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 <complex>
using namespace std;
typedef complex<int> P;

int main()
{
  int W, H;
  while (cin >> W >> H && !(W == 0 && H == 0)) {
    vector<string> grid(H);
    P start;
    for (int i = 0; i < H; i++) {
      cin >> grid[i];
      for (int j = 0; j < W; j++) {
        if (grid[i][j] == '@') {
          start = P(i, j);
          grid[i][j] = '.';
        }
      }
    }

    queue<P> q;
    q.push(start);
    vector<vector<bool> > visited(H, vector<bool>(W, false));
    visited[start.real()][start.imag()] = true;
    int ans = 0;
    while (!q.empty()) {
      const P p = q.front();
      q.pop();
      ans++;

      static const P dir[] = {P(-1,0), P(1,0), P(0,-1), P(0,1)};
      for (int d = 0; d < 4; d++) {
        const P next = p + dir[d];
        if (0 <= next.real() && next.real() < H && 0 <= next.imag() && next.imag() < W
            && grid[next.real()][next.imag()] == '.' && !visited[next.real()][next.imag()]) {
          visited[next.real()][next.imag()] = true;
          q.push(next);
        }
      }
    }
    cout << ans << endl;
  }
  return 0;
}

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

#define range(i,a,b) for(int (i)=a;(i)<(b);(i)++)
#define rep(i,n) range(i,0,n)

using namespace std;

char table[30][30];

int dw[4]={1,0,-1,0};
int dh[4]={0,1,0,-1};

void dfs(int w,int h){
	table[h][w]='@';
	rep(i,4){
		int nw=w+dw[i];
		int nh=h+dh[i];
		if(table[nh][nw]=='.')
			dfs(nw,nh);
	}
}

int main(void){
	int w,h;
	while(cin >> w >> h,w|h){
		int sw,sh;
		rep(i,30)rep(j,30) table[i][j]='#';
		range(i,1,h+1) range(j,1,w+1){
			cin >> table[i][j];
			if(table[i][j]=='@'){
				sh=i;
				sw=j;
			}
		}
		dfs(sw,sh);
		int ans=0;
		range(i,1,h+1) range(j,1,w+1) if(table[i][j]=='@') ans++;
		cout << ans << endl;
	}
	return 0;
}

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

#define rep(i,n) rep2(i,0,n)
#define rep2(i,m,n) for(int i=m;i<n;i++)

int w,h,ans,dy[]={-1,0,1,0},dx[]={0,-1,0,1};
char c[22][22];

void dfs(int y,int x){
	ans++;
	c[y][x]='#';
	rep(i,4){
		int ny=y+dy[i],nx=x+dx[i];
		if(c[ny][nx]=='-')dfs(ny,nx);
	}
}

int main(){
	while(cin>>w>>h&&w){
		ans=0;
		int y,x;
		rep2(i,1,h+1)rep2(j,1,w+1){
			cin>>c[i][j];
			if(c[i][j]=='@')y=i,x=j;
		}
		dfs(y,x);
		cout<<ans<<endl;
	}
}

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

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

void bfs(vector< vector<char> > &plane, int x, int y, int &ans){  
  plane[x][y] = '#';
  ans++;
  for(int i=0; i<4; i++){
    if(0 <= x+dx[i] && x+dx[i] < w && 0 <= y+dy[i] && y+dy[i] < h)
      if(plane[x + dx[i]][y + dy[i]] != '#')
        bfs(plane, x + dx[i], y + dy[i], ans);
  }
  
  return;
}

int main(){
  while(true){
    cin >> w >> h;
    if(w == 0)
      break;

    int sx, sy;
    vector< vector<char> > plane(w, vector<char>(h));
    for(int i=0; i<h; i++){
      for(int j=0; j<w; j++){
        cin >> plane[j][i];
        if(plane[j][i] == '@'){
          sx = j;
          sy = i;
        }
      }
    }

    int ans = 0;
    bfs(plane, sx, sy, ans);
    cout << ans << endl;
  }
  
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int main(){
  int w,h,a[20][20],str1,str2,count=0,m;
  char in;
  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>>in;
	if(in=='#') a[i][j]=0;
	else if(in=='@') {
	  a[i][j]=2;
	}
	else a[i][j]=1;
      }
    }
    count=0;
    while(1){
      m=0;
      for(int i=0;i<h;i++){
	for(int j=0;j<w;j++){
	  if(a[i][j]==2){
	    m=1;
	    count++;
	    a[i][j]=3;
	    if(i==0){
	      if(j==0){
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	      }
	      else {
		if(a[i][j-1]==1) a[i][j-1]=2;
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	      }
	    }
	    else if(j==0){
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	    }
	    else if(i==h-1){
	      if(j==w-1){
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
	      }
	      else {
		if(a[i][j+1]==1) a[i][j+1]=2;
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
	      }
	    }
	    else if(j==w-1){
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
	    }
	    else {
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	    }
	  }
	}
      }
      if(m==0) break;
    }
    cout<<count<<endl;
  }
    
    
  return 0;
}