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;
const int inf = 1001001001;


int w, h;
int a[20][20];
int sw, sh;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
int D[20][20];
int Ans = 1;


int main()
{
    while(cin >> w, w != 0)
    {
        Ans = 1;
        cin >> h;
        
        for(int i = 0; i < 20; i++)
        {
            for(int j = 0; j < 20; j++)
            {
                D[i][j] = inf;
            }
        }
        
        for(int i = 0; i < h; i++)
        {
            for(int j = 0; j < w; j++)
            {
                char x;
                cin >> x;
                if(x == '.')
                {
                    a[j][i] = 1;
                }
                else if(x == '#')
                {
                    a[j][i] = 0;
                }
                else
                {
                    sw = j;
                    sh = i;
                    a[j][i] = 100;  //  ????????§?????????
                }
            }
        }
        queue< pair<int, int> > Q;
        pair<int, int> S;
        S = make_pair(sw, sh);
        Q.push(S);
        D[sw][sh] = 0;
        int k = 0;
        while(! Q.empty())
        {
            pair<int, int> cur = Q.front();
            Q.pop();
            int x = cur.first;
            int y = cur.second;
            for(int i = 0; i < 4; i++)
            {
                if(a[x+dx[i]][y+dy[i]] == 1 && 0 <= x+dx[i] && x+dx[i] < w && 0 <= y+dy[i] && y+dy[i] < h && D[x+dx[i]][y+dy[i]] == inf)    //  ????????????????¢????????????????????????????????????°???
                {
                    D[x+dx[i]][y+dy[i]] = D[x][y] + 1;
                    Ans = Ans + 1;
                    pair<int, int> T;
                    T = make_pair(x+dx[i], y+dy[i]);
                    Q.push(T);
                }
            }
        }
        cout << Ans << endl;
    }
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <cmath>
#include <numeric>
#include <queue>
using namespace std;

int main(){
    bool end = false ;
    int w,h ;
    cin >> w >> h ;
    if (w == 0 && h == 0)
    {
        end = true ;
    }
    
    //外側が全て壁になるように、元の迷路より2マス大きく取る（エラー回避）
    
    while (end == 0)
    {
    vector<vector<int> > a(h+2,vector<int>(w+2,-2)) ;
    pair<int,int> start ;
        for (size_t i = 1; i < h+1; i++)
        {
            for (size_t j = 1; j < w+1; j++)
            {
                char status ;
                cin >> status ;
                if (status == '.')
                {
                    a.at(i).at(j) = -1 ;
                }else if (status == '@')
                {
                    a.at(i).at(j) = 0 ;
                    start.first = i ;
                    start.second = j ;
                }else if (status == '#')
                {
                    a.at(i).at(j) = -2 ;
                }
                
                
                
            }
            
        }
        
        queue<pair<int,int> > wait ;
        int del[4][2] = {
            {1,0},
            {0,1},
            {-1,0},
            {0,-1}
        };
        wait.push(start) ;
        pair<int,int> now ;
        int ans = 0 ;
        while (wait.empty() == 0)
        {
            now = wait.front() ;
            wait.pop() ;
            ans ++;
            for (size_t i = 0; i < 4; i++)
            {
                int look1 = now.first + del[i][0] ;
                int look2 = now.second + del[i][1] ;
                pair <int,int> look = make_pair(look1,look2) ;
                if (a.at(look.first).at(look.second) == -1)
                {
                    a.at(look.first).at(look.second) = 0 ;
                    wait.push(look) ;

                }
                
            }
            
        }
        
        cout << ans << endl ;

        cin >> w >> h ;
        if (w == 0 && h == 0)
        {
            end = true ;
        }
        
    }
    
}

C++: #include <iostream>
using namespace std;
int w,h;
int tx[4] = {0,-1,0,1};
int ty[4] = {-1,0,1,0};
char map[21][21];

int slv(int x,int y){
    map[y][x]='#';
    int ans=0;
    for(int i=0;i<4;++i)
        if( x+tx[i]!=-1 && y+ty[i]!=-1 && x+tx[i]!=w && y+ty[i]!=h)
            if( map[y+ty[i]][x+tx[i]] == '.' )
                ans+=slv( x+tx[i], y+ty[i] ) + 1 ;
    return ans;
}

int main()
{
    int ix,iy;
    while(1){
        cin >> w >> h;
        if(w==0&&h==0)break;
        for(int i=0;i<h;++i)
            for(int j=0;j<w;++j){
                cin >> map[i][j];
                if(map[i][j]=='@'){ix=j;iy=i;}
            }
        cout << slv( ix , iy ) + 1 << endl;
    }
    return 0;
}

C++: #include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
int check(int j){
	if (j==2 || j==4 ||j==3)
	return j;
	else
	return 3;
}
int main(){
	int board[22][22];
	int w,h;
	char t;
	while(1){
		cin >>w >>h;
		if(w==0 && h==0) break;
		for(int i=0;i<=21;i++){
			for(int j = 0;j<=21;j++){
				board[j][i]=4;
			}
		}//initialization
		for(int i=1;i<=h;i++){
			for(int j=1;j<=w;j++){
				cin >> t ;
				if(t == '.')board[j][i] = 1;
				else if(t=='#')board[j][i]=2;
				else board[j][i] = 3;
			}
		}// input end
		int cmem = 2000;
		int c  = 0;
		while(cmem-c !=0){
				cmem = c;
			for(int i=1;i<=h;i++){
				for (int j= 1;j<=w;j++){
					if(board[j][i] ==3){
					board[j-1][i] =check(board[j-1][i]);
					board[j+1][i] =check(board[j+1][i]);
					board[j][i-1] =check(board[j][i-1]);
					board[j][i+1] =check(board[j][i+1]);
					}
				}
			}
			c = 0;
			for(int i=1;i<=h;i++){
				for (int j= 1;j<=w;j++){
					if (board[j][i] ==3) c+=1;
				}
			}
	
		}	
	
		cout <<c << endl;	
	}//while loop end
	return 0;
}