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;
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;
}

C++: #include<iostream>
#include<queue>
using namespace std;

int w,h;
char rb[20][20];	//redblack
int n = 0;	//乗れる
void rob(int,int);	//red or black

int main(){

	int k = 0;	//セットの数
	queue<int> able;	//乗れる数メモ
	while(1){
		int x,y;
		cin >> w >> h ;
		if (w == 0 && h == 0){
			break;
		}

		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				cin >> rb[j][i];
				if(rb[j][i] == '@'){
					x = j;
					y = i;
				}
			}
		}	//入力終了

	rob(x,y);
	able.push(n);
	n = 0;
	k++;
	}
	for(int i = 0; i < k; i++){
		cout << able.front() << endl;
		able.pop();
	}
	return 0;
}

	void rob(int x,int y){
		rb[x][y] = '#';
		n++;
		if(0 <= x-1 && rb[x-1][y] == '.'){
			rob(x-1,y);
		}
		if(x+1 < w && rb[x+1][y] == '.'){
			rob(x+1,y);
		}
		if(0 <= y-1 && rb[x][y-1] == '.'){
			rob(x,y-1);
		}
		if(y+1 < h && rb[x][y+1] == '.'){
			rob(x,y+1);
		}	//十字

		return ;
	}

C++: #include <iostream>

using namespace std;

int table[22][22],count;


void DFS(int,int);

int main(){
  int W,H,firstW,firstH;
  char a;


  while(cin >> W && cin >> H && W && H){
      count = 0;
  for(int i= 0;i < H+2;i++){
    for(int j = 0;j < W+2;j++){
      if(i == 0 || i == H+1)table[j][i] = 0;
      else table[0][i] = table[W+1][i] = 0;
    }
  }

  
    for(int i=1;i<H+1;i++){
      for(int j=1;j<W+1;j++){
          cin >> a;
          if(a == '.') table[j][i] = 1;
          if(a == '#') table[j][i] = 0;
          if(a == '@') table[j][i] = 1,firstW = j,firstH = i;
        }
      
    }


    DFS(firstW,firstH);
    cout << count << endl;
  }
  return 0;
}

void DFS(int W,int H) {
  count++;
  table[W][H] = 0;
  if(table[W-1][H] == 1) DFS(W-1,H);
  if(table[W][H-1] == 1) DFS(W,H-1);
  if(table[W+1][H] == 1) DFS(W+1,H);
  if(table[W][H+1] == 1) DFS(W,H+1);
}

C++: #include <iostream>
#include <math.h>
#include <algorithm>
#include <vector>
using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;

int main(){
	int w,h;
	vector<string> f(30);
	vector< vector<int> > g(30,vector<int>(30,0));
	int count = 0;
	int count2 = 0;

	while(1){
		cin >> w >> h;
		if(w == 0 && h == 0){
			break;
		}

		for(int i = 0;i < h; i++){
			cin >> f[i];
		}



		for(int i = 0;i < h; i++){
			for(int j = 0;j < w;j++){
				g[i][j] = 0;
			}
		}
		count = 0;
		count2 = 0;
		while(1){
			for(int i = 0; i < h; i++){
				for(int j = 0; j < w;j++){
					if(f[i][j] == '@' && count== 0){
						g[i][j] = 1;
						count2++;
					}else if(g[i][j] == 1){
						if(i != 0){
							if(f[i-1][j] == '.'){
								if(g[i-1][j] == 0){
									count2++;
								}
								g[i-1][j] = 1;

							}
						}
						if(i != h-1){
							if(f[i+1][j] == '.'){
								if(g[i+1][j] == 0){
									count2++;
								}
								g[i+1][j] = 1;
							}
						}
						if(j != 0){
							if(f[i][j-1] == '.'){
								if(g[i][j-1] == 0){
									count2++;
								}
								g[i][j-1] = 1;
							}
						}
						if(j != w-1){
							if(f[i][j+1] == '.'){
								if(g[i][j+1] == 0){
									count2++;
								}
								g[i][j+1] = 1;
							}
						}
					}else{}
				}
			}

			if(count == count2){
				cout << count << endl;
				break;
			}
			count = count2;
		}

	}
}