Go: package main

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

var strByColor = map[Color]string{
	ColorBlack: "black",
	ColorRed:   "red",
}

func (c Color) String() string {
	return strByColor[c]
}

type Node struct {
	ID      int
	X, Y    int
	Color   Color
	NextIDs map[int]struct{}
}

func (n *Node) String() string {
	d, _ := json.Marshal(n)
	return string(d)
}

type Map struct {
	Width, Height int
	Nodes         map[int]*Node
}

func (m *Map) GetByID(id int) *Node {
	return m.Nodes[id]
}

func (m *Map) Get(x, y int) *Node {
	return m.Nodes[m.getID(x, y)]
}

func (m *Map) SetColor(x, y int, c Color) {
	m.Nodes[m.getID(x, y)].Color = c
}

func (m *Map) getID(x, y int) int {
	return x + y*m.Width
}

func (m *Map) init() {
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			id := m.getID(x, y)
			m.Nodes[id] = &Node{ID: id, X: x, Y: y}
		}
	}
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			nextIDs := map[int]struct{}{}
			if x > 1 {
				nextIDs[m.getID(x-1, y)] = struct{}{}
			}
			if x < m.Width {
				nextIDs[m.getID(x+1, y)] = struct{}{}
			}
			if y > 1 {
				nextIDs[m.getID(x, y-1)] = struct{}{}
			}
			if y < m.Height {
				nextIDs[m.getID(x, y+1)] = struct{}{}
			}
			m.Get(x, y).NextIDs = nextIDs
		}
	}
}

func NewMap(w, h int) *Map {
	m := &Map{Width: w, Height: h, Nodes: map[int]*Node{}}
	m.init()
	return m
}

type Searcher struct {
	Map        *Map
	queue      []*Node
	touchedIDs map[int]struct{}
	visitedIDs map[int]struct{}
}

func NewSearcher(m *Map) *Searcher {
	return &Searcher{
		Map:        m,
		queue:      []*Node{},
		touchedIDs: map[int]struct{}{},
		visitedIDs: map[int]struct{}{},
	}
}

func (s *Searcher) Search(start *Node) {
	s.enqueue(start)

	for {
		if len(s.queue) == 0 {
			break
		}
		for nextID := range s.dequeue().NextIDs {
			n := s.Map.GetByID(nextID)
			if n.Color == ColorRed || s.hasTouched(n) {
				continue
			}
			s.enqueue(n)
		}
	}
}

func (s *Searcher) VisitedNodeCount() int {
	return len(s.visitedIDs)
}

func (s *Searcher) hasTouched(n *Node) bool {
	_, ok := s.touchedIDs[n.ID]
	return ok
}

func (s *Searcher) enqueue(n *Node) {
	s.queue = append(s.queue, n)
	s.touchedIDs[n.ID] = struct{}{}
}

func (s *Searcher) dequeue() *Node {
	n := s.queue[0]
	s.queue = s.queue[1:]
	s.visitedIDs[n.ID] = struct{}{}
	return n
}

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])

		m := NewMap(w, h)

		// Set colors
		var start *Node
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					m.SetColor(i+1, y, ColorBlack)
					if c == '@' {
						start = m.Get(i+1, y)
					}
				case '#':
					m.SetColor(i+1, y, ColorRed)
				}
			}
		}

		// Search
		s := NewSearcher(m)
		s.Search(start)
		fmt.Println(s.VisitedNodeCount())
	}
}

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>
//#include<fstream>

#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){
    n++;
    map[starty][startx]=false;
    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){
        dfs(x,y,map);}
    }
    
}

int main(){
    int x,y;
    //ofstream outputfile("test.txt");
    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);
        //outputfile<<n;
        //outputfile<<"\n";
        cout<<n<<endl;
        
    }
    //outputfile.close();
    return 0;
};

C++: #include <iostream>
using namespace std;
#define rep(i, n) for (int i = 0; i < int(n); ++i)

char table[20][20];
	int W,H;

void dfs(int x,int y){
	if(table[x][y]!= '#')
		table[x][y]='#';
	int dx[4]={1,0,-1,0};
	int dy[4]={0,1,0,-1};
	rep(i,4)
		if(0<=x+dx[i]&&x+dx[i]<W&&0<=y+dy[i]&&y+dy[i]<H&&table[x+dx[i]][y+dy[i]]!='#')
			dfs(x+dx[i],y+dy[i]);
	return ;
}


int main(void){
	while(cin >> W >> H,W|H){
		int pre=0,sx,sy;
		rep(y,H){
			rep(x,W){
				cin >> table[x][y];
				if(table[x][y]=='#')
					pre++;
				if(table[x][y]=='@')
					sx=x,sy=y;
			}
		}
		dfs(sx,sy);
		int post=0;
		rep(y,H)
			rep(x,W)
				if(table[x][y]=='#')
					post++;
		cout << post-pre << endl;
	}
	return 0;
}

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

using namespace std;

struct locate{
  int x;
  int y;
};
 
char graph[20][20];
int cou = 0;
int H,W;

void bfs(locate z){
  queue<locate> Q;
  locate u;
  locate v;
  Q.push(z);
 

  while(!Q.empty()){

    u = Q.front();Q.pop();
    if(u.y < H - 1){
      if(graph[u.y + 1][u.x] == '.' && graph[u.y + 1][u.x] != 'N'){
	graph[u.y + 1][u.x] = 'N';
	cou++;
	v.y = u.y + 1;
	v.x = u.x;
	Q.push(v);
      }
    }
    if(u.x < W - 1){
      if(graph[u.y][u.x + 1] == '.' && graph[u.y][u.x + 1] != 'N'){
	graph[u.y][u.x + 1] = 'N';
	cou++;
	v.x  = u.x + 1;
	v.y = u.y;
	Q.push(v);
      }
    }
    if(u.y > 0){
      if(graph[u.y - 1][u.x] == '.' && graph[u.y - 1][u.x] != 'N'){
	graph[u.y - 1][u.x] = 'N';
	cou++;
	v.x = u.x;
	v.y = u.y - 1;
	Q.push(v);
      }
    }
    if(u.x > 0){
      if(graph[u.y][u.x - 1] == '.' && graph[u.y][u.x - 1] != 'N'){
	graph[u.y][u.x - 1] = 'N';
	cou++;
	v.x = u.x - 1;
	v.y = u.y;
	Q.push(v);
      }
    }

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


int main(){

  int a1,a2;
  locate a;
  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;
      }
    */

    a.y = a1;
    a.x = a2;
    bfs(a);

    cout << cou+1 << endl;

    cou = 0;
    for(int i = 0;i < H ;i++){
      for(int j = 0;j < W; j++){
	graph[i][j] = 'N';
      }
    }
    cou = 0;
  }
  return 0;
}