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 <algorithm>
#include <stdlib.h>
#include <string>
#include <queue>
#include <map>
#include <stack>
using namespace std;

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


int main() {
int yoko,tate,px,py;
while(cin>>yoko>>tate &&yoko){

	int m[25][25];

string st;
//cin>>yoko>>tate;
for(int i=1;i<tate+1;i++){
cin>>st;
for(int j=1;j<yoko+1;j++){
if(st[j-1]=='#') m[i][j]=-1;
else if(st[j-1]=='.') m[i][j]=0;
else if(st[j-1]=='@'){
 m[i][j]=1;
px=i;
py=j;
}
}
}
int ans=1;
stack<pair<int,int> > s;
s.push(make_pair(px,py));
//cout<<"pxpy "<<px<<" "<<py<<endl;
while(s.empty()==false){
px=s.top().first;
py=s.top().second;
s.pop();

for(int i=0;i<4;i++){
//cout<<px+dx[i]<<" "<<py+dy[i]<<endl;
	if(0<px+dx[i] && px+dx[i]<tate+1 && py+dy[i]>0 && py+dy[i]<yoko+1){
	
		if(m[px+dx[i]][py+dy[i]]==0){
			m[px+dx[i]][py+dy[i]]=1;
			s.push(make_pair(px+dx[i],py+dy[i]));
			ans++;
//cout<<px+dx[i]<<" "<<py+dy[i]<<" "<<ans<<endl;
		}

	}}
}

cout<<ans<<endl;

}



	return 0;
}

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

using namespace std;

string fld[21];
int w,h;
bool flg[21][21];
int ans;

void solve(int x, int y){
    if(flg[y][x]||fld[y][x]=='#'){
        return;
    }

    ans++;
    flg[y][x] = true;

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

    for(int i=0;i<4;i++){
        int nx = x+dx[i];
        int ny = y+dy[i];
        if(nx<0||nx>=w||ny<0||ny>=h){
            continue;
        }

        solve(nx, ny);
    }
}


int main(){
    while(cin>>w>>h,w!=0){
        for(int i=0;i<21;i++){
            for(int k=0;k<21;k++){
                flg[k][i] = false;
            }
        }
        ans = 0;

        for(int y=0;y<h;y++){
            cin >> fld[y];
        }

        int sx,sy;
        for(int y=0;y<h;y++){
            for(int x=0;x<w;x++){
                if(fld[y][x]=='@'){
                    sx = x;
                    sy = y;
                }
            }
        }

        solve(sx, sy);
        cout << ans << endl;
    }
    return 0;
}

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

using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
char fld[20][20];
int d[20][20];
const int inf=1e8;

int main(){

   while(1){
      int W, H;
      cin>> W>> H;
      if(!(H||W)) break;
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
            }
         }
      }

      for(int i=0; i<H; i++)for(int j=0; j<W; j++)d[i][j]=inf;
      queue<pii> Q;
      Q.push(pii(sy, sx));
      d[sy][sx]=0;
      while(!Q.empty()){
         pii now=Q.front(); Q.pop();
         for(pii dd: dxdy){
            int ni=now.first+dd.first;
            int nj=now.second+dd.second;
            if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
            if(fld[ni][nj]=='#'||fld[ni][nj]=='@') continue;
            if(d[ni][nj]<=d[now.first][now.second]+1) continue;
            d[ni][nj]=d[now.first][now.second]+1;
            Q.push(pii(ni, nj));
         }
      }

      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=(d[i][j]!=inf);
      }

      cout<< ans<< endl;
   }

   return 0;
}

C++: #include <iostream>

#define inRange(x, y, w, h) (0 <= x && x < w && 0 <= y && y < h)

using namespace std;

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

void search( int x, int y, char map[20][20] )
{
	map[y][x] = '@';
	for(int i=0; i < 4; ++i) {
		int mx = x + dx[i];
		int my = y + dy[i];
		if(inRange(mx, my, w, h) && map[my][mx] == '.') {
			search(mx, my, map);
		}
	}
}

int main( void )
{
	char map[20][20];

	while(cin >> w >> h, w | h) {
		int sx, sy;
		for(int y=0; y < h; ++y)
			for(int x=0; x < w; ++x) {
				cin >> map[y][x];
				if(map[y][x] == '@') {
					sx = x;	sy = y;
				}
			}
		search(sx, sy, map);
		int count = 0;
		for(int y=0; y < h; ++y)
			for(int x=0; x < w; ++x) {
				if(map[y][x] == '@') ++count;
			}
		cout << count << endl;
	}
	return 0;
}