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 used[23][23];
int answer;
int h,w;
string s[23];

void dfs(int y,int x){
  answer++;
  used[y][x] = 1;
  
  if( y-1 >= 0 && used[y-1][x]==0 && s[y-1][x]!='#') dfs( y-1,x );
  
  if( y+1 <  h && used[y+1][x]==0 && s[y+1][x]!='#') dfs( y+1,x );
  
  if( x-1 >= 0 && used[y][x-1]==0 && s[y][x-1]!='#') dfs( y,x-1 );
  
  if( x+1 <  w && used[y][x+1]==0 && s[y][x+1]!='#') dfs( y,x+1 );
}

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

    answer = 0;
    
    memset( used,0,sizeof(used) );
    for(int i=0;i<h;i++) cin>>s[i];
    
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	if(s[i][j]=='@'){
	  y = i;
	  x = j;
	}
      }
    }    
    dfs(y,x);
    cout<< answer <<endl;
  }
}

C++: #include <bits/stdc++.h>
using namespace std;
string place[20];
int dx[4]={0,1,0,-1},dy[4]={-1,0,1,0};
int serch(int n,int m,int y,int x)
{
	int ans=1;
	place[y][x]='#';
	for(int i=0,ax,ay;i<4;i++)if((ay=dy[i]+y)<m&&(ax=dx[i]+x)<n&&ay>=0&&ax>=0&&place[ay][ax]=='.')ans+=serch(n,m,ay,ax);
	return ans;
}
int main() {
	int n,m,y,x;
	while(cin>>n>>m,n,m){
		for(int i=0;i<m;i++){
			cin>>place[i];
			for(int j=0;j<n;j++){
				if(place[i][j]=='@'){
					x=j;
					y=i;
				}
			}
		}
		cout<<serch(n,m,y,x)<<endl;
	}
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <math.h>
#include <complex>
#include <queue>
#include <deque>
#include <stack>
#include <map>
#include <unordered_map>
#include <set>
#include <bitset>
#include <functional>
#include <assert.h>
using ll=long long;
#define int ll
#define rng(i,a,b) for(int i=int(a);i<int(b);i++)
#define rep(i,b) rng(i,0,b)
#define gnr(i,a,b) for(int i=int(b)-1;i>=int(a);i--)
#define per(i,b) gnr(i,0,b)
#define eb emplace_back
#define all(x) x.begin(),x.end()
#define SZ(x) ((int)x.size())
#define fi first
#define se second
const int INF = 1e15;
using namespace std;

template<class T> using vc=vector<T>;
template<class T> using vvc=vc<vc<T>>;

using pi=pair<int,int>;
using vi=vc<int>;

template<class T>
inline bool chmax(T &a,T b){if(a<b){a=b;return true;}return false;}
template<class T>
inline bool chmin(T &a,T b){if(a>b){a=b;return true;}return false;}

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

signed main() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout<<fixed<<setprecision(20);

    int N,M;
    while(true){
        cin >> N >> M;
        if(N==0 && M==0) break;
        int x,y;
        char C[M+2][N+2];
        rep(i,M+2) rep(j,N+2) C[i][j]='#';
        rep(i,M){
            rep(j,N){
                cin >> C[i+1][j+1];
                if(C[i+1][j+1]=='@'){
                    x=i+1; y=j+1;
                }
            }
        }
        queue<pi> que;
        que.emplace(x,y);
        int ans=0;
        while(!que.empty()){
            int a=que.front().fi,b=que.front().se;
            que.pop();
            if(C[a][b]=='#') continue;
            ans++;
            C[a][b]='#';
            rep(i,4){
                if(C[a+dx[i]][b+dy[i]]=='.'){
                    que.emplace(a+dx[i],b+dy[i]);
                }
            }
        }
        cout << ans << "\n";
    }
}

C++: #include <iostream>
#include <vector>
#include <unordered_map>
#include <unordered_set>
#include <string>
#include <sstream>

using namespace std;

int count_black (vector<vector<bool>> &is_black, int i, int j, int rows, int columns)
{
	if (i < 0 || rows <= i || j < 0 || columns <= j)
	{
		return 0;
	}
	
	if (is_black[i][j])
	{
		is_black[i][j] = false;
		int sum = 1;
		sum += count_black(is_black, i-1, j, rows, columns);
		sum += count_black(is_black, i+1, j, rows, columns);
		sum += count_black(is_black, i, j-1, rows, columns);
		sum += count_black(is_black, i, j+1, rows, columns);
		return sum;
	}
	else
	{
		return 0;
	}
}

int main ()
{
	while (true)
	{
		int rows, columns;
		cin >> columns >> rows;

		if (0 == rows && 0 == columns) break;

		vector<vector<bool>> is_black(rows);
		int start_i = 0, start_j = 0;
		for (int i=0; i<rows; ++i)
		{
			is_black[i].resize(columns, false);
			for (int j=0; j<columns; ++j)
			{
				char c;
				cin >> c;

				if ('.' == c)
				{
					is_black[i][j] = true;
				}
				else if ('@' == c)
				{
					is_black[i][j] = true;
					start_i = i;
					start_j = j;
				}
			}
		}

		cout << count_black(is_black, start_i, start_j, rows, columns) << endl;
	}
	
	return 0;
}

C++: #include <iostream>
#include <cstring>
#include <vector>
#include <string>

using namespace std;

int W, H;
vector<string> map;
int ans;

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

void dfs(int x, int y) {
	map[y][x] = '*';
	ans++;
	
	for (int d=0; d<4; d++) {
		if (!(0 <= y + dy[d] && y + dy[d] < H && 0 <= x + dx[d] && x + dx[d] < W)) continue;
		if (map[ y + dy[d] ][ x + dx[d] ] == '.') {
			dfs(x + dx[d], y + dy[d]);
		}
	}
}

int main() {
	while (1) {
		cin >> W >> H; if (W == 0 && H == 0) break;
		map.assign(H, "");
		
		int sx, sy;
		for (int i=0; i<H; i++) {
			cin >> map[i];
			for (int j=0; j<W; j++) if (map[i][j] == '@') { sx = j; sy = i; }
		}
		
		ans = 0;
		dfs(sx, sy);
		cout << ans << endl;
	}
	
	return 0;
}