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 <algorithm>
#define MAX_INT  100005
using namespace std;

//??\???????????§???
#define MAX_X  22
#define MAX_Y  22

//?????°
char field[MAX_X][MAX_Y];
int X, Y;
int px, py;                     // players position


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


void initialize_field() {
  for (int y = 0; y < Y; y++ ) {
    for (int x = 0; x < X; x++ ) {
      field[x][y] = '#';
    }
  }
}

void print_field() {
  for (int y = 0; y < Y; y++ ) {
    for (int x = 0; x < X; x++ ) {
      cout << field[x][y];
    }
    cout << endl;
  }
  cout << endl;
}


void dfs(int x, int y) {

  field[x][y] = '$';            // ?¨????????????¨?????°

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

    if(field[nx][ny] == '.') {
      dfs(nx, ny);
    }
  }

}


int main() {

  while(true) {

    initialize_field();

    cin >> X >> Y;

    if(X == 0 && Y == 0) break;

    for(int y = 0; y < Y; y++ ){
      for(int x = 0; x < X; x++ ){
        cin >> field[x][y];
        if( field[x][y] == '@' ) { px = x; py = y; }
      }
    }

    dfs(px, py);

    // ?¨?????????´????????°??????
    int res = 0;
    for(int y = 0; y < Y; y++ ){
      for(int x = 0; x < X; x++ ){
        if ( field[x][y] == '$' ) { res++; }
      }
    }
    cout << res << endl;
  }

  return 0;

}

C++: #include<iostream>

using namespace std;

char ban[21][21];
int cont = 0;

	int w, h;
	
	int MAX = 0;

void saiki(int y, int x)
{
	cont++;
	ban[y][x] = '_';
	/*
	for(int i = 0; i < h; i++){
		for(int j = 0; j < w; j++){
			cout << ban[i][j];
		}
		cout << endl;
	}
	
	cout << w << h << endl;
	
	getchar();
	
	cout << cont << endl;;
	*/
	
	if(ban[y+1][x] == '.'){
		saiki(y+1,x);
		ban[y+1][x] == '.';
	}
	if(ban[y-1][x] == '.'){
		saiki(y-1,x);
		ban[y-1][x] == '.';
	} 
	if(ban[y][x-1] == '.'){
		saiki(y,x-1);
		ban[y][x-1] == '.';
	}
	if(ban[y][x+1] == '.'){
		saiki(y,x+1);
		ban[y][x+1] == '.';
	}
	
	if(cont > MAX){
		MAX = cont;
	}
}

int main()
{
	for(;;){
		int st_x, st_y;
		MAX = cont = 0;
		
		cin >> w >> h;
		
		if( w == 0){
			break;
		}
		
		for(int i = 0; i < h; i++){
			for(int j = 0; j < w; j++){
				cin >> ban[i][j];
				if(ban[i][j] == '@'){
					st_x = j;
					st_y = i;
				}
			}
		}
		
		
		saiki(st_y,st_x);
		
		cout << MAX << endl;
	}
}

C++: #include<iostream>
#include<algorithm>
#include<vector>

using namespace std;

int xs[] = {0, 1, 0, -1}, ys[] = {1, 0, -1, 0};
char c[108][108];
int w, h;
bool come[108][108];
int dfs(int x, int y){
	if(x < 0 || y < 0 || x >= h || y >= w)return 0;
	if(c[x][y] == '#')return 0;
	if(come[x][y])return 0;
	come[x][y] = true;
	int res = 1;
	for(int i = 0;i < 4;i++){
		res += dfs(x + xs[i], y + ys[i]);
	}
	return res;
}

int main(){
	int si, sj;
	while(cin >> w >> h, w | h){
		for(int i = 0;i < h;i++){
			for(int j = 0;j < w;j++){
				cin >> c[i][j];
				if(c[i][j] == '@'){
					si = i;
					sj = j;
				}
				come[i][j] = false;
			}
		}
		cout << dfs(si, sj) << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <map>
using namespace std;
int main(int argc, char *argv[]) {
	int w,h;
	while (true) {
		cin>>w>>h;
		cin.ignore();
		if (w == 0 && h == 0) break;
		vector<string> map(h);
		for (int i=0;i<h;i++) getline(cin,map[i]);
		//for (int i=0;i<h;i++) cout<<map[i]<<endl;
		// search @
		int si,sj;
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (map[i][j] == '@') {
					si = i; sj = j;
					break;
				}
			}
		}
		// bfs
		queue<pair<int,int> > mQue;
		int count = 0;
		mQue.push(make_pair(si,sj));
		vector<vector<bool> > check(h,vector<bool>(w,true));
		while (mQue.size() > 0) {
			int ti,tj;
			pair<int,int> tp = mQue.front();
			mQue.pop();
			ti = tp.first;
			tj = tp.second;
			//cout<<tj<<":"<<ti<<endl;
			// count
			if (check[ti][tj]) {
				count++;
				check[ti][tj] = false;
			}
			// deque
			if (ti-1>=0 && map[ti-1][tj] == '.' && check[ti-1][tj]) {
				mQue.push(make_pair(ti-1,tj));
			}
			if (tj+1<w && map[ti][tj+1] == '.' && check[ti][tj+1]) {
				mQue.push(make_pair(ti,tj+1));
			}
			if (ti+1<h && map[ti+1][tj] == '.' && check[ti+1][tj]) {
				mQue.push(make_pair(ti+1,tj));
			}
			if (tj-1>=0 && map[ti][tj-1] == '.' && check[ti][tj-1]) {
				mQue.push(make_pair(ti,tj-1));
			}
		}
		cout<<count<<endl;
	}
	return 0;
}

C++: 
#include <stdio.h>
int x,y,p,q,i,w,h,r=1;
int j,k[101][101];
char s[101][101];
int a[]={1,-1,0,0}, b[]={0,0,1,-1};
void func(int x, int y){
    for(i=0;i<4;i++){
        if (x+a[i]<=h&&x+a[i]>0&&y+b[i]<=w&&y+b[i]>0){
            if(k[x+a[i]][y+b[i]]==1);
            else if(s[x+a[i]][y+b[i]]=='.'){
                r++;
                k[x+a[i]][y+b[i]]=1;
                func(x+a[i],y+b[i]);
            }
            
        }
    }
}

int main(){
        while(1){
        scanf("%d %d",&w,&h);
        if (w==0&&h==0) break;
        for (i=1;i<=h;i++){
            for (j=1;j<=w;j++){
                scanf("%c",&s[i][j]);
                if (s[i][j]=='@') k[i][j]=1;
                else k[i][j]=0;
                if (s[i][j]=='@') {
                    p=i; q=j;
                }
            }
        }
        func(p,q);
        printf("%d\n",r);
        
    }
    return 0;
}