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<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;

void dfs(int, int);
int limx,limy,cnt=0,ni,nj;
char data[20][21];

int main(){
  int i,j;
  while(1){
    scanf("%d%d",&limx,&limy);
    if(limx==0 && limy==0)
      break;
    for(i=0;i<limy;i++)
      scanf("%s",data[i]);

    for(i=0;i<limy;i++)
      for(j=0;j<limx;j++)
	if(data[i][j]=='@') ni=i,nj=j;
    dfs(ni,nj);
    printf("%d\n",cnt);
    cnt=0;
  }
  return 0;
}

void dfs(int i, int j){
  int di[4]={0,-1,0,1};
  int dj[4]={1,0,-1,0};

  data[i][j]='#';
  cnt++;
  for(int r=0;r<4;r++){
    ni=i+di[r];
    nj=j+dj[r];
    if(data[ni][nj]=='.') dfs(ni,nj);
  }
}

C++: #include <iostream>
#include <stack>
#include <utility>

using namespace std;

using Point = pair<int,int>;

int main(){
  int W,H;
  while((cin>>W>>H)&&W>0&&H>0){
    int map[22][22]={0};
    int sy,sx;
    for(int i=1;i<=H;++i){
      for(int j=1;j<=W;++j){
        char c;
        cin >> c;
        if(c=='.'){
          map[i][j] = 1;
        }else if(c=='@'){
          map[i][j] = 1;
          sy = i ; sx = j;
        }
      }
    }

    stack<Point> s;
    s.push(make_pair(sy,sx));
    int tile = 0;
    while(!s.empty()){
      auto p = s.top();s.pop();
      int y = p.first , x = p.second;
      if(map[y][x]){
        map[y][x] = 0;
        tile++;
      }else{
        continue;
      }

      s.push(make_pair(y-1,x));
      s.push(make_pair(y+1,x));
      s.push(make_pair(y,x-1));
      s.push(make_pair(y,x+1));
    }

    cout << tile << endl;
  }
  return 0;
}

C++: //前置き
#pragma region

#include<iostream>
#include<cstdio>
#include<cmath>
#include<string>
#include<vector>
#include<list>
#include<algorithm>
#include<set>
#include<map>
#include<stack>
#include<queue>

using namespace std;

typedef  long long ll;

#define pb(a) push_back(a)
#define all(x) (x).begin(),(x).end()

#define FOR(i, a, n) for(int i = (int)(a); i < (int)(n); ++i)
#define REP(i, n) FOR(i, 0, n)
#define SORT(a) sort(all(a))
#define REVERSE(a) reverse(all(a))

#define PRINT(str) printf(#str "\n")
#define yOUT PRINT(Yes)
#define nOUT PRINT(No)
#define YOUT PRINT(YES)
#define NOUT PRINT(NO)

#pragma endregion

//関数定義
#pragma region 
//偶数 ->　true :: 奇数　-> false
bool EVEN(ll a){
    return a%2-1;
}

//最大公約数
ll gcd(ll a, ll b) {  
    if (!b)  
        return a;  
    return gcd(b, a%b);  
}  

//最小公倍数
ll lcm(ll a, ll b) {  
    return a*b/gcd(a,b);  
} 

#pragma endregion


int main(){
    while(1){
        int w, h, cnt =1;
        cin >> w >> h;
        if(w == 0 && h == 0) return 0;

        bool vis[22][22];
        char gragh[22][22];

        REP(i,22){
            REP(j,22){
                vis[i][j] = false;
                gragh[i][j] = '#';
            }
        }

        


        int a,b;
        for(int i=1;i<h+1;i++){
            for(int j=1;j<w+1;j++){
                cin >> gragh[i][j];

                if(gragh[i][j] == '@'){
                    a = i;b = j;
                }
            }   
        }

        
        stack<int> st;
        st.push(a); st.push(b);
        int dx[4] = {0,1,-1,0};
        int dy[4] = {1,0,0,-1};
 

        while (!st.empty()){
            int x,y;
            y = st.top(); st.pop();
            x = st.top(); st.pop();

            for(int i = 0;i<4;i++){
                int ix = x + dx[i];
                int iy = y + dy[i];
            
                if(gragh[ix][iy] == '.' && vis[ix][iy] == false){
                    st.push(ix); st.push(iy);
                    vis[ix][iy] = true;
                    cnt ++;
                }
            }
        }
        cout << cnt <<endl;
    }
}

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

int w,h;
string field[21]; // ¶ðµ¤ÌÅstring^Ìzñðg¤ÆÖÅ·B

// 4ßT(ãº¶E)ÌÆ«ÍÌæ¤ÈzñðpÓ·éÆÖ
int dx[4] = {0,0,1,-1};
int dy[4] = {1,-1,0,0};

// »Ýn(x,y)
void dfs(int x, int y){
	// ¡¢éÆ±ëð'B'Éu«·¦é
	field[y][x] = 'B';

	// Ú®·é4ûüð[v
	for(int i=0 ; i < 4 ; i++ ){
		// xûüÉdx[i], yûüÉdy[i], Ú®µ½êð(mx,my)Æ·é
		int mx = x + dx[i];
		int my = y + dy[i];
		// ÍÍOÉo½Æ«ÍÌÖ
		if( mx < 0 || my < 0 || mx >= w || my >= h ) continue;
		// ¢^C'.'¾Á½çÄAÄÑoµ
		if( field[my][mx] == '.' ){
			dfs( mx , my );
		}
	}
}

int main(){
	while( cin >> w >> h , w||h ){
		for(int y=0 ; y < h ; y++ ){
			cin >> field[y];
		}

		for(int y=0 ; y < h ; y++ ){
			for(int x=0 ; x < w ; x++ ){
				if( field[y][x] == '@' ){ // l'@'¾Á½çÄAÖðÄÑo·
					dfs( x , y );
				}
			}
		}
		int ans = 0;
		// [vÅ'B'Ìð¦é(BÂ\È¢^CÌ)
		for(int y=0 ; y < h ; y++ ){
			for(int x=0 ; x < w ; x++ ){
				if( field[y][x] == 'B' )
					ans++;
			}
		}
		cout << ans << endl;
	}
}

C++: #include <iostream>
#include <queue>

using namespace std;

char graph[20][20];
int cou = 0;

void bfs(int a1,int a2){
  queue<int> Q;
  int u1,u2;
  Q.push(a1);
  Q.push(a2);

  while(!Q.empty()){

    u1 = Q.front();Q.pop();
    u2 = Q.front();Q.pop();

    if(graph[u1+1][u2] == '.'){
      graph[u1+1][u2] = 'N';
      cou++;
      Q.push(u1+1);Q.push(u2);
    }

    if(graph[u1][u2+1] == '.'){
      graph[u1][u2+1] = 'N';
      cou++;
      Q.push(u1);Q.push(u2+1);
    }

    if(graph[u1-1][u2] == '.'){
      graph[u1-1][u2] = 'N';
      cou++;
      Q.push(u1-1);Q.push(u2);
    }

    if(graph[u1][u2-1] == '.'){
      graph[u1][u2-1] = 'N';
      cou++;
      Q.push(u1);Q.push(u2-1);
    }

    /* for(int i = 0; i< 9 ;i++){
       for(int j = 0; j < 6; j++){
       cout << graph[i][j];
       }
       cout << endl;
       }
       }*/
  }
}

int main(){

  int W,H,a1,a2;

  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 >> graph[i][j];
	if(graph[i][j] == '@'){
	  a1 = i;
	  a2 = j;
	}
      }
    }

    /*
      for(int i = 0; i< H ;i++){
      for(int j = 0; j < W; j++){
      cout << graph[i][j];
      }
      cout << endl;
      }
    */

    bfs(a1,a2);

    cout << cou+1 << endl;
    cou = 0;
  }
  return 0;
}