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 <queue>
using namespace std;

int G[1010];
int R[110][110];
int D[1010];
int bfs(int src,int num);

int main() {
	int W,H,root;
	char c;


	while (true) {
		for(int i=0;i<100;i++) {
			for(int j=0;j<100;j++) {
				R[i][j] = 0;
			}
		}
		for(int i=0;i<1000;i++) {
			G[i] = 0;
		}
		for(int i=0;i<1000;i++) {
			D[i] = 100;
		}
		cin >> W >> H;
		if((W == 0) && (H == 0)) break;
		for(int i=0;i<W*H;i++) {
			cin >> c;
			if(c == '.') G[i] = 1;
			if(c == '#') G[i] = 0;
			if(c == '@') {
				G[i] = 1;
				root = i;
			}
		}
		for(int i=0;i<W*H;i++) {
			if(G[i] == 1) {
				if(i%W != 0) { //????????????
					if(G[i-1] == 1) {//??????????????? 
						R[i][i-1] = 1;
					}
				}
				if(i%W != W-1) { //????????????
					if(G[i+1] == 1) { //???????????????
						R[i][i+1] = 1;
					}			

				}
				if(i > W-1) { //???????????????
					if(G[i-W] == 1) { //???????????????
						R[i][i-W] = 1;
					}	

				}
				if(i < W*H-W) {//???????????????
					if(G[i+W] == 1){
						R[i][i+W] = 1;
					}	

				}
			}
		}
		cout << bfs(root,W*H) << endl;
	}
	/*
	for(int i=0;i<99;i++) {
		cout << i << " " << G[i] << endl;
	}
	cout << G[98] << endl;
	cout << R[87][98] << endl;
	*/
	
}





int bfs(int src,int num) {
	int cur,dst;
	int count = 1;
	//cerr << "bfs root = " << src << endl;
	queue<int> Q; // ??´??°????????????????????\????????????
	Q.push(src);
	D[src] = 0; // ?????????
	while (!Q.empty()) {
		
		int cur = Q.front(); // ??????????´????????????????
		Q.pop();
// ???????¢??????¨??¨???
		//cerr << cur << " " << D[cur] << endl;
		for (int dst=0;dst<num;dst++) { // ???????????? dst ????????????
			if ((R[cur][dst] == 1) && (D[dst] == 100)) { // cur ?????? dst ??????????????????dst ???????¨??????????
				D[dst] = D[cur]+1; //
				Q.push(dst); // dst ????¨???????????????????
				count++;
			}
		}
	}
	return count;
}

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

using namespace std;
using vec = vector<int>;
using mat = vector<vec>;

void init(vec & parent, vec & size)
{
	for (int i = 0; i < parent.size(); ++i)
	{
		parent[i] = i;
		size[i] = 1;
	}
}


int find(vec & parent, int x)
{
	if (parent[x] == x)
	{
		return x;
	}
	return x = find(parent,parent[x]);
}


void unite(vec & parent, vec & size, int x, int y)
{
	x = find(parent, x);
	y = find(parent, y);

	if (x == y)return;

	parent[y] = x;
	size[x] += size[y];
}


int change(int y,int x,int W)
{
	return y*W + x;
}


int main()
{
	while (true)
	{
		int W, H;
		cin >> W >> H;
		if (W == 0 && H == 0)return 0;

		mat color(H, vec(W));
		vec parent(W*H);
		vec size(W*H);

		init(parent, size);

		int start;

		for (int y = 0; y < H; ++y)
		{
			string c;
			cin >> c;

			for (int x = 0; x < W; ++x)
			{
				
				if (c[x] == '.') { 
					
					color[y][x] = 0; 
					
				}
				else if (c[x] == '#') { color[y][x] = 1; }
				else
				{
					color[y][x] = 0;
					start = change(y, x, W);
				}

				if (x != 0 && color[y][x] == color[y][x - 1])
				{
					unite(parent, size, change(y, x, W), change(y, x - 1, W));
				}
				if (y != 0 && color[y][x] == color[y - 1][x])
				{	
					unite(parent, size, change(y, x, W), change(y - 1, x, W));
				}
				
			}
		}

		cout << size[find(parent, start)] << endl;
	}
}

C++: #include<iostream>
#include<algorithm>
#include<stack>
#include<queue>
#include<string>
#include<string.h>
#include<functional>
using namespace std;

#define INF 1<<21
#define DIV 1000000007

typedef pair<int, int>P;


int w, h;
int cnt;
char f[21][21];
int dx[] = { 1,-1,0,0 }, dy[] = { 0,0,1,-1 };

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

  for (int i = 0; i < 4; i++) {
	int nx = x + dx[i];
	int ny = y + dy[i];

	if (nx <= 0 || w < nx || ny <= 0 || h < ny||f[y][x]!=',')continue;

	rec(nx, ny);
  }
}

int main(){
  while (cin >> w >> h,(w || h)) {
	int sx, sy;
	cnt = 0;
	for (int y = 1; y <= h; y++) {
	  for (int x = 1; x <= w; x++) {
		cin >> f[y][x];
		if (f[y][x] == '@') {
		  sx = x;
		  sy = y;
		}
	  }
	}
	rec(sx, sy);
  }
}

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

char map[21][21];
int count;
int x, y;

void dfs(int nx, int ny){
  map[ny][nx] = '#';
  count++;
  if (nx+1 < x && map[ny][nx+1] == '.'){
    dfs(nx+1, ny);
  }
  if (nx > 0 && map[ny][nx-1] == '.'){
    dfs(nx-1, ny);
  }
  if (ny+1 < y && map[ny+1][nx] == '.'){
    dfs(nx, ny+1);
  }
  if (ny > 0 && map[ny-1][nx] == '.'){
    dfs(nx, ny-1);
  }
  return;
}

int main(){
  while (1){
    cin >> x >> y;
    if (!(x || y)) break;
    int ix, iy;
    count = 0;
    for (int i = 0; i < y; i++){
      for (int j = 0; j < x; j++){
	cin >> map[i][j];
	if (map[i][j] == '@'){ix = j; iy = i;}
      }
    }
    dfs(ix, iy);
    cout << count << endl;
  }
}