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>

using namespace std;

static int W, H;
static vector<string> field;

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

int dfs(int x, int y) {
    if (x < 0 || x >= W || y < 0 || y >= H || field[y][x] == '#')
        return 0;

    field[y][x] = '#';
    int sum = 0;
    for (int k = 0; k < 4; k++) {
        sum += dfs(x + dx[k], y + dy[k]);
    }

    return sum + 1;
}

int main() {
    for (;;) {
        cin >> W >> H;
        if (W == 0) break;

        field.resize(H);
        int sx, sy;
        for (int y = 0; y < H; y++) {
            cin >> field[y];
            for (int x = 0; x < W; x++) {
                if (field[y][x] == '@') {
                    sx = x;
                    sy = y;
                    field[y][x] = '.';
                }
            }
        }

        cout << dfs(sx, sy) << endl;
    }
}

C++: #include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;

void dfs(int, int);
int limx,limy,cnt=0,ni,nj;
char data[21][21];

int main(){
  int i,j;
  while(1){
    scanf("%d%d",&limx,&limy);
    if(limx==0 && limy==0)
      break;
    for(i=0;i<limy;i++)
      scanf("%s",data[i]);

    for(i=0;i<limy;i++)
      for(j=0;j<limx;j++)
	if(data[i][j]=='@') ni=i,nj=j;
    dfs(ni,nj);
    printf("%d\n",cnt);
    cnt=0;
  }
  return 0;
}

void dfs(int i, int j){
  int di[4]={0,-1,0,1};
  int dj[4]={1,0,-1,0};

  data[i][j]='#';
  cnt++;
  for(int r=0;r<4;r++){
    ni=i+di[r];
    nj=j+dj[r];
    if(data[ni][nj]=='.' && ni >= 0 && ni < limy && nj >= 0 && nj < limx) 
      dfs(ni,nj);
  }
}

C++: #include <stdio.h>
int A[4][2]={-1,0,0,1,1,0,0,-1},ans,N,M;char RB[25][25];void bfs(int x, int y) {RB[x][y]='#';ans++;for (int i=0;i<4;i++) {int nx=x+A[i][0],ny=y+A[i][1];if (nx>=0 && nx<N && ny>=0 && ny<M && RB[nx][ny]=='.') bfs(nx,ny);}}
int main() {int i,j;while (1) {scanf("%d %d",&M,&N);if (!N && !M) return 0;for (i=0;i<N;scanf("%s",&RB[i++]));ans=0;for (i=0;i<N;i++) for (j=0;j<M;j++) if (RB[i][j]=='@') bfs(i,j);printf("%d\n",ans);}}

C++: #include<iostream>
#include<algorithm>
#include<queue>
#include<cstring>
using namespace std;
char cm[200][200];
int im[200][200];
 int W,H;
bool visited[200][200];
int sy,sx;
int dx[4] = {1,0,-1,0};
int dy[4] = {0,1,0,-1};
typedef pair<int,int> P;
int maxx = 1;
int q = 1;

int dp(int x,int y){
 

  for(int i=0;i<4;i++)
    {
      int ny = y+dy[i],nx = x + dx[i];
      if(0 <= nx && nx < W && 0 <= ny && ny < H && cm[ny][nx] != '#' && !visited[ny][nx]){
	q++;
	im[ny][nx] = q;
	maxx = max(maxx,q);
	visited[ny][nx] = true;
	dp(nx,ny);
      }
    }
}


int main(){
 

  while(1)
    {
      cin >> W >> H;
      if(W == 0 && H == 0)break;
      maxx = 1;
      memset(im,0,sizeof(im));
      memset(visited,false,sizeof(visited));
      for(int i=0;i<H;i++)
	{
	  for(int j=0;j<W;j++)
	    {
	      cin >> cm[i][j];
	      if(cm[i][j] == '#'){
		im[i][j] = 0;
	      }
	      else if(cm[i][j] == '@')
		{
		  sy = i;
		  sx = j;
		}
	    }
	}

      im[sy][sx]=1;
      q = 1;
      visited[sy][sx] = true;
      dp(sx,sy);
      cout << maxx  << endl;
      /*          
 for(int i=0;i<H;i++){
	for(int j=0;j<W;j++)cout << im[i][j];
	cout << 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;
    }
}