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<vector>
#include<algorithm>

using namespace std;

int h,w;
char grid[21][21];
int dx[4]={0,1,0,-1};
int dy[4]={1,0,-1,0};

int dfs(int x,int y){
  if(grid[y][x]=='#')return 0;
  grid[y][x]='#';
  int sum=1;
  for(int i=0;i<4;i++){
    int nx=x+dx[i],ny=y+dy[i];
    if(0<=nx && nx<w && 0<=ny && ny<h)sum+=dfs(nx,ny);
  }
  return sum;
}

int main(void){

  while(cin >> w >> h,w|h){
    for(int i=0;i<h;i++)cin >> grid[i];
    
    for(int y=0;y<h;y++)
      for(int x=0;x<w;x++)
	if(grid[y][x]=='@')cout << dfs(x,y) << endl;
  }

  return 0;
}

C++: #include <iostream>
#include <string.h>
#include <queue>

using namespace std;

struct P
{
	int x, y;
};

int W, H;

int field[20][20];

int main()
{
	int x, y;
	char c;
	while( 1 )
	{
		cin >> W >> H;
		if(W==0 && H==0) break;

		memset(field, 0, sizeof(int) * 400);
		for(int j=0; j<H; j++)
		{
			for(int i=0; i<W; i++)
			{
				cin >> c;
				if(c == '.')
	  				field[i][j] = 1;
				else if(c == '#')
					field[i][j] = 2;
				else if(c == '@')
				{
					field[i][j] = 3;
					x = i;
					y = j;
				}
			}
		}

		int offset[4][2] = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
		int total = 1;
		queue<P> que;
		P init_p = {x, y};
		que.push(init_p);

		while(!que.empty())
		{
			P p = que.front();
			que.pop();

			int cx = p.x;
			int cy = p.y;

			for(int i=0; i<4; i++)
			{
				int xx = cx + offset[i][0];
				int yy = cy + offset[i][1];
				if(xx>=0 && yy>=0 && xx<W && yy<H)
				{
					if(field[xx][yy] == 1)
					{
						P new_p = {xx, yy};
						que.push(new_p);
						field[xx][yy] = 4;
						total++;
					}
				}
			}
		}

		printf("%d\n", total);
	}
}

C++: #include <iostream>
using namespace std;
#define REP(i,n) for (int i = 0; i < n; i++)
#define FORI(i,a,b) for (int i = (a); i <= (b); i++)
const int MAX_W = 20;
const int MAX_H = 20;
int W,H,res;
int dx[4] = {1,0,-1,0},dy[4]= {0,1,0,-1};
char field[MAX_W][MAX_H];
void solve();

int main() {solve(); return 0;}

void dfs(int x, int y) {
  field[x][y] = '#';
  REP(i,4) {
    int X = x + dx[i], Y = y + dy[i];
    if (0 <= X && 0 <= Y && X < W && Y < H && field[X][Y] == '.') {
      dfs(X,Y);
    }
  }
  return ;
}
void solve() {
  while (true) {
    res = 1;
    cin >> W >> H;
    if (W==0&&H==0) break;
    int x0,y0;
    REP(i,H) {REP(j,W) {
      cin >> field[j][i];
      if (field[j][i] == '@') x0=j,y0=i;
    } }


    dfs(x0,y0);
    cout << res << endl;
  }
  return ;
}

C++: #include <iostream>
#include <queue>
using namespace std;
int w,h;
int g[21][21];
bool visited[21][21];
int moves[4][2]={{-1,0},{1,0},{0,-1},{0,1}};
int count;
bool legal(int x, int y)
{
  if(visited[x][y]!=true&&0<=x&&x<h&&0<=y&&y<w&&g[x][y]==0)
    return true;
  else
    return false;
}

void dfs(int x,int y)
{
  visited[x][y]=true;
  count++;
  for(int i=0;i<4;i++)
  {
    int newx=x+moves[i][0];
    int newy=y+moves[i][1];
    if(legal(newx,newy))
    {
      dfs(newx,newy);
    }
  }
}

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++)
      {
        g[i][j]=-1;
        visited[i][j]=false;
      }
    int psx,psy;
    for(int i=0;i<h;i++)
      for(int j=0;j<w;j++)
      {
        char s;
        cin>>s;
        switch(s)
        {
          case '.':
            g[i][j]=0;break;
          case '#':
            g[i][j]=1;break;
          case '@':
            g[i][j]=2;psx=i;psy=j;break;
        }
      }
    count=0;
    dfs(psx,psy);
/*    int count=0;
    for(int i=0;i<h;i++)
      for(int j=0;j<w;j++)
      {
        count+=visited[i][j];
      }
*/  cout<<count<<endl;

  }
}