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 <bits/stdc++.h>

using namespace std;

template<typename T>
class UF
{
public:
	UF()
	{
		count = 0;
	}

	void push(T a)
	{
		M[a] = count;
		V.push_back(count);
		rank.push_back(0);
		count++;
	}

	void unite(T a , T b)
	{
		long long int x = find(M[a]);
		long long int y = find(M[b]);
		if(x == y)return;
		if(rank[x] < rank[y])
		{
			V[x] = y;
		} else
		{
			V[y] = x;
			if(rank[x] == rank[y])rank[x]++;
		}
	}
	void merge(T a , T b)
	{
		unite(a , b);
	}
	bool same(T a , T b)
	{
		return find(M[a]) == find(M[b]);
	}
	long long int group()
	{
		set<long long int>S;
		for(long long int i = 0; i < V.size(); i++)
		{
			S.insert(find(V[i]));
		}
		return S.size();
	}

private:
	vector<long long int> V , rank;
	map < T , long long int>M;
	long long int count;

	long long int find(long long int x)
	{
		if(V[x] == x)return x;
		else return V[x] = find(V[x]);
	}
};

int main()
{
	int W , H;
	while(cin >> W >> H , W || H)
	{
		vector<vector<char>>VVC(H , vector<char>(W));
		UF<pair<int , int>>UFP;
		pair<int,int>at;
		for(size_t i = 0; i < H; i++)
		{
			for(size_t j = 0; j < W; j++)
			{
				cin >> VVC[i][j];
				UFP.push(make_pair(i , j));
				if(VVC[i][j]=='@')
				{
					at=make_pair(i,j);
				}
				if(VVC[i][j] != '#')
				{
					if(i != 0)
					{
						if(VVC[i - 1][j] != '#')
						{
							UFP.merge(make_pair(i , j) , make_pair(i - 1 , j));
						}
					}
					if(j != 0)
					{
						if(VVC[i][j - 1] != '#')
						{
							UFP.merge(make_pair(i , j) , make_pair(i , j - 1));
						}
					}
				}

			}
		}
		int ans=0;
		for(size_t i = 0; i < H; i++)
		{
			for(size_t j = 0; j < W; j++)
			{
				if(UFP.same(at,make_pair(i,j)))
				{
					ans++;
				}
			}
		}
		cout<<ans<<endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
#define REP(i,a) for(int i=0;i<(a);i++)
#define MOD 1000000007

int main(){
  while(1){
    int h,w;
    cin>>w>>h;
    if(h==0&&w==0) break;
    char a[h][w];
    queue<pair<int,int>> q;
    bool used[h][w];
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
        cin>>a[i][j];
        used[i][j]=0;
        if(a[i][j]=='@') q.push(make_pair(i,j));
      }
    }
    int ans=0;
    while(!q.empty()){
      pair<int,int> p=q.front();
      q.pop();
      int x=p.first, y=p.second;
      if(used[x][y]) continue;
      used[x][y]=1;
      ans++;
      if(x>0){
        if(!used[x-1][y]&&a[x-1][y]=='.'){
          q.push(make_pair(x-1,y));
        }
      }
      if(x<h-1){
        if(!used[x+1][y]&&a[x+1][y]=='.'){
          q.push(make_pair(x+1,y));
        }
      }
      if(y>0){
        if(!used[x][y-1]&&a[x][y-1]=='.'){
          q.push(make_pair(x,y-1));
        }
      }
      if(y<w-1){
        if(!used[x][y+1]&&a[x][y+1]=='.'){
          q.push(make_pair(x,y+1));
        }
      }
    }
    cout<<ans<<endl;
  }
  return 0;
}

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;
}