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,a,n) for(ll i=a;i<n;i++)
#define ALL(s) s.begin(),s.end()
#define P pair<ll,ll>
#define vl vector<ll>
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vvl vector<vector<ll>>
#define print(n) cout<<n<<endl
const int M=100010;
const int MOD=1000000007;
const int inf=1000000007;
const long long INF=1000000000000000007;
using ll=long long;
int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};

int ans=0;
int w,h;
char a[22][22];
bool f[22][22];
void dfs(int H,int W){
	if(W<0||W>=w||H<0||H>=h||a[H][W]=='#'||f[H][W]==true)return;
	if(a[H][W]=='.'&&f[H][W]==false){
		f[H][W]=true;
		ans++;
	}
	rep(i,0,4)dfs(H+dy[i],W+dx[i]);
	return;
}

int main(){
	while(1){
		ans=0;
		rep(i,0,22)rep(j,0,22)f[i][j]=false;
		cin>>w>>h;
		int start_x,start_y;
		if(w==0)break;
		rep(i,0,h)rep(j,0,w){
			cin>>a[i][j];
			if(a[i][j]=='@')start_y=i,start_x=j;
		}
		dfs(start_y,start_x);
		print(ans+1);
	}
	return 0;
}

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

using ll = long long;
#define debug(x) cerr << #x << ": " << x << endl;
#define line() cerr << "---------------" << endl;

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

void dfs(int x, int y) {
   for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= H || ny < 0 || ny >= W) continue;
      if (board[nx][ny] == '#') continue;
      if (visited[nx][ny]) continue; // これ必要な理由がわからない
      visited[nx][ny] = true;
      dfs(nx, ny);
   }
}

int main() {
   while (cin >> W >> H, W && H) {
      memset(visited, false, sizeof(visited));

      int sx = 0, sy = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            cin >> board[i][j];
            if (board[i][j] == '@') {
               sx = i, sy = j;
            }
         }
      }

      visited[sx][sy] = true;
      dfs(sx, sy);

      int cnt = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (visited[i][j]) cnt++;
         }
      }
      cout << cnt << endl;

   }

   return 0;
}

C++: #include<iostream>
#include<string>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
using namespace std;
typedef long long ll;
const ll MOD = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
typedef pair<int, int> P;
typedef unsigned int ui;
int main(){
	int w, h;
	string s;
	int i, j;
	while (cin >> w >> h) {
		if (w == 0)break;
		char tile[22][22]; int used[22][22];
		for (i = 0; i < 22; i++) {
			for (j = 0; j < 22; j++) {
				used[i][j] = 0; tile[i][j] = '#';
			}
		}
		vector<int> q1,q2;
		for (j = 1; j <= h; j++) {
			cin >> s;
			for (i = 1; i <= w; i++) {
				tile[i][j] = s[i - 1];
				if (s[i - 1] == '@') {
					q1.push_back(i); q2.push_back(j); used[i][j] = 1;
				}
			}
		}
		while (!q1.empty()) {
			int x = q1[0]; int y = q2[0];
			q1.erase(q1.begin() + 0); q2.erase(q2.begin() + 0);
			if (used[x - 1][y] == 0 && tile[x - 1][y]=='.') {
				q1.push_back(x - 1); q2.push_back(y); used[x - 1][y] = 1;
			}
			if (used[x + 1][y] == 0 && tile[x + 1][y] == '.') {
				q1.push_back(x + 1); q2.push_back(y); used[x + 1][y] = 1;
			}
			if (used[x][y-1] == 0 && tile[x][y-1] == '.') {
				q1.push_back(x); q2.push_back(y - 1); used[x][y - 1] = 1;
			}
			if (used[x][y+1] == 0 && tile[x][y+1] == '.') {
				q1.push_back(x); q2.push_back(y + 1); used[x][y + 1] = 1;
			}
		}
		int count = 0;
		for (i = 0; i < 22; i++) {
			for (j = 0; j < 22; j++) {
				if (used[i][j] == 1)count++;
			}
		}
		cout << count << endl;
	}
	return 0;
}

C++: #include <stdio.h>
#include <string.h>
int cnt = 0;
int W, H;

void search(char data[][22], int self_i, int self_j) {
  //  printf("self_i = %d  self_j = %d\n", self_i, self_j);
  data[self_i][self_j] = '#';
  cnt++;
  if (self_i-1 >= 0 && data[self_i-1][self_j] == '.') {
	search(data, self_i-1, self_j);
  }
  if (self_j-1 >= 0 && data[self_i][self_j-1] == '.') {
	search(data, self_i, self_j-1);
  }
  if (self_j+1 <= W && data[self_i][self_j+1] == '.') {
	search(data, self_i, self_j+1);
  }
  if (self_i+1 <= H && data[self_i+1][self_j] == '.') {
	search(data, self_i+1, self_j);
  }
}

int main(void) {
  while(1) {
	cnt = 0;
	scanf("%d%d", &W, &H);
	if (W == H && W == 0) break;
	char data[22][22] = {{0}};
	for (int i = 0; i < H; i++) {
	  scanf("%s", data[i]);
	}
	int self_i, self_j;
	for (int i = 0; i < H; i++) {
	  for (int j = 0; j < W; j++) {
		if (data[i][j] == '@') {
		  self_j = j;
		  self_i = i;
		  goto L1;
		}
	  }
	}
  L1:
	search(data, self_i, self_j);
	printf("%d\n", cnt);


  }
  return 0;
}

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

typedef pair<int,int> pii;

int main(void){
	int h,w;
	stack<pii> s;
	string m[21];
	while(cin>>w>>h,w||h){
		for(int i=0;i<h;i++)cin>>m[i];
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(m[i][j]=='@'){
					s.push(pii(i,j));
					i=j=999;
				}
			}
		}
		int c=0;
		while(!s.empty()){
			pii a=s.top();s.pop();
			int i=a.first;
			int j=a.second;
			if(!(0<=i&&i<h&&0<=j&&j<w))continue;
			if(m[i][j]=='#')continue;
			c++;
			m[i][j]='#';
			s.push(pii(i+1,j));
			s.push(pii(i-1,j));
			s.push(pii(i,j+1));
			s.push(pii(i,j-1));
		}
		cout<<c<<endl;
	}
	return 0;
}