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>
#include<queue>
#include<cstring>
// #include<stdio.h>
// 

/*
..#.#..
..#.#..
###.###
...@...
###.###
..#.#..
..#.#..
 */

using namespace std;

int W,H;
char c[100][101]; // c[H][W]

bool visited[100][100]; // <----

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

void compute() {

  memset(visited,false,sizeof visited); //memset(???????????????????????????1bit???????????????????????????) ?????????1bit????????§?????????

  queue<int> que;
  int i,j;
  for(i=0;i<H;++i) {
    for(j=0;j<W;++j) {
      if( c[i][j] == '@' ) {
	que.push(j+i*W);
	visited[i][j] = true;
	//break;
	goto Skip;
      }
    }
  }
 Skip:;

  int answer = 1; // <---???????????????
  while( !que.empty() ) { // !0 => 1  !1 => 0
    int cur = que.front(); que.pop(); // que -> [1,2,3,4], front -> 1, pop -> [2,3,4]
    int x = cur % W, y = cur / W; //???????????§?¨?
    
    for(i=0;i<4;++i) {
      int nx = x + dx[i], ny = y + dy[i]; //?¬??????§?¨?????¨?????????§?¨?

      if( !( 0 <= nx && nx < W && 0 <= ny && ny < H ) ) continue; // 0 && 0 => 0, 1 && 0 => 0, 0 && 1 => 0, 1 && 1 => 1
      if( c[ny][nx] == '#' || visited[ny][nx] ) continue;         // 0 || 0 => 0, 1 || 0 => 1, 0 || 1 => 1, 1 || 1 => 1
      
      que.push(nx+ny*W);
      visited[ny][nx] = true;
      ++answer;// <---???????????????

    }
    
  }

  cout << answer << endl;// <---???????????????

}

int main() {
  while( scanf("%d %d",&W,&H), W | H ) {
    //while( scanf("%d %d",&W,&H), W == 0 && H == 0 ) {
    int i;
    for(i=0;i<H;++i) {
      scanf(" %s",&c[i][0]);  // c[y][x]
    }
    compute();
  }
  return 0;
}

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

int w,h;
string s[100];
int ans;

void solve(int y,int x){
  int dx[] = {0,0,1,-1};
  int dy[] = {1,-1,0,0};

  if(x < 0 || y < 0 || x == w || y == h) return;

  if(s[y][x] == '#') return;

  s[y][x] = '#';

  ans++;

  for(int i=0;i<4;i++) solve(y+dy[i],x+dx[i]);

}

int main(){

  while(true){
    cin >> w >> h;
    if(!w && !h) break;
    ans = 0;

    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] == '@') solve(i,j);
      }
    }

    cout << ans << endl;

  }
}

C++: #include <iostream>
#include <math.h>
#include <queue>
#include <cstdio>
using namespace std;
int W, H;
char c[100][101];
bool visited[100][100];

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

void computer(){
    memset(visited,false,sizeof visited);
    queue<int> que;
    int i, j;
    for(i=0;i<H;i++){
        for(j=0;j<W;j++){
            if(c[i][j]=='@'){
                que.push(j+i*W);
                visited[i][j] = true;
                goto Skip;
            }
        }
    }
Skip:;
    
    int answer = 1;
    while( !que.empty()){
        int cur = que.front(); que.pop();
        int x = cur % W, y = cur / W;
        
        for(i=0;i<4;++i){
            int nx = x + dx[i],ny = y + dy[i];
            if(!(0<=nx && nx < W && 0<=ny && ny<H))continue;
            if(c[ny][nx]=='#' || visited[ny][nx])continue;
            que.push(nx+ny*W);
            visited[ny][nx]=true;
            ++answer;
        }
        
        
        
    }
    
    cout << answer << endl;
    
    
}

// abcdd
// efgeg

int main(){
    while(cin >> W >> H, W==0&&H==0){
        int i;
        for(i=0;i<H;++i){
            //cin >> c[i][0];
            scanf(" %s",&c[i][0]);
        }
        computer();
        
    }
    
    return 0;
}

C++: import java.util.*;
import java.awt.Point;

public class Main {
	static int w,h,sx,sy;
	static int[] dx = {-1, 0, 1, 0}, dy = {0, -1, 0, 1};
	static String[] field;
	static boolean[][] visited;
	static Scanner sc = new Scanner(System.in);
	public static void main(String[] args) {
		while(read()) {
			System.out.println(solve());
		}
	}
	
	static boolean read() {
		w = sc.nextInt(); h = sc.nextInt();
		if(w == 0 && h == 0) {
			return false;
		}
		field = new String[h];
		
		for(int i = 0; i < h; i++) {
			field[i] = sc.next();
			int tmp = field[i].indexOf('@');
			if(tmp != -1) {
				sy = i; sx = tmp;
			}
		}
		return true;
	}
	
	static int solve() {
		visited = new boolean[h][w];
		Queue<Point> que = new LinkedList<Point>();
		Point s = new Point(sx, sy);
		
		que.add(s);
		visited[sy][sx] = true;
		int ans = 1;
		while(!que.isEmpty()) {
			Point p = que.poll();
			for(int i = 0; i < 4; i++) {
				Point next = new Point(p.x + dx[i], p.y + dy[i]);
				if(!inField(next) || field[next.y].charAt(next.x) == '#' || visited[next.y][next.x] == true) {
					continue;
				} else {
					que.add(next);
					visited[next.y][next.x] = true;
					ans++;
				}
			}
		}
		return ans;
	}
	
	static boolean inField(Point p) {
		if(p.x >= 0 && p.x < w && p.y >= 0 && p.y < h) {
			return true;
		} else {
			return false;
		}
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define all(x) (x).begin(),(x).end()
const int mod=1000000007,MAX=55,INF=1<<30;
int H,W;
int cnt;
int state[MAX][MAX];
vector<int> dx={1,0,-1,0},dy={0,-1,0,1};

void DFS(int us,int ut){
    for(int i=0;i<4;i++){
        if(0<=us+dx[i]&&us+dx[i]<H&&0<=ut+dy[i]&&ut+dy[i]<W){
            if(state[us+dx[i]][ut+dy[i]]==0){
                cnt++;
                state[us+dx[i]][ut+dy[i]]=1;
                DFS(us+dx[i],ut+dy[i]);
            }
        }
    }
}

int main(){
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    while(1){
        cin>>W>>H;
        if(H==0&&W==0) break;
        
        int s,t;
        cnt=1;
        memset(state,0,sizeof(state));
        
        for(int i=0;i<H;i++){
            for(int j=0;j<W;j++){
                char c;cin>>c;
                if(c=='@'){
                    s=i;
                    t=j;
                    state[i][j]=1;
                }else if(c=='#'){
                    state[i][j]=1;
                }
            }
        }
        
        DFS(s,t);
        
        cout<<cnt<<endl;
    }
}