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++: #include <iostream>
#include <queue>
#include <utility>
#include <array>

using namespace std;

typedef pair<int, int> pii;

int height, width;
int sx, sy;
int ans = 0;
array<int, 4> dx = {0, 0, 1, -1};
array<int, 4> dy = {1, -1, 0, 0};


void bfs()
{
    queue<pii> p;
    p.push({sx, sy});
    
    while (!p.empty()) {
        pii q = p.front(); p.pop();
        
        //??????
        int cx = p.first;
        int cy = p.second;

        for (int i = 0; i < 4; ++i) {
            int nx = cx + dx.at(i);
            int ny = cy + dy.at(i);

            if (nx >= 0 && nx < width && ny >= 0 && ny < height 
                    && field.at(ny).at(nx) == '.') {

                ans++;
                p.push({nx, ny});
            }
        }
    }

    std::cout << ans << std::endl;
}


int main()
{
    while (cin >> height >> width) {
        if (height == 0 && width == 0) break;

        ans = 0;
        
        vector<vector<char>> field(height, vector<char>(width));

        for (int y = 0; y < height; ++y) {
            for (int x = 0; x < width; ++x) {
                cin >> field.at(y).at(x);

                if (field.at(y).at(x) == '@') {
                    sx = x; sy = y;
                }
            }
        }

        bfs();
    }

    return 0;
}

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

char mas[111][111];
int dx[4]={0,1,0,-1};
int dy[4]={1,0,-1,0};
int sx,sy;
int w,h,cou;

int dfs(int x,int y);
int main(){
  while(1){
    cou=0;
    cin>>w>>h;
    if(w==0 && h==0) break;
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin>>mas[i][j];
	if(mas[i][j]=='@'){
	  sx=j;
	  sy=i;
	}
      }
    }
    dfs(sx,sy);
    cout<<cou<<endl;
  }
}
int dfs(int x,int y){
  mas[y][x]='#';
  cou++;
  for(int i=0;i<4;i++){
    int nextx=x+dx[i];
    int nexty=y+dy[i];
    if(0<=nextx && nextx<w && 0<=nexty && nexty<h && mas[nexty][nextx]=='.'){
      dfs(nextx,nexty);
    }
  }
}

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

using namespace std;
typedef pair<int, int> pii;

#define W_MAX 20
#define H_MAX 20

#define B '.'
#define R '#'
#define M '@'

char map[W_MAX][H_MAX];

int main(){

  while(true){
    int W,H;
    cin >> W >> H;

    if(W==0 && H==0) break;

    pii s; //start

    for(int i=0; i<H; i++){
      getchar();
      for(int j=0; j<W; j++){
	scanf("%c", &map[i][j]);
	if(map[i][j]==M){
	  s = make_pair(i,j);
	  map[i][j] = R;
	}
      }
    }

    stack<pii> st;
    st.push(s);
    int dx[] = {-1,0,0,1}; int dy[] = {0,1,-1,0};
    int ans=0;
    while(!st.empty()){
      pii p = st.top(); st.pop();
      int x = p.first; int y = p.second;
      ans++;
      for(int i=0; i<4; i++){
	int nx = x+dx[i]; int ny = y+dy[i];
	if(nx>=0 && nx<H &&
	   ny>=0 && ny<W &&
	   map[nx][ny]==B){
	  st.push(make_pair(nx,ny));
	  map[nx][ny] = R;
	}
      }
    }

    cout << ans << endl;

  }
}

C++: #include<iostream>
#include<vector>
#include<queue>
#include<utility>
using namespace std;
int w,h;
string matrix[20];

const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1};
bool valid(int x,int y)
{
    return (x>=0)&&(x<w)&&(y>=0)&&(y<h)&&(matrix[y][x]!='#');
}
int main()
{
    while(true)
    {
        queue<pair<int,int> > q;
        pair<int,int> man;
        int amount=1;
        
        cin>>w>>h;
        if(w==0&&h==0)
            break;
        for(int i=0;i<h;i++)
        {
            cin>>matrix[i];
            //cout<<matrix[i]<<endl;
        }
        
        for(int i=0;i<h;i++)
        {
            for(int j=0;j<w;j++)
            {
                if(matrix[i][j]=='@')
                {
                    man.first=j;
                    man.second=i;
                }
            }
        }
        
        q.push(man);
        while(!q.empty())
        {
            pair<int,int> current;
            current=q.front();
            q.pop();
            
            for(int i=0;i<4;i++)
            {

                if(valid(current.first+dx[i],current.second+dy[i]))
                {
                    q.push(make_pair(current.first+dx[i],current.second+dy[i]));
                    matrix[current.second+dy[i]][current.first+dx[i]]='#';
                    amount++;
                    //cout<<current.first+dx[i]<<","<<current.second+dy[i]<<endl;
                }
            }
            
            matrix[current.second][current.first]='#';
        }
        
        cout<<amount<<endl;
    }
}