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 <fstream>
#include <algorithm>
#include <iomanip>
#include <map>
#include <queue>
#include <stack>
#include <set>
#include <ctime>
#include <iterator>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
using namespace std;

int main()
{
    int nb_lignes,nb_colonnes;
    while(cin>>nb_lignes>>nb_colonnes)
    {
        if(nb_lignes == 0)
            break;
        int res = 1;
        vector<string> lab(nb_lignes);
        vector<vector<bool> > visites(nb_lignes,vector<bool>(nb_colonnes,false));
        for(int c=0;c<nb_lignes;c++) cin>>lab[c];
        int xdep,ydep=0;
        for(xdep=0;xdep<nb_lignes&&lab[xdep][ydep]!='@';xdep++){ for(ydep=0;ydep<nb_colonnes&&lab[xdep][ydep]!='@';ydep++);if(lab[xdep][ydep]=='@')break;}
        vector<pair<int,int> > parcours;
        visites[xdep][ydep]=true;
        parcours.push_back(make_pair(xdep,ydep));
        while(parcours.size())
        {
            int xact = parcours.back().first;
            int yact = parcours.back().second;
            parcours.pop_back();
            int dx[4]={1,-1,0,0};
            int dy[4]={0,0,1,-1};
            for(int c=0;c<4;c++)
            {
                int x2 = xact + dx[c];
                int y2 = yact + dy[c];
                if(x2>=0&&x2<nb_lignes&&y2>=0&&y2<nb_colonnes&&lab[x2][y2]!='#'&&!visites[x2][y2])
                {
                    visites[x2][y2]=true;
                    res++;
                    parcours.push_back(make_pair(x2,y2));
                }
            }
        }
        cout<<res<<endl;
    }
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <queue>

using namespace std;
using Tiles = vector<string>;

struct Point {
	int x;
	int y;

	Point() :
		x{0}, y{0}
	{}

	Point(int _x, int _y) :
		x{_x}, y{_y}
	{}

	const Point operator+ (const Point& rhs) const
	{
		Point temp{x + rhs.x, y + rhs.y};
		return temp;
	}
};

const Point Arround[4] {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};

int dfs(const Tiles& tiles, const Point& start, int W, int H)
{
	int count{0};
	queue<Point> Q;

	// ????????¨????????????????????????????????????
	vector<vector<bool>> done(H, vector<bool>(W, false));

	Q.push(start);
	done[start.y][start.x] = true;

	while(true) {
		if(Q.empty()) break;

		const auto& pos = Q.front();
		Q.pop();

		if(tiles[pos.y][pos.x] == '#') continue;

		++count;

		for(const auto& a : Arround) {
			const auto& p = pos + a;

			if(p.x < 0 || W <= p.x) continue;
			if(p.y < 0 || H <= p.y) continue;
			if(done[p.y][p.x]) continue;

			done[p.y][p.x] = true;
			Q.push(p);
		}
	}

	return count;
}

int main()
{
	int W, H;

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

		Tiles tiles(H);
		Point start;

		for(int i = 0; i < H; ++i) {
			cin >> tiles[i];

			for(int j = 0; j < tiles[i].size(); ++j) {
				if(tiles[i][j] == '@') {
					start = Point{j, i};
					break;
				}
			}
		}

		cout << dfs(tiles, start, W, H) << endl;
	}

	return 0;
}

C++: #include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (int)n; ++i)
#define REP(i, a, b) for (int i = (int)a; i < (int)b; ++i)
#define rer(i, a, b) for (int i = (int)a; i <= (int)b; ++i)
#define each(i,c) for(__typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define all(v) v.begin(), v.end()
#define mset(a, n) memset(a, n, sizeof(a))

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<vector<int> > vvi;
typedef vector<pair<int, int> > vpii;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;

const int inf = 1000000000;
const int mod = 1000000007;
const double eps = 1e-9;
const int dx[] = { -1, 0, 1, 0};
const int dy[] = { 0, -1, 0, 1};

string field[100];

void dfs(int y, int x) {
	if (x < 0 || y < 0 || x == w || y == h) return;
	if (field[y][x] == '#') return;
	ans++;
	field[y][x] = '#';
	rep(i, 4) dfs(y + dy[i], x + dx[i]);
}

int main() {
	int W, H;
	while (cin >> W >> H, W) {
		int ans = 0;
		rep(i, H) cin >> s[i];
		rep(i, H) rep(i, W) {
			if (s[i][j] == '@') solve(i, j);
		}
	}
	return 0;
}

C++: #include <iostream>
#include <string.h>
#include <queue>

using namespace std;
typedef pair<int, int> P;

char t[20][20];
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
int ans,h,w;
queue<P> q;

int bfs(int x, int y){

  q.push( P(x, y) );
  
  while( !q.empty() ){
    P now = q.front();
    q.pop();
    for( int i = 0 ; i < 4 ; i++ ){
      int nx = now.first + dx[i];
      int ny = now.second + dy[i];
      if( 0 <= nx && nx < w && 0 <= ny && ny <= h 
	  && t[ny][nx] == '.' ){
	t[ny][nx] = '#';
	q.push( P(nx, ny) );
	ans++;
      }
    }
  }
  return ans;
}

int main(void){

  int x,y;
  while( cin >> w >> h, h + w ){

    memset( t, 0, sizeof(t) );
    ans = 1;

    for( int i = 0 ; i < h ; i++ ){
      for( int j = 0 ; j < w ; j++ ){
	cin >> t[i][j];
	if( t[i][j] == '@' ){
	  x = j;
	  y = i;
	}
      }
    }
    cout << bfs(x,y) << endl;
  }
  return 0;
}

C++: #include<stdio.h>
#include<string.h>

char map[20][20]={0};
int number=1;
int h,w;

void debug(void) {
	for(int i=0;i<h;i++) { for(int j=0;j<w;j++) putchar(map[i][j]); putchar('\n'); }
}

void trace(int x,int y) {
	if(x-1 >= 0 && map[y][x-1]=='.') {
		map[y][x-1] = '@';
		trace(x-1,y);

		number++;
	}

	if(x+1 < w && map[y][x+1]=='.') {
		map[y][x+1] = '@';
		trace(x+1,y);
		number++;
	}
	if(y-1 >= 0 && map[y-1][x]=='.') {
		map[y-1][x] = '@';
		trace(x,y-1);
		number++;
	}

	if(y+1 < h && map[y+1][x]=='.') {
		map[y+1][x] = '@';
		trace(x,y+1);
		number++;
	}
}

int main(void) {
	char buf[21];

	int x,y;


	for(;;) {

		number=1;
		scanf("%d %d\n",&w,&h);

		if(w==0 || h==0) break;

		for(int i=0;i<h;i++) {
			fgets(buf,1024,stdin);

			memcpy(&map[i],buf,w);
		}


		for(int j=0;j<h;j++) {
			for(int i=0;i<w;i++) {
				if(map[j][i] == '@'){
					x=i;y=j;
				}
			}
		}


		trace(x,y);

		printf("%d\n",number);

	}

	//debug();

	return 0;
}