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<cstdio>
#include <iostream>
#include<stack>

using namespace std;

struct Coord {
  int row;
  int col;
};

int solve (int W, int H);
  
int main () {
  int W, H;

  while(W!=0 && H!=0)
    {
      scanf("%d%d", &W, &H);
      if(W!=0 && H!=0)
	{
          printf("%d\n", solve(W,H));
	}
    }
  return 0;
}

int solve (int W, int H) {
  int n=0;
  char M[H+2][W+2];
  Coord start;
  for (int i=0; i<=W+1; i++) { M[0][i] = '#'; M[H+1][i] = '#'; }
  for (int i=1; i<=H; i++) { M[i][0] = '#'; M[i][W+1] = '#'; } 
  for (int i=1; i<=H; i++) {
    for (int j=1; j<=W; j++) {
      scanf(" %c", &M[i][j]);
      if (M[i][j]=='@') {
	start.row = i;
	start.col = j;
      }
    }
  }

  
  stack<Coord> S;
  S.push(start);
  while (!S.empty()) {
    Coord current = S.top(); S.pop();
    n++;
    int D1[]={1,0,-1,0}; int D2[]={0,-1,0,1};
    for (int d=0; d<4; d++) {
      Coord nc = current;
      nc.row += D1[d];
      nc.col += D2[d];
      if(M[nc.row][nc.col]=='.')
	{
	  M[nc.row][nc.col] = '*';
	  S.push(nc);
	}
    } 
    
  }
  return n;
}

C++: #include <bits/stdc++.h>
using namespace std;

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

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

int W, H;
char B[30][30];

int dfs(int y, int x) {
   int ret = 1;
   B[y][x] = '#';
   rep (i, 4) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= W || ny < 0 || ny >= H || B[ny][nx] == '#') continue;
      ret += dfs(ny, nx);
   }
   return ret;
}

int main() {
   while (1) {
      cin >> W >> H;
      if (H*W == 0) break;

      rep (i, H) cin >> B[i];

      rep (i, H) {
         rep (j, W) {
            if (B[i][j] == '@') {
               cout << dfs(i, j) << endl;
            }
         }
      }
   }

   return 0;
}

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

char a[22][22];
int w, h;
int yy[4] = {-1, 0, 0, 1}, xx[4] = {0, 1, -1, 0};
int solve(int y, int x){
	if(a[y][x] == '#')return 0;
	a[y][x] = '#';
	int ans = 0;
	for(int i = 0;i < 4;i++){
		ans += solve(y+yy[i], x+xx[i]);
	}
	return ans+1;
}

int main(){
	int y, x;
	while(cin >> w >> h){
		fill((char*)a, (char*)(a+22), '#');
		for(int i = 1;i <= h;i++)
		for(int j = 1;j <= w;j++){
			cin >> a[i][j];
			if(a[i][j] == '@'){y = i;x = j;}
		}
		cout << solve(y, x) << endl;
	}
	return 0;
}

C++: #include<cstdio>
#include<cstdlib>
#include<cmath>
#include<climits>
#include<iostream>
#include<algorithm>
#include<stack>
#include<queue>
#include<vector>
#include<list>
#include<map>
#include<set>
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
#define pb push_back
int dy[]={0, 0, 1, -1, 0};
int dx[]={1, -1, 0, 0, 0};
 
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
char room[100][100];
bool fin[100][100];

int dfs(int i,int j){
    int sum = 1;

    fin[i][j] = false;

    for(int k = 0;k < 4;k++){
        int ii = i + dy[k];int jj = j + dx[k];
        if(fin[ii][jj] && room[ii][jj] == '.')
            sum += dfs(ii,jj);
    }
    return sum;
}

int main(){
    int w,h;
    while(1){
        scanf("%d%d",&w,&h);

        if(w == 0 && h == 0)
            break;

        for(int i = 0;i < h;i++)
            scanf("%s",&room[i][0]);

        for(int i = 0;i < h;i++)
            for(int j = 0;j < w;j++)
                fin[i][j] = true;

        for(int i = 0;i < h;i++){
            for(int j = 0;j < w;j++){
                if(room[i][j] == '@')
                    printf("%d\n",dfs(i,j));
            }
        }
    }
    return 0;
}

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

using namespace std;
char S[21][21];
int count = 0;

void a(int i, int j){
	if(S[i][j] == '.'){
		count++;
		S[i][j] = '#';
		a(i - 1, j);
		a(i + 1, j);
		a(i, j + 1);
		a(i, j - 1);
	}
}


int main(){
	while(true){
		memset(S,'#',sizeof(S));
		count = 0;
		int x, y;
		cin >> x >> y;
		if(x == 0 && y == 0){
			break;
		}
		for(int i = 0; i < y; i++){
			for(int j = 0; j < x; j++){
				cin >> S[i][j];
			}
		}

		int startx, starty;
		for(int i = 0; i < y; i++){
			for(int j = 0; j < x; j++){
				if(S[i][j] == '@'){
					startx = j;
					starty = i;
					S[i][j] = '.';
					break;
				}
			}
		}
		a(starty, startx);
		cout << count << endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
string place[20];
int dx[4]={0,1,0,-1},dy[4]={-1,0,1,0},n,m;
int serch(int y,int x)
{
	int ans=1;
	place[y][x]='#';
	for(int i=0,ax,ay;i<4;i++)if((ay=dy[i]+y)<m&&(ax=dx[i]+x)<n&&ay>=0&&ax>=0&&place[ay][ax]=='.')ans+=serch(ay,ax);
	return ans;
}
int main() {
	while(cin>>n>>m,n,m){
		for(int i=0;i<m;i++)cin>>place[i];
		for(int i=0;i<m;i++)for(int j=0;j<n;j++)if(place[i][j]=='@')cout<<serch(i,j)<<endl;
	}
}