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;

int dfs(char tebl[][20],int y,int x,int h,int w,int ans){
	tebl[y][x]='$';
	if(y>0&&tebl[y-1][x]=='.'){//up
		ans=max(ans,dfs(tebl,y-1,x,h,w,ans+1));
	}
	if(y<h&&tebl[y+1][x]=='.'){//down
		ans=max(ans,dfs(tebl,y+1,x,h,w,ans+1));
	}
	if(x>0&&tebl[y][x-1]=='.'){//left
		ans=max(ans,dfs(tebl,y,x-1,h,w,ans+1));
	}
	if(x<w&&tebl[y][x+1]=='.'){//right
		ans=max(ans,dfs(tebl,y,x+1,h,w,ans+1));
	}
	return ans;
}
	




int main(){
	while(true){
		char tebl[20][20]={};
		int w,h,sy,sx;
		cin>>w>>h;
		if(w==0||h==0){
			break;
		}
		for(int n=0;n<h;n++){
			for(int m=0;m<w;m++){
				cin>>tebl[n][m];
				if(tebl[n][m]=='@'){
					sy=n;
					sx=m;
				}
			}
		}
		cout<<dfs(tebl,sy,sx,h-1,w-1,1)<<endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
typedef pair<P,P> PP;
const ll MOD = 1000000007;
const int IINF = INT_MAX;
const ll LLINF = LLONG_MAX;
const int MAX_N = int(1e5 + 5);
const double EPS = 1e-10;
const int di[] = {0, 1, 0, -1}, dj[] = {1, 0, -1, 0};
#define REP(i, n) for (int i = 0; i < n; i++)

struct UnionFindTree {
    vector<int> par;
    vector<int> rank;
    vector<int> siz;

    void init(int n) {
        par.resize(n);
        rank.resize(n);
        siz.resize(n);
        for (int i = 0; i < n; i++) {
            par[i] = i;
            rank[i] = 0;
            siz[i] = 1;
        }
    }
    int find(int x) {
        if (par[x] == x) {
            return x;
        } else {
            return par[x] = find(par[x]);
        }
    }
    void unite(int x, int y) {
        x = find(x);
        y = find(y);
        if (x == y) return;
        if (rank[x] < rank[y]) swap(x, y);
        if (rank[x] == rank[y]) rank[x]++;
        par[y] = x;
        siz[x] += siz[y];
    }
    bool is_same(int x, int y) {
        return find(x) == find(y);
    }
    int size(int x) {
        x = find(x);
        return siz[x];
    }
};

int main() {
    int h, w;
    while(cin >> w >> h, h){
        string s[25];
        int si, sj;
        UnionFindTree g;
        g.init(h*w);
        REP(i,h) cin >> s[i];
        REP(i,h){
            REP(j,w){
                if(s[i][j]=='#')continue;
                if(s[i][j]=='@'){
                    si = i;
                    sj = j;
                }
                REP(k,4){
                    int ni = i+di[k], nj = j+dj[k];
                    if(ni < 0 || ni >= h) continue;
                    if(nj < 0 || nj >= w) continue;
                    if(s[ni][nj]=='#') continue;
                    g.unite(i*w+j,ni*w+nj);
                }
            }
        }
        cout << g.size(si*w+sj) << endl;
    }
    return 0;
}

C++: 
#include <iostream>
#include <stdio.h>
#include <string>
#include <sstream>
#include <vector>
#include <stdlib.h>
#include <math.h>

using namespace std;

//マップ探索
void check(int w, int h, int x, int y, int map[20][20], int *ans){

	if(map[x][y] != 0) *ans = *ans + 1;
	if(map[x][y] == 0) return;
	map[x][y] = 0;

	if(x > 0) check(w, h, x - 1, y, map, ans);
	if(x < w - 1) check(w, h, x + 1, y, map, ans);
	if(y > 0) check(w, h, x, y - 1, map, ans);
	if(y < h - 1) check(w, h, x, y + 1, map, ans);

}

int main(void){
	//FILE* fp_in = freopen("data.txt", "r", stdin);

	while(1) {
		//変数宣言
		int w, h ;
		int x, y ;

		int map[20][20];
		cin >> w >> h;

		if( w == 0 && h == 0 ) break;	//終了処理

		//マップ初期化
		for(int i = 0; i < h; i++) {
			string tmp;
			cin >> tmp;
			for(int j = 0; j < w; j++) {
				char ch = tmp.at(j);
				switch(ch) {
					case '.':
						map[j][i] = 1;
						break;
					case '#':
						map[j][i] = 0;
						break;
					case '@':
						x = j;
						y = i;
						map[j][i] = 2;
						break;
				}
			}
		}

		int ans = 0;
		check(w,h,x,y,map,&ans);

		cout << ans << endl;
	}

	
	//while(1){}
	return 0;
}

C++: #include <cstdio>
#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;
char heya[22][22];
char vis[22][22];
int H,W;

int solve(int x,int y){
  if(y<0||y>=W) return 0;
  if(x<0||x>=H) return 0;
  if(heya[x][y]!='.') return 0;
  if(vis[x][y]==1) return 0;
  vis[x][y]=1;
  int res=1;
  res+=solve(x+1,y);
  res+=solve(x-1,y);
  res+=solve(x,y+1);
  res+=solve(x,y-1);
  return res;
}

int main(void){
  int i,j;
  int sx,sy;
  while(cin>>W>>H,W||H){
    sx=sy=0;
    for(i=0;i<H;i++){
      for(j=0;j<W;j++){
        cin>>heya[i][j];
        vis[i][j]=0;
        if(heya[i][j]=='@'){
          sx=i;sy=j;
          heya[i][j]='.';
        }
      }
    }
    cout<<solve(sx,sy)<<endl;
  }
  return 0;
}

C++: #pragma GCC optimize("O3")
#include<bits/stdc++.h> 
using namespace std;
using ll=long long;
typedef  unsigned long long int ull;
typedef pair<ll,ll> P;
template<class T> using V=vector<T>; 
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
const ll inf=(1e18);
//const ll mod=998244353;
const ll mod=1000000007;
//ios_base::sync_with_stdio(false);
//cin.tie(NULL);
ll gcd(ll a,ll b) {return b ? gcd(b,a%b):a;}
ll lcm(ll c,ll d){return c/gcd(c,d)*d;}
struct __INIT{__INIT(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}} __init;
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
V<int> dx={0,-1,0,1},dy={-1,0,1,0};
int main(){
while(1){
	int h,w;
	cin>>w>>h;
	if(w==0)return 0;
	V<V<bool>> used(h,V<bool>(w,false));
	V<string> s(h);
	for(int i=0;i<h;i++)cin>>s[i];
	queue<P> q;
	for(int i=0;i<h;i++){
		for(int j=0;j<w;j++){
			if(s[i][j]=='@'){
				q.push(P(i,j));
				used[i][j]=true;
			}
		}
	}
	while(!q.empty()){
		P now=q.front();
		q.pop();
		for(int i=0;i<4;i++){
			int nx=dx[i]+now.fi,ny=dy[i]+now.se;
			if(nx<0||nx>=h||ny<0||ny>=w)continue;
			if(s[nx][ny]=='#'||used[nx][ny])continue;
			used[nx][ny]=true;
			q.push(P(nx,ny));
		}
	}
	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;
}
}