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++: //C++14 (Clang 3.8.0)

#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <string>
#define rp(i,a,b) for(int (i)=(int)(a);i<(int)(b);++i)

typedef long long ll;

int mx[]={-1,1,0,0};
int my[]={0,0,-1,1};
using namespace std;
int n=0;
void dfs(int startx,int starty,vector<vector<bool> > &map){
    for(int i=0;i<4;i++){
        int x=startx+mx[i],y=starty+my[i];
        if(x>=0 and y>=0 and y<map.size() and x<map[0].size() and map[y][x]==true){
        map[y][x]=false;
        n++;
        dfs(x,y,map);}
    }
    
}

int main(){
    int x,y;
    while(cin>>x>>y,x|y){
        int startx,starty;
        vector<vector<bool> > map;

        if(x==0 and y==0) return 0;
        for(int i=0;i<y;i++){
            vector<bool> tmp;
            string s;
            cin>>s;
            for(int j=0;j<x;j++){

                if(s[j]=='@') {startx=j;starty=i;tmp.push_back(true);}
                else if(s[j]=='.') tmp.push_back(true);
                else tmp.push_back(false);    
            }
            map.push_back((tmp));
        }
        n=0;
        dfs(startx,starty,map);
        cout<<n<<endl;
        
    }

    return 0;
};

C++: #include<iostream>
#include<iomanip>
#include<algorithm>
#include<bitset>
#include<cctype>
#include<cmath>
#include<cstdio>
#include<cstring>
#include<functional>
#include<limits>
#include<list>
#include<map>
#include<set>
#include<stack>
#include<string>
#include<sstream>
#include<queue>
#include<vector>
using namespace std;

#define MOD 1000000007
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL

#define MAX 20
#define NIL -1

static char M[MAX][MAX];
static int color[MAX][MAX];
int W, H;
int cnt;

void dfs(int h, int w) {
	cnt++;
	color[h][w] = 0;
	if (h != H - 1 && M[h + 1][w] == '.'&&color[h + 1][w] == NIL)
		dfs(h + 1, w);
	if (h != 0 && M[h - 1][w] == '.'&&color[h - 1][w] == NIL)
		dfs(h - 1, w);
	if (w != W - 1 && M[h][w + 1] == '.'&&color[h][w + 1] == NIL)
		dfs(h, w + 1);
	if (w != 0 && M[h][w - 1] == '.'&&color[h][w - 1] == NIL)
		dfs(h, w - 1);
}

struct { int x; int y; } s;

int main() {
	while (cin >> W >> H&&H&&W) {
		for (int h = 0; h < H; h++) {
			for (int w = 0; w < W; w++) {
				cin >> M[h][w];
				if (M[h][w] == '@')s.x = w, s.y = h;
			}
		}
		fill(color[0], color[MAX], NIL);
		cnt = 0;
		dfs(s.y, s.x);
		cout << cnt << endl;
	}
	return 0;
}

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

int dfs(int x,int y);
int w,h;
int dx[4]={0,1,0,-1};
int dy[4]={-1,0,1,0};
int sum=1;
char maps[21][21];

int main(){
  int sx,sy;
  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 >> maps[i][j];
	if(maps[i][j]=='@'){
	  sy=i;
	  sx=j;
	}
      }
    }
    
    dfs(sx,sy);
    cout << sum << endl;
    sum=1;
  }
  return(0);
}

int dfs(int x,int y){
  int nx,ny;
  for(int i=0;i<4;i++){
    nx=x+dx[i];
    ny=y+dy[i];
    maps[y][x]='#';
    if(0<=nx && nx<w && 0<=ny && ny<h && maps[ny][nx]!='#'){
      dfs(nx,ny);
      sum++;
    } 
  }
  return 0;
}

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

char c[25][25];
void dfs(int x,int y,int w,int h) {
	c[x][y] = '!';

	for (int i = -1;i <= 1;i++) {
		int ax = x + i;
		if (ax >= 0 && ax < h&&c[ax][y] == '.')dfs(ax, y, w, h);
	}
	for (int i = -1;i<= 1;i++) {
		int ay = y + i;
		if (ay >= 0 && ay < w&&c[x][ay] == '.')dfs(x, ay, w, h);
	}
}

int main() {
	int w, h;
	while (cin >> w >> h,w,h) {
		for (int i = 0;i < h;i++) {
			for (int j = 0;j < w;j++)cin >> c[i][j];
		}

		for (int i = 0;i < h;i++) {
			for (int j = 0;j < w;j++) {
				if (c[i][j] == '@')dfs(i,j,w,h);
			}
		}
		int al = 0;
		for (int i = 0;i < h;i++) {
			for (int j = 0;j < w;j++) {
				if (c[i][j] == '!')al++;
			}
		}
		cout << al << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
int nx[]={0,1,0,-1};
int ny[]={-1,0,1,0};
int w,h;
char mp[101][101];
int dfs(int y,int x);
int a,b;
int main()
{
  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 >> mp[i][j];
	if(mp[i][j]=='@'){
	  a=i;
	  b=j;
	}
      }
    }
    cout << dfs(a,b) << endl;
  }
  return(0);
}

int dfs(int y,int x){
  if(0>y||0>x||y>=h||x>=w){
    return(0);
  }
  if(mp[y][x]=='#'){
    return(0);
  }
  mp[y][x]='#';
  int ans=0;
  for(int i=0;i<4;i++){
    ans=ans+dfs(y+ny[i],x+nx[i]);
  }
  return ans+1;
}

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

using namespace std;

int data[22][22];
int vis[22][22];
int cnt = 0;

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

void dfs(int x, int y){
  vis[x][y] = 1;
  cnt++;
  for(int i = 0; i < 4; i++){
    if(data[x + dx[i]][y + dy[i]] == 1 && vis[x + dx[i]][y + dy[i]] == 0){
      dfs(x + dx[i], y + dy[i]);
    }
  }
}

int main(){
  int w, h;
  while(cin >> w >> h && w + h){
    cnt = 0;
    for(int i = 0; i < 22; i++){
      for(int k = 0; k < 22; k++){
	data[i][k] = vis[i][k] = 0;
      }
    }
    pair<int,int> start;
    for(int i = 0; i < h; i++){
      string s;
      cin >> s;
      for(int k = 0; k < w; k++){
	if(s[k] == '.')
	  data[i + 1][k + 1] = 1;
	else if(s[k] == '@')
	  start.first = i + 1,start.second = k + 1;
      }
    }
    dfs(start.first, start.second);
    cout << cnt << endl;
  }
}