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>
using namespace std;
#define BLACK '.'
#define RED '#'
#define PEOPLE '@'
#define N 25

char room[N][N];
int h,w;

int dx[4] = {1,0,-1,0};
int dy[4] = {0,1,0,-1};
int count(int x, int y){
  if( x<1 || x>w || y<1 || y>h )
    return 0;

  if( room[y][x] == RED )
    return 0;
  room[y][x] = RED;

  int r=1;
  for( int i=0;i<4;i++ )
    r += count(x+dx[i], y+dy[i]);
  return r;
}


int main(){
  int st_x, st_y;
  while( cin >> w >> h,(w|h) ){
    for( int i=1;i<=h;i++ ){
      for( int j=1;j<=w;j++ ){
	cin >> room[i][j];
	if( room[i][j] == PEOPLE ){
	  st_x=j;st_y=i;
	  room[i][j] = BLACK;
	}
      }
    }
    cout << count(st_x, st_y) << endl;
  }
}

C++: #include <stdio.h>
#include <stdlib.h>
using namespace std;


const int MAX=30;

int count=0;

int w,h;
int tile[MAX][MAX];//0->rot 1->schwarz 2->besucht

int search(int a,int b)
{
    if(a<0 || a>=h || b<0 || b>=w)
    {
        return 0;
    }
    switch(tile[a][b])
    {
        case 0:
        case 2:
            return 0;
        case 1:
        {
            count++;
            tile[a][b]=2;
            search(a-1,b);
            search(a+1,b);
            search(a,b-1);
            search(a,b+1);
        }
    }
    return 1;
}


int main(void)
{
    while(scanf("%d%d",&w,&h)==2 &&(w||h))
    {
        int rc=0;
        int sa,sb;
        while(rc<w*h)
        {
            int ch=getchar();
            switch(ch)
            {
                case '.':
                    tile[rc/w][rc%w]=1;
                    rc++;
                    break;
                case '#':
                    tile[rc/w][rc%w]=0;
                    rc++;
                    break;
                case '@':
                    tile[rc/w][rc%w]=1;
                    sa=rc/w;
                    sb=rc%w;
                    rc++;
                    break;
            }
        }
        count=0;
        search(sa,sb);
        printf("%d\n",count);
    }
    return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<vector>
#include<queue>
#include<algorithm>
#include<cmath>
#include<climits>
#include<string>
#include<set>
#include<numeric>
#include<map>
#include<iostream>
using namespace std;
#define rep(i,n) for(int i = 0;i<((int)(n));i++)
#define reg(i,a,b) for(int i = ((int)(a));i<=((int)(b));i++)
#define irep(i,n) for(int i = ((int)(n)-1);i>=0;i--)
#define ireg(i,a,b) for(int i = ((int)(b));i>=((int)(a));i--)
typedef long long ll;
typedef pair<ll, ll> mp;

int W,H,f[20][20],ans=0,sx,sy;//finded
string m[20][20],t;

void find(int x,int y){
	if(x>=W || x<0 || y>=H || y<0)return;
	t = m[x][y];
	if(t=="#")return;
	if(f[x][y]!=0)return;
	f[x][y] = 1;
	ans++;
	cout<<x<<","<<y<<","<<ans<<endl;
	find(x+1,y);
	find(x-1,y);
	find(x,y+1);
	find(x,y-1);
}

int main(void){
	cin>>W>>H;
	rep(i,H){
		rep(j,W){
			cin>>m[j][i];
			t=m[j][i];
			if(t=="@"){
				sx=j;
				sy=i;
			}
		}
	}
	find(sx,sy);
	
	cout<<ans<<endl;
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef unsigned int uint;
typedef long long int lint;
typedef unsigned long long int ulint;

//#define DEBUG

#ifdef DEBUG
#	define dump(i) cout << "[*] " #i " : " << i << endl;
#	define debug(i) i
#else
#	define dump(i)
#	define debug(i)
#endif
#define rep(i, MAX) for (int i = 0; i < (MAX); ++i)

struct UnionFind
{
	vector<int> par;

	UnionFind(int n): par(n) {
		for (int i = 0; i < n; ++i) {
			par[i] = i;
		}
	}

	int find(int x) {
		if (x == par[x]) return x;
		return par[x] = find(par[x]);
	}

	bool same(int x, int y) {
		return find(x) == find(y);
	}

	void unite(int x, int y) {
		x = find(x);
		y = find(y);
		if (x == y) return;
		par[y] = x;
	}
};

inline void solve() {
	int w, h;
	constexpr int MAX = 20;
	char tile[MAX][MAX];
	dump(MAX);

	while (true) {
		pair<int, int> at;
		cin >> w >> h;
		if (!w && !h) return;
		int table[MAX] = {};
		UnionFind uf = UnionFind(200);

		const int asdf = MAX * MAX;
		dump(asdf);
		rep(i, h) {
			rep(j, w) {
				cin >> tile[i][j];
				if (tile[i][j] == '@') {
					at.first = i;
					at.second = j;
					tile[i][j] = '.';
				}
			}
		}
		int csi = 1;
		if (tile[0][0] == '.') {
			tile[0][0] = csi++;
		} else {
			tile[0][0] = 0;
		}
		table[tile[0][0]]++;
		for (int i = 1; i < w; ++i) {
			if (tile[0][i] == '.') {
				tile[0][i] = tile[0][i-1] != 0 ? tile[0][i-1] : csi++;
			} else {
				tile[0][i] = 0;
			}
			table[tile[0][i]]++;
		}
		rep(i, h) {
			if (i == 0) continue;
			rep(j, w) {
				if (j == 0) {
					if (tile[i][j] == '.') {
						tile[i][j] = tile[i-1][j] != 0 ? tile[i-1][j] : csi++;
					} else {
						tile[i][j] = 0;
					}
				} else {
					if (tile[i][j] == '.') {
						int tmp = tile[i-1][j];
						if (tmp == 0) {
							tmp = tile[i][j-1];
						} else if (tile[i][j-1] != 0) {
							uf.unite(tmp, tile[i][j-1]);
							if (tmp > tile[i][j-1]) {
								tmp = tile[i][j-1];
							}
						}
						tile[i][j] = tmp != 0 ? tmp : csi++;
					} else {
						tile[i][j] = 0;
					}
				}
				table[tile[i][j]]++;
			}
		}
		int ans = 0;
		int iv = tile[at.first][at.second];
		rep(i, csi) {
			if (uf.same(iv, i)) {
				ans += table[i];
			}
		}
		dump("tile");
		dump("table");
		dump("ans");
		cout << ans << endl;
	}
}

int main() {
	//*
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	// */
#ifdef DEBUG
	const auto start = chrono::high_resolution_clock::now();
#endif
	solve();
#ifdef DEBUG
	const auto stop = chrono::high_resolution_clock::now();
	cerr << "[*] time: " << chrono::duration_cast<chrono::milliseconds>(stop-start).count()
		<< " [ms]" << endl;
#endif
	return 0;
}