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++: // ICPC 2004 à\I B
// Red and Black
// C++ queue Å

#include <cstdio>
#include <queue>

using namespace std;

struct Coord {
  int col;
  int row;
};

int solve (int W, int H);
  
int main () {
  while (1) {
    int W, H;
    scanf("%d%d", &W, &H);
    if (W==0 && H==0) break;
    printf("%d\n", solve(W,H));
  }

  return 0;
}

int solve (int W, int H) {
  char M[H+2][W+2];
  Coord start;
  //  ÇiÔºjÌÝè 
  for (int i=0; i<=W+1; i++) { M[0][i] = '#'; M[H+1][i] = '#'; }
  for (int i=1; i<=H; i++) { M[i][0] = '#'; M[i][W+1] = '#'; }
  //  f[^ÌÇÝÝ
  for (int i=1; i<=H; i++) {
    for (int j=1; j<=W; j++) {
      scanf(" %c", &M[i][j]);
      if (M[i][j]=='@') {
	start.row = i;	start.col = j;
      }
    }
  }


  // X^[gn_ÌÝè
  queue<Coord> Q;                              // BZðÛ·éL[
  Q.push(start);                               // X^[gn_ð enqueue
  int c=1;                                     // BZJE^

  // ÈºTõi±ê©çìéj
	while( !Q.empty()){
		Coord current = Q.front();Q.pop();
		int D1[] ={1,0,-1,0};int D2[]={0,-1,0,1};
		for ( int d=0;d<4;d++){
			Coord nc = current;
			nc.row += D1[d]; nc.col += D2[d];
		printf("r:%d , c:%d\n",nc.row ,nc.col);
			if (M[ nc.row ][ nc.col ]=='.'){
				M[ nc.row ][ nc.col ]='*';
				Q.push(nc);
				c++;
			}
		}
	}

  return c;
}

C++: #include<iostream>
#include<algorithm>
#include<string>
#include<cstring>
#include<sstream>
#include<iomanip>
#include<limits>
#include<deque>
#include<map>
#include<list>
#include<set>
#include <unordered_set>
#include<vector>
#include<cmath>
#include<cstdio>
#include<memory>
#include<bitset>
#include<stack>
#include<functional>
#include<queue>
#include<regex>
#include<time.h>

using namespace std;


#define YES cout<<"YES"<<endl
#define NO cout<<"NO"<<endl
#define Yes cout<<"Yes"<<endl
#define No cout<<"No"<<endl
#define yes cout<<"yes"<<endl
#define no cout<<"no"<<endl

#define Tof(x) (x) ? Yes : No
#define TOF(x) (x) ? YES : NO
#define tof(x) (x) ? yes : no

using ll = long long;

constexpr ll MOD = 1000000007;
constexpr ll INF = 1050000000;


/*vectorの要素を全部確認するやつだよ。普通に拡張for文したほうがいいよ。

for (vector<int>::iterator itr = ans.begin(); itr != ans.end(); ++itr) {
	cout << *itr << " ";
}

*/


/*for文でx,yを上下左右確認するやつだよ。正直使いづらいよ。

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


for (int i = 0; i < 4; i++) {
	int ny = y + dy[i];
	int nx = x + dx[i];
}

*/


/*繰り返し二乗法だよ。modもとってくれるよ。n^kをmodで割った余りでやってくれるよ。

ll POW_MOD(ll n, ll k, ll mod) {
	ll r = 1;

	for (; k > 0; k >>= 1) {
		if (k & 1) {
			r = (r * n) % mod;
		}
		n = (n * n) % mod;
	}
	return r;
}

*/


/*実行開始からの経過時間を表すよ。

while (clock() < 1900000) {  }

*/


/*まだ使えないテクニックのメモだよ。

for (int bit = 0; bit < (1<<n); ++bit)//bit全探索用のfor文だよ

*/


/*2重for文書くのめんどくなった時用だよ。

for (int i = 0; i < h; ++i) {
	for (int j = 0; j < w; ++j) {

	}
}

*/

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

int main() {
	int w, h;

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

		char a[22][22];
		memset(a, '!', sizeof(a));

		bool check[22][22];

		for (int i = 1; i <= h; i++) {
			for (int j = 1; j <= w; j++) {
				check[i][j] = false;
			}
		}

		queue<pair<int, int>> q;

		for (int i = 1; i <= h; i++) {
			for (int j = 1; j <= w; j++) {
				cin >> a[i][j];
				if (a[i][j] == '@') { q.push(make_pair(i, j)); check[i][j] = true; }
			}
		}


		while (!q.empty()) {
			pair<int, int> n = q.front(); q.pop();

			int x = n.first;
			int y = n.second;

			for (int i = 0; i < 4; i++) {
				if (a[x + dx[i]][y + dy[i]] == '.' && check[x + dx[i]][y + dy[i]] == false) {
					check[x + dx[i]][y + dy[i]] = true; q.push(make_pair(x + dx[i], y + dy[i]));
				}
			}
		}

		int ans = 0;

		for (int i = 1; i <= h; i++) {
			for (int j = 1; j <= w; j++) {
				if (check[i][j]) { ans++; }
			}
		}

		cout << ans << endl;

	}

	return 0;
}

C++: #include <bits/stdc++.h>

using namespace std;


int main()
{
  int W, H, i, j, f, d, count;
  int dx[4] = {0, 1, 0, -1};
  int dy[4] = {1, 0, -1, 0};
  char map[32][32];
  char s[32];

  while (1) {
    gets(s);
    sscanf(s, "%d %d", &W, &H); if (W == 0 && H == 0) { break; }
    for (i = 0; i < 30; i++) {
      strcpy(map[i], "##############################");
    }
    
    for (i = 0; i < H; i++) {
      gets(&map[i+1][1]);
      map[i+1][W+1] = '#';
      map[i+1][W+2] = '\0';
    }

    count = 1;
    while (1) {
      f = 0;
      for (i = 1; i <= H; i++) {
        for (j = 1; j <= W; j++) {
          if (map[i][j] == '@') {
            for (d = 0; d < 4; d++) {
              if (map[i+dy[d]][j+dx[d]] == '.') {
                map[i+dy[d]][j+dx[d]] = '@';
                count++;
                f = 1;
              }
            }
          }
        }
      }
      if (!f) {
        break;
      }
    }
    printf("%d\n", count);
  }
  return 0;
}