for (int i=0;i<x.size();i++){
    if (field[x[i]-1][y[i]]==1&&x[i]-1>=0&&check(x[i]-1,y[i])){
      nextx.push_back(x[i]-1);
      nexty.push_back(y[i]);
    }
    if (field[x[i]+1][y[i]]==1&&x[i]+1<h&&check(x[i]+1,y[i])){
      nextx.push_back(x[i]+1);
      nexty.push_back(y[i]);
    }
    if (field[x[i]][y[i]-1]==1&&y[i]-1>=0&&check(x[i],y[i]-1)){
      nextx.push_back(x[i]);
      nexty.push_back(y[i]-1);
    }
    if (field[x[i]][y[i]+1]==1&&y[i]+1<w&&check(x[i],y[i]+1)){
      nextx.push_back(x[i]);
      nexty.push_back(y[i]+1);
    }
  }
  if (nextx.empty()){
    return -1;
  }
  int *f[h];
  for (int i=0;i<h;i++)
    f[i]=field[i];
  return search(f);
}
int main(){
  string temp;
  while (cin>>w>>h,w|h!=0){
    count=0;
    int field[h][w];
    for (int i=0;i<h;i++){
      cin>>temp;
      for (int j=0;j<w;j++){
	if (temp[j]=='.')
	  field[i][j]=1;
	if (temp[j]=='#')
	  field[i][j]=0;
	if (temp[j]=='@'){
	  field[i][j]=1;
	  nextx.push_back(i);
	  nexty.push_back(j);
	}
      }
    }
    int *f[h];
    for (int i=0;i<h;i++)
      f[i]=field[i];
    search(f);
    cout<<count<<endl;
  }
}

C++: #include <bits/stdc++.h>
using namespace std;

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

int dfs(int y, int x) {
   board[y][x] = '#';
   int ret = 1;
   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 && board[ny][nx] == '.') {
         ret += dfs(ny, nx);
      }
   }
   return ret;
}

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

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

   return 0;
}

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

char map[20][20];

int W, H, start_x, start_y, ans = 0;

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

void dfs(int x, int y)
{
  ans++;
  if(map[x][y] != '#'){
    map[x][y] = '#';
    for(int i = 0; i < 4; i++){
      int next_x = x + dx[i], next_y = y + dy[i];
      if(next_x < W && next_x >= 0 && next_y < H && next_y >= 0){
	if(map[next_x][next_y] == '#')
	  continue;
	dfs(next_x, next_y);
      }else{
	continue;
      }
    }
  }
}

int main()
{
  while(1){
    cin >> W >> H;
    if(W == 0 && H == 0){
      break;
    }else{
      for(int i = 0; i < H; i++){
	for(int j = 0; j < W; j++){
	  cin >> map[j][i];
	  if(map[j][i] == '@'){
	    start_x = j;
	    start_y = i;
	  }
	}
      }
      dfs(start_x, start_y);
      cout << ans << endl;
    }
  }
}