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 <deque>
#include <string>
#include <utility>

#define phb push_back
#define ppf pop_front
#define mkp make_pair
#define r first
#define c second
using namespace std;

typedef pair< int, int > Pr;
const int kDir[4][2] = {{1, 0}, {0, 1}, {-1, 0}, {0, -1}};

int R, C, SR, SC;
char M[25][25];

void read();
void solve();

int main() {
  while (cin >> C >> R, R != 0 && C != 0)
    read(), solve();

  return 0;
}

void read() {
  fill(&M[0][0], &M[25][0], '#');
  for (int i = 1; i <= R; ++i)
    for (int j = 1; j <= C; ++j) {
      cin >> M[i][j];
      if (M[i][j] == '@')
        SR = i, SC = j;
    }
}

void solve() {
  static int vst[25][25], vst_cnt;
  int res = 0;
  deque< Pr > que;

  que.phb(mkp(SR, SC)), vst[SR][SC] = ++vst_cnt;
  while (!que.empty()) {
    Pr cur = que.front(); que.ppf(), ++res;
    for (int di = 0; di < 4; ++di) {
      Pr nxt(cur.r + kDir[di][0], cur.c + kDir[di][1]);
      if (M[nxt.r][nxt.c] == '.' && vst[nxt.r][nxt.c] != vst_cnt)
        que.phb(nxt), vst[nxt.r][nxt.c] = vst_cnt;
    }
  }

  cout << res << "\n";
}

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

bool map[20][20];
int w,h;


int Search(int x,int y, int sum){

  sum++;
  map[y][x]=false;

//  cout << "x,y=" << x << ',' << y << endl;

  if(x!=w-1){
    if(map[y][x+1]==true){
      sum = Search(x+1,y,sum);
    }
  }
  if(x!=0){
    if(map[y][x-1]==true){
      sum = Search(x-1,y,sum);
    }
  }
  if(y!=h-1){
    if(map[y+1][x]==true){
      sum = Search(x,y+1,sum);
    }
  }
  if(y!=0){
    if(map[y-1][x]==true){
      sum = Search(x,y-1,sum);
    }
  }

  return sum;

}


int main(){

  int x,y;
  char info;

  while(cin >> w >> h){
    if(w==0 && h==0){ break; }

    for(int i=0; i<h; i++){
        for(int j=0; j<w; j++){
          cin >> info;
          if(info == '.'){ map[i][j]=true;}
          else if(info == '#'){ map[i][j]=false;}
          else{ x=j; y=i; map[i][j]=false;}
        }
    }

    cout << Search(x,y,0) << endl;
  }

  return 0;

}

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

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

vector<vector<int>> M;
vector<vector<int>> checked;
int ans;
int w, h;

void dfs(int row, int col) {
  ans++;
  checked[row][col] = 1;
  for (int i = 0; i < 4; i++) {
    int x = col + dx[i];
    int y = row + dy[i];
    if (0 <= x && x < w && 0 <= y && y < h && M[y][x] == 1 &&
        checked[y][x] == 0) {
      dfs(y, x);
    }
  }
}
int main() {
  while (cin >> w >> h) {
    if (!w && !h)
      break;

    ans = 0;
    M = vector<vector<int>>(h, vector<int>(w));
    checked = vector<vector<int>>(h, vector<int>(w, 0));

    int s_row, s_col;
    for (int i = 0; i < h; i++) {
      string line;
      cin >> line;
      for (int j = 0; j < w; j++) {
        char c = line[j];
        if (c == '.') {
          M[i][j] = 1;
        } else if (c == '#') {
          M[i][j] = 0;
        } else {
          M[i][j] = 1;
          s_row = i;
          s_col = j;
        }
      }
    }
    dfs(s_row, s_col);
    cout<<ans<<endl;
  }
}

C++: #include "bits/stdc++.h"

using namespace std;

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

int main(){
    int w,h;
    while(cin>>w>>h){
        if(w==0 && h==0)break;
        int sx,sy;
        int sum=0;
        int dy[]={-1,0,1,0};
        int dx[]={0,-1,0,1};
    
    
        string data[30]={};
    
        for(int i=0;i<h;i++){
            cin>>data[i];
        }
    
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                if(data[i][j]=='@'){
                    sx=j;sy=i;
                    break;
                }
            }
        }
    
        bool visited[30][30]={};
        rep(i,30)rep(j,30)visited[i][j]=false;
        
    
        queue<pair<int,int> > que;
    
        que.push( pair<int,int>(sy,sx) );
        visited[sy][sx]=true;
    
        while(que.size()){
            int qff=que.front().first,qfs=que.front().second;
            sum++;
            rep(i,4){
                if( qff+dy[i]<h && qff+dy[i]>=0 && qfs+dx[i]<w && qfs>=0 && visited[ qff+dy[i] ][ qfs+dx[i] ]==false && data[ qff+dy[i] ][ qfs+dx[i] ]!='#' ){
                    que.push( pair<int,int>(qff+dy[i] , qfs+dx[i]) );
                    visited[ qff+dy[i] ][ qfs+dx[i] ]=true;
                }
            }
            
            que.pop();
        }
        cout<<sum<<endl;
    }
    
    return 0;
}