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 FOR(i,a,b) for(int i=(a);i<(int)(b);i++)
#define rep(i,n) FOR(i,0,n)
#define ZERO(a) memset(a,0,sizeof(a))

int w,h,ans;
char f[22][22];
bool visited[22][22];
int dx[] = {1,0,-1,0}, dy[] = {0,1,0,-1};

bool ng(int x, int y) {
    return (x<0 or x>=w or y<0 or y>=h);
}

void dfs(int x, int y) {
    if(x>=w or y>=h) return ;
    visited[y][x] = true;
    ans++;
    rep(i,4) {
        int nx = x+dx[i], ny = y+dy[i];
        if(!ng(nx,ny) and !visited[ny][nx] and f[ny][nx]!='#')
            dfs(nx,ny);
    }

    return ;
}

int main() {
    while(cin>>w>>h && w) {
        int sx=0, sy=0;
        ans = 0;
        ZERO(visited);
        rep(y,h) rep(x,w) f[y][x] = '$';
        rep(y,h) rep(x,w) {
            cin>>f[y][x];
            if(f[y][x] == '@') {sx=x; sy=y;}
        }

        dfs(sx,sy);
        cout << ans << endl;
    }

    return 0;
}

C++: 1>------ テ」ツδ禿」ツδォテ」ツδ嘉ゥツ鳴凝・ツァツ? テ」ツδ療」ツδュテ」ツつクテ」ツつァテ」ツつッテ」ツδ?4_3, テヲツァツ凝ヲツ按?Debug Win32 ------
1>  Source3.cpp
1>  4_3.vcxproj -> C:\Users\hokiken\Documents\programming\contest5\Debug\4_3.exe
========== テ」ツδ禿」ツδォテ」ツδ? 1 テヲツュツ」テ・ツクツクテァツオツづ、ツコツ?」ツ?? テ・ツ、ツアテヲツ閉療」ツ?? テヲツ崢エテヲツ鳴ーテ、ツクツ催ィツヲツ?」ツ?? テ」ツつケテ」ツつュテ」ツδε」ツδ?==========

C++: #include <iostream>
#include <fstream>
#include <algorithm>
#include <vector>
#include <string>
#include <functional>
#include <array>
#include <stack>
#include <queue>
#include <chrono>
#include <numeric>

#pragma warning(disable:4996)
#define REP(i,n) for(int i = 0 ; i < n ; ++i)
typedef long long Int;

using namespace std;

struct point{
	int x;
	int y;
	point(int _x, int _y) :x(_x), y(_y){}
};

char tile[20][20];
stack<point> st;

int main()
{
	cin.tie(0); ios::sync_with_stdio(false); 
	fstream fs("input.txt");
	////////////ここから開始///////////

	
	int x(-1), y(-1);
	while (x != 0 && y != 0){
		REP(i, 20)REP(j, 20)tile[i][j] = '#';
		cin >> x >> y;
		//fs >> x >> y;
		REP(i, y){
			REP(j, x){
				cin >> tile[i][j];
				//fs >> tile[i][j];
			}
		}

		REP(i, y){
			REP(j, x){
				if (tile[i][j] == '@')
					st.emplace(i, j);
			}
		}

		int dx[]{0, 1, 0, -1};
		int dy[]{1, 0, -1, 0};
		int ans(1);

		while (!st.empty()){
			point now = st.top();
			st.pop();
			REP(i, 4){
				int x1 = now.x + dx[i];
				int y1 = now.y + dy[i];
				if (x1 > 19 || x1 < 0 || y1 > 19 || y1 < 0)
					continue;

				if (tile[x1][y1] == '.'){
					st.emplace(x1, y1);
					tile[x1][y1] = '#';
					++ans;
				}
			}
		}

		cout << ans << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<string>
#include<vector>
#include<algorithm> 
#include<functional> 
#include<cassert>
#include<numeric> // std::accumulate(vec.begin(),vec.end(),0) sum of element
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<queue>
#include<map>
#include<set>


using namespace std;


typedef long long ll;
typedef std::pair<int, int> P;
typedef vector<int> vi;


#define rep(i,n) for(int i=0;i<n;i++)
#define rep1(i,n) for(int i=1;i<=(int)(n);i++)
#define rrep(i,n) for(int i=n-1;i>=0;i--)
#define all(c) c.begin(),c.end()
#define show(x) cout << #x << " = " << x << endl

#define fi first
#define se second
#define pb push_back



#define DEBUG


// void print_vector(const std::vector<int> &t);
// void unique_vector(std:: vector<int> &t);

template <class X>
void print_vector(const std::vector<X> &t);
template <class X>
void unique_vector(std:: vector<X> &t);


int w, h;

vector<int> xl={-1, 0, 1, 0};
vector<int> yl={0, 1, 0, -1};

int main(){

  vi ans;
  while(true){
    cin >> w >> h;
    if(w == 0 && h == 0) break;
    vector<vector<char>> f(h, vector<char>(w));
    queue<P> ser;
    for(int i = 0; i < h; i++){
      for(int j = 0; j < w; j++) {
	cin >> f[i][j];
	if(f[i][j] == '@')
	  ser.push(make_pair(i, j));
      }
    }
    int c = 0;
    while(!ser.empty()){
      P t = ser.front();
      ser.pop();
      int y = t.fi, x = t.se;
      // show(x); show(y);
      if(f[y][x] == '@') f[y][x] = '#';
      for(int i = 0; i < 4; i++) {
	int dx = x+xl[i], dy=y+yl[i];
	// cout << dx << ' ' << dy << endl;
	// show(dx); show(dy);
	if(dy < 0 || dy >= h || dx < 0 || dx >= w || f[dy][dx] == '#')
	  continue;
	
	if(f[dy][dx] == '.'){
	  c++;
	  ser.push(make_pair(dy, dx));
	  f[dy][dx] = '#';
	}
      }	
      // for(int i = 0; i < h; i++) {
      // 	for(int j = 0; j < w; j++) {
      // 	  cout << f[i][j] << ' ';
      // 	}
      // 	cout << endl;
      // }
      // cout << endl;
    }
    ans.pb(c+1);
  }
  for(auto &i : ans) cout << i << endl;
}

template <class X>
void unique_vector(std::vector<X> &t)
{
  std::sort(t.begin(), t.end());
  t.erase( std::unique(t.begin(), t.end()), t.end() );
}

template <class X>
void print_vector(const std::vector<X> &t){
  for(auto itr=t.begin(); itr != t.end(); itr++){
    std::cout << *itr;
    if(itr != t.end()-1) std::cout << ' ';
  }
  std::cout << std::endl;
}

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

#define REP(i, n) for(int i = 0;i < (n);i++)
#define FOR(i, a, b) for(int i = (a);i < (b);i++)
#define all(v) v.begin(),v.end()

using int64 = long long;

int dx[4] = {-1, 1, 0, 0};
int dy[4] = {0, 0, -1, 1};

int main(void){
	int H, W;
	while(cin >> H >> W && H && W){
		vector<string> f(H+2, string(W+2, '#'));
		int sy, sx;
		REP(i, H){
			cin >> f[i+1];
			REP(j, W+2)
				if(f[i+1][j] == '@'){
					sy = i+1;
					sx = j;
				}
			f[i+1][0] = f[i+1][W+1] = '#';
		}
		bool visited[30][30] = {};
		int cnt = 0;
		
		function<void(int, int)> dfs = [&](int y, int x){
			visited[y][x] = 1;
			cnt++;
			REP(i, 4){
				int yy = y+dy[i], xx = x+dx[i];
				if(f[yy][xx] != '#' && !visited[yy][xx]){
					dfs(yy, xx);
				}
			}
		};
		dfs(sy, sx);
		cout << cnt << endl;
	}
}'