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>
#include <utility>
#include <array>

using namespace std;

typedef pair<int, int> pii;

int height, width;
int sx, sy;


void bfs(vector<vector<char>>& field)
{
    int ans = 1;
    array<int, 4> dx = {0, 0, 1, -1};
    array<int, 4> dy = {1, -1, 0, 0};

    queue<pii> p;
    p.push({sx, sy});
    
    while (!p.empty()) {
        pii q = p.front(); p.pop();
        
        //??????
        int cx = q.first;
        int cy = q.second;

        for (int i = 0; i < 4; ++i) {
            int nx = cx + dx.at(i);
            int ny = cy + dy.at(i);

            if (nx >= 0 && nx < width && ny >= 0 && ny < height 
                    && field.at(ny).at(nx) == '.') {

                ans++;
                field.at(ny).at(nx) = '#';
                p.push({nx, ny});
            }
        }
    }

    std::cout << ans << std::endl;
}


int main()
{
    while (cin >> width >> height) {
        if (height == 0 && width == 0) break;

        vector<vector<char>> field(height, vector<char>(width));

        for (int y = 0; y < height; ++y) {
            for (int x = 0; x < width; ++x) {
                cin >> field.at(y).at(x);

                if (field.at(y).at(x) == '@') {
                    sx = x; sy = y;
                }
            }
        }

        bfs(field);
    }

    return 0;
}

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

void RB(int x,int y){
    int W,H;
    long count = 0;
    char area[22][22]={};
    char temp;
    using P = pair <int,int>;
    queue <P> que;
    const int direction[4][2] = {{1,0},{-1,0},{0,1},{0,-1}}; 

    W = x;
    H = y;
    for(int i=1;i<=H;i++){
        for(int j=1;j<=W;j++){
            cin >> temp;
            switch (temp){
                case '.':
                    area[i][j] = 1;
                    break;
                case '#':
                    break;
                case '@':
                    area[i][j] = 0;
                    que.push(P(i,j));
                    break;
            }
        }
    }

    while(!que.empty()){
        P temppair = que.front();
        que.pop();
        for(int i=0;i<4;i++){
            int tempx = temppair.first + direction[i][0];
            int tempy = temppair.second + direction[i][1];
            if(area[tempx][tempy]==1){
                que.push(P(tempx,tempy));
                area[tempx][tempy] = 0;
            }
        }
        count++;
    }

    cout << count << endl;

}

int main(){
    int x,y;
    while(1){
        cin >> x >> y;
        if(x == 0 && y == 0) break;
        RB(x,y);
    }
    return 0;
}

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

int W,H;

void dfs(string *m, int x, int y){
	if(y-1 >= 0 && m[y-1][x] == '.'){
		m[y-1][x] = '@';
		dfs(m, x, y-1);
	}
	if(y+1 < H && m[y+1][x] == '.'){
		m[y+1][x] = '@';
		dfs(m, x, y+1);
	}
	if(x-1 >= 0 && m[y][x-1] == '.'){
		m[y][x-1] = '@';
		dfs(m, x-1, y);
	}
	if(x+1 < W && m[y][x+1] == '.'){
		m[y][x+1] = '@';
		return dfs(m, x+1, y);
	}
}
int main(){
	cin >> W >> H;
	while(W!=0 || H!=0){
		string m[H];
		int x,y;
		for(int i = 0; i < H; i++){
			string tmp;
			cin >> tmp;
			m[i] = tmp;
			for(int j = 0; j < W; j++){
				if(tmp[j] == '@'){
					x = j, y = i;
				}
			}
		}
		dfs(m,x,y);
		int ans = 0;
		for(int i = 0; i < H; i++){
			for(int j = 0; j < W; j++){
				if(m[i][j] == '@'){
					ans++;
				}
			}
		}
		cout << ans << endl;
		cin >> W >> H;
	}
}

C++: #include <iostream>

using namespace std;

int w, h;
char tile[21][21];

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

int dfs(int x, int y){
	if(x<0 || w<=x || y<0 || h<=y) return 0;
	if(tile[y][x] == '#') return 0;

	tile[y][x] = '#';
	int count = 1;
	for(int i=0; i<4; i++){
		count += dfs(x+dx[i], y+dy[i]);
	}
	return count;
}


int main(void){
	while(1){
		cin >> w >> h;
		if(!w && !h) break;

		int x=0, y=0; //pos
		for(int i=0; i<h; i++){
			for(int j=0; j<w; j++){
				cin >> tile[i][j];
				if(tile[i][j] == '@'){
					x = j;
					y = i;
				}
			}
		}
		cout << dfs(x, y) << endl;
	}
	return 0;
}