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 <bits/stdc++.h>
using namespace std;

#define rep(i, j, n) for(int i=j;i<n;++i)
#define all(i) i.begin(),i.end()
#define rall(i) i.rbegin(),i.rend()
#define inf 1e9
#define Puts(x) puts(x? "Yes":"No")
const int mod = 1e9 + 7;

int gcd(int n, int m) {return (m == 0? n : gcd(m, n % m));}
int dx[] = {0, 1, 0, -1};
int dy[] = {1, 0, -1, 0};

typedef pair<int, int> pi;
typedef long long i64;

template <class T> using vt = vector<T>;
template <class T> using vvt = vector<vector<T>>;

int w, h;
int cou;

void dfs(int y, int x, auto &grid, auto &visited) {
  if(!grid[y][x] || visited[y][x]) return;

  cou++;
  visited[y][x] = true;
  rep(i, 0, 4) {
    dfs(y + dy[i], x + dx[i], grid, visited);
  }
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  while(cin >> w >> h, w && h) {
    vvt<bool> grid(h + 2, vt<bool>(w + 2, false));
    vvt<bool> visited(h + 2, vt<bool>(w + 2, false));
    int sx, sy;

    rep(i, 0, h) {
      string s;
      cin >> s;
      rep(j, 0, w) {
        if(s[j] == '.') grid[i + 1][j + 1] = true;
        if(s[j] == '@') {
          sx = j + 1;
          sy = i + 1;
          grid[sy][sx] = true;
        }
      }
    }

    cou = 0;
    dfs(sy, sx, grid, visited);
    cout << cou << endl;
  }
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <cctype>

#include <iostream>
#include <vector>
#include <deque>
#include <queue>
#include <map>
#include <algorithm>

using namespace std;

#define rep(i, n) for(int i = 0; i < n; i++)
#define repi(i,l,n) for(int (i)=(int)(l);(i)<(int)(n);(i)++)
//#define d_arr(arr, h, w) rep(i,( (arr).size() ) ){ cout << "["; rep(j,(w) ) { cout << (arr)[i][j] << ", "; } cout << "]" << endl;}
#define d_arr(arr, type) rep(i, sizeof(arr)/sizeof(arr)[0] ){ cout << "["; rep(j,sizeof(arr[0])/sizeof(type)) { cout << (arr)[i][j] << ", "; } cout << "]" << endl;}

class Coordinates
{
    public:
        int x;
        int y;

        Coordinates(){ x = 0; y = 0;};
        Coordinates(int a, int b) { x = a; y = b;}

        Coordinates operator+(Coordinates p)
        {
            Coordinates tmp;
            tmp.x = x + p.x;
            tmp.y = y + p.y;
            return tmp;
        }

        friend bool operator==(const Coordinates a, const Coordinates b)
        {
            return (a.x == b.x && a.y == b.y);
        }
        friend bool operator<(const Coordinates a, const Coordinates b)
        {
            return (a.x < b.x && a.y < b.y);
        }
        friend bool operator<=(const Coordinates a, const Coordinates b)
        {
            return (a.x <= b.x && a.y <= b.y);
        }
        void show() { cout << "(" <<  y << ", " << x << ")" << endl; }
};
typedef Coordinates Coo;


Coo dir[4] = { Coo(0,-1), Coo(1,0), Coo(0,1), Coo(-1,0) };


char grid[30][30];
bool isVisit[30][30];
//start_pos
Coo start;
int h,w;

int four_max(int a, int b, int c, int d);
int max(int a, int b);
void dfs(Coo s);

int main()
{
    while (cin >> w >> h) 
    {
        if( w == 0 && h ==0) {return 0;}
        for (int i = 0; i < h; i++) 
        {
            for (int j = 0; j < w; j++) 
            {
                cin >> grid[i][j];
                if (grid[i][j] == '@') 
                {
                    //Coo(x, y)
                    start = Coo(j, i);
                }
                //ついでにisVisit初期化
                isVisit[i][j]=false;
            }
        }
        isVisit[start.y][start.x] = true;
        dfs(start);
        int cnt = 0;
        rep(i,h)
        {
            rep(j,w)
            {
                if( isVisit[i][j] == true ) { cnt++; }
            }
        }

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

//x,yから始まって最大行けるredの数返す関数
void dfs(Coo s)
{
    //s.show();
    vector<int> maxnum;
    rep(i, 4) 
    {
        Coo next = s + dir[i];
        //外に出てたら
        if ( next.y < 0 || next.x < 0  ||  h <= next.y || w <= next.x ) { continue; }
        //壁じゃなかったら
        if ( grid[next.y][next.x] == '#') { continue; }
        //既に訪れてたら
        if (isVisit[next.y][next.x] == true ) { continue; }

        isVisit[next.y][next.x] = true;
        dfs(next);
    }
    return;
}



int four_max(int a, int b, int c, int d)
{
    return max( max(a, b), max(c, d) );
}

int max(int a, int b) 
{
    if ( a > b)  { return a; }
    else         { return b; }
}

C++: #include <iostream>
#include <map>
#include <queue>
using namespace std;
typedef pair<int,int> P;

int w, h;
int dx[4] = {1,0,-1,0},dy[4] = {0,1,0,-1};
char cell[20][20];
bool used[20][20];

int main(){
	while(cin >> w >> h){
		if(w == 0 && h == 0) break;
		queue<P> que;
		for(int i = 0; i < 20; i++){
			for(int j = 0; j < 20; j++) used[i][j] = 0;
		}
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				cin >> cell[i][j];
				if(cell[i][j] == '@'){
					que.push(P(i,j));
					used[i][j] = 1;
				}
			}
		}
		while(!que.empty()){
			P p = que.front(); que.pop();
			for(int i = 0; i < 4; i++){
				int px = p.second+dx[i],py = p.first+dy[i];
				if(0<=px&&px<w&&0<=py&&py<h&&cell[py][px]=='.'&&used[py][px]==false){
					used[py][px] = true;
					que.push(P(py,px));
				}
			}
		}
		int ans = 0;
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				if(used[i][j]) ans++;
			}
		}
		cout << ans << endl;
	}
}