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<string>
#include<queue>
#include <utility>
#include <functional>
//#include<>
using namespace std;

const int dx[] = {1, 0, -1, 0}, dy[] = {0, -1, 0, 1};
//#define int long long;
signed main(){
  int w, h;
  while(cin >> w >> h, w){
    int cnt{};
    vector<string> v(h);
    for(int i = 0; i < h; ++i){
      cin >> v[i];
    }
    //initialize
    pair<int, int> p;
    queue<pair<int, int>> q;	
    for(int i = 0; i < h; ++i){
      for(int j = 0; j < w; ++j){
	if(v[i][j] == '@'){
	  p = make_pair(i,j);
	  q.push(p);
	}
      }
    }
    while(!q.empty()){
      pair<int, int> p;
      p = q.front();
      q.pop();

      for(int k = 0; k < 4; ++k){
	int x = p.first + dx[k]; int y = p.second + dy[k];
	if(0 > x || h <= x || 0 > y || w <= y)continue;
	if('.' == v[x][y]){
	  v[x][y] = '@';
	  ++cnt;
	  q.push(make_pair(x, y));
	}
      }
    }
    cout << cnt+1 << endl;
  }
  return 0;
}

C++: #include <iostream>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <cstring>
#include <climits>
#include <algorithm>
#include <map>
#include <set>
#include <stack>
#include <vector>
#include <queue>
#include <string>
using namespace std;

#define rep(i,n) for(int i=0; i<n; i++)
#define repa(i,s,e) for(int i=s; i<=e; i++)
#define repd(i,s,e) for(int i=s; i>=e; i--)

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef pair<int, int> pii;

int W, H, x, y;
string f[20];
char v[20][20];

int ofs[4][2] = { {1, 0}, {-1, 0}, {0, 1}, {0, -1} };

int bfs() {
	int n = 0;
	memset(&v[0][0], 0, sizeof(char) * 20 * 20);

	queue<pii> que;
	que.push(pii(x, y));
	while(!que.empty()) {
		pii p = que.front();
		que.pop();
		if(v[p.second][p.first] == 0) {
			v[p.second][p.first] = 1;
			n++;
		} else {
			continue;
		}

		rep(i,4) {
			int nx = p.first + ofs[i][0];
			int ny = p.second + ofs[i][1];
			if(nx >= 0 && ny >= 0 && nx < W && ny < H && v[ny][nx] == 0 && f[ny][nx] == '.') {
				que.push(pii(nx, ny));
			}
		}
	}
	return n;
}

int main() {
	while(cin>>W>>H, W|H) {
		rep(i,H) {
			cin >> f[i];
			int j = f[i].find_first_of('@');
			if(j != string::npos) {
				x = j;
				y = i;
			}
		}
		cout << bfs() << endl;
	}
}

C++: #include<iostream>
#include<cstdio>
#include<vector>
#include<algorithm>
#include<functional>
#include<string>
#include<cstring>
#include<cmath>
using namespace std;
int cnt;
int w,h;
char map[24][24];
void func(int y,int x)
{
    cnt++;
    map[y][x]='#';
    int ny,nx;
    int dx[]={0,0,1,-1};
    int dy[]={1,-1,0,0};
    for(int i=0;i<4;i++)
    {
        ny=y+dy[i];
        nx=x+dx[i];
        if(map[ny][nx]=='.'&&0<=ny&&0<=nx&&ny<h&&nx<w)func(ny,nx);
    }
    return;
}
int main()
{
    while(cin>>w>>h,w)
    {
        cnt=0;
        int x,y;
        for(int i=0;i<24;i++)
        {
            for(int j=0;j<24;j++)
            {
                map[i][j]='#';
            }
        }
        for(int i=0;i<h;i++)
        {
            for(int j=0;j<w;j++)
            {
                cin>>map[i][j];
                if(map[i][j]=='@')
                {
                    y=i;
                    x=j;
                }
            }
        }
        func(y,x);
        cout<<cnt<<endl;
    }
    return 0;
}

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

#define rep(i, n) for(int i = 0 ; i < n ; i++)
using namespace std;
void dfs(int x, int y);

int cnt = 0;
string data[20];
int dx[] = {-1, 0, 1, 0};
int dy[] = {0, 1, 0, -1};
int w, h;

int main()
{
    while(true){
      cin >> w >> h;
      if(w == 0 && h == 0) break;
      
      int sx, sy;
      for(int i = 0 ; i < h ; i++){
	cin >> data[i];
	for(int j = 0 ; j < w ; j++){
	  if(data[i][j] == '@'){
	    sx = j;
	    sy = i;
	  }
	}
      }
      dfs(sx, sy);
      cout << cnt << '\n';
      cnt = 0;
  }
}

void dfs(int x, int y)
{
  data[y][x] = '#';
  cnt++;

  rep(i, 4){
    int nx = x + dx[i], ny = y + dy[i];
    if(0 <= nx && nx < w && 0 <= ny && ny < h && data[ny][nx] == '.'){
      dfs(nx, ny);
    }
      }
}