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>
#define _ ios_base::sync_with_stdio(0);cin.tie(0);
#define REP(i,n) for(int i=0;i<(int)(n);i++)

using namespace std;
bool check[22][22];
char c[22][22];
int w,h;

int recursion(pair<int,int> pos){
  if(c[pos.first][pos.second]=='#'){return 0;}
  if(check[pos.first][pos.second]){return 0;}
  check[pos.first][pos.second]=true;
  int count=1;
  if(pos.first>0){
    count+=recursion(make_pair(pos.first-1,pos.second));
  }
  if(pos.second>0){
    count+=recursion(make_pair(pos.first,pos.second-1));
  }
  if(pos.first<w-1){
    count+=recursion(make_pair(pos.first+1,pos.second));
  }
  if(pos.second<h-1){
    count+=recursion(make_pair(pos.first,pos.second+1));
  }
  return count;
}

int main(){ _;
  pair<int,int> pos;
  while(cin>>w>>h,!(w==0&&h==0)){
    REP(y,h)REP(x,w){
      cin>>c[x][y];
      check[x][y]=false;
      if(c[x][y]=='@')
	pos=make_pair(x,y);
    }
    cout<<recursion(pos)<<endl;
  }
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <set>
#include <queue>
#include <stack>
#include <string>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>

#define rep(i, n) for(int i = 0; i < n; i ++)
#define ALL(T) T.begin(), T.end()
#define mp make_pair
#define pb push_back
#define X first
#define Y second
#define MEMSET(v, h) memset((v), h, sizeof(v))

using namespace std;

typedef pair<int, int> pii;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;

const int INF = 1 << 24;
const int dx[4] = {1, -1, 0, 0};
const int dy[4] = {0, 0, 1, -1};

int cnt, w, h;
char m[100][100];

void dfs(pii);

int main(void){
    while(cin >> w >> h, w){
        MEMSET(m, '#');

        cnt = 0;
        pii p;

        rep(y, h){
            cin >> m[y];
            rep(x, w) if(m[y][x] == '@') m[y][x] = '.', p = mp(x, y);
        }

        dfs(p);
        cout << cnt << endl;
    }

    return 0;
}

void dfs(pii p){
    if((p.X < 0 || w <= p.X) ||
       (p.Y < 0 || h <= p.Y)) return;

    if(m[p.Y][p.X] == '.') cnt ++;
    else return;
    m[p.Y][p.X] = '#';

    rep(i, 4){
        pii next = mp(p.X + dx[i], p.Y + dy[i]);
        dfs(next);
    }
}

C++: /*
 * http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=1130&lang=jp
 */


#include <iostream>
#include <limits>

constexpr size_t MAX_HEIGHT = 20;
const char START_TILE = '@';
const char PASSABLE_TILE = '.';
const char PROHIBITED_TILE = '#';
const char PASSED_TILE = 'P';

bool is_wall(std::string world[MAX_HEIGHT], size_t x, size_t y) {
	char tile = world[y][x];
	return tile == PROHIBITED_TILE || tile == PASSED_TILE;
}

bool valid(std::string world[MAX_HEIGHT], size_t width, size_t height, size_t x, size_t y) {
	return x < width && y < height // x and y is unsigned
		   && !is_wall(world, x, y);
}

int walk(std::string world[MAX_HEIGHT], size_t width, size_t height, size_t x, size_t y) {
	if (!valid(world, width, height, x, y)) {
		return 0; // ????????????????????\?????????????£?or??¢?´¢??????
	}
	int total = 1; // ????????????????????\?????????1?????¨??????????????????
	world[y][x] = PASSED_TILE; // ??????????????¨????????????????????¨?????????????????????????????????
	// ?????????????????§??????????????????valid???????????????walk????????§
	total += walk(world, width, height, x - 1, y);
	total += walk(world, width, height, x + 1, y);
	total += walk(world, width, height, x, y - 1);
	total += walk(world, width, height, x, y + 1);
	return total;
}

void process(size_t width, size_t height) {
	std::string world[MAX_HEIGHT];
	size_t x = std::numeric_limits<size_t>::max(), y = std::numeric_limits<size_t>::max();
	for (size_t i = 0; i < height; ++i) {
		std::cin >> world[i];
		size_t start_index = world[i].find(START_TILE);
		if (start_index != std::string::npos) {
			x = start_index;
			y = i;
		}
	}

	std::cout << walk(world, width, height, x, y);
}

int main() {
	size_t width, height;
	while (true) {
		std::cin >> width >> height;
		if (width == 0 || height == 0) {
			break;
		}
		else {
			process(width, height);
		}
	}
}

C++: #include<bits/stdc++.h>
using namespace std;
#define inf 1e9
#define ll long long
#define ull unsigned long long
#define M 1000000007
#define P pair<int,int>
#define PLL pair<ll,ll>
#define FOR(i,m,n) for(int i=m;i<n;i++)
#define RFOR(i,m,n) for(int i=m;i>=n;i--)
#define rep(i,n) FOR(i,0,n)
#define rrep(i,n) RFOR(i,n,0)
#define all(a) a.begin(),a.end()
const int vx[4] = {0,1,0,-1};
const int vy[4] = {1,0,-1,0};
#define PI 3.14159265


void f(int w,int h){
  string s[100];
  int sx,sy;

  rep(i,h){
    cin>>s[i];
    rep(j,w){
      if(s[i][j]=='@'){
        sx=j;sy=i;
      }
    }
  }

  bool a[100][100]={};
  queue<P> p;
  p.push(P(sx,sy));
  int ans=1;
  while(p.size()){
    P q = p.front(); p.pop();
    rep(i,4){
      int x = q.first+vx[i];
      int y = q.second+vy[i];
      if(0<=x&&x<w&&0<=y&&y<h){
        if(s[y][x]=='.'&&!a[y][x]){
          ans++;
          a[y][x]=true;
          p.push(P(x,y));
        }
      }
    }
  }

  cout<<ans<<endl;
}



int main(){
  
  while(1){
    int n,m;
    cin>>n>>m;
    if(n==0) break;
    f(n,m);
  }

  return 0;
}