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;
using lint     = long long;
const lint inf = 1LL << 60;
const lint mod = 1000000007;

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    while (true) {
        int h, w;
        cin >> w >> h;
        if (w == 0 && h == 0)
            return 0;
        vector<string> s(h);
        for (int i = 0; i < h; ++i) {
            cin >> s[i];
        }
        int sy, sx;
        [&] {
            for (int i = 0; i < h; ++i) {
                for (int j = 0; j < w; ++j) {
                    if (s[i][j] == '@') {
                        sy = i;
                        sx = j;
                        return;
                    }
                }
            }
        }();
        queue<pair<int, int>> q;
        q.emplace(sy, sx);
        const int dx[4] = {-1, 1, 0, 0};
        const int dy[4] = {0, 0, -1, 1};
        auto inside     = [&](int y, int x) { return 0 <= y && y < h && 0 <= x && x < w; };
        int ret         = 1;
        while (q.size() != 0) {
            auto p = q.front();
            q.pop();
            for (int i = 0; i < 4; ++i) {
                int nx = p.second + dx[i];
                int ny = p.first + dy[i];
                if (inside(ny, nx) && s[ny][nx] == '.') {
                    ret++;
                    s[ny][nx] = '#';
                    q.emplace(ny, nx);
                }
            }
        }
        cout << ret << "\n";
    }
    return 0;
}

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

int serch(int,int);
char tile[20][20];
int W,H;

int main(){
	int posx,posy;

	while(1){
		cin >> W >> H;
		if(W==0&&H==0) break;

		//^CÇÝÝ
		for(int j=0;j<H;j++){
			for(int i=0;i<W;i++){
				cin >> tile[i][j];
			}
		}

		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				if(tile[i][j]=='@'){
					posx=i,posy=j;
				}
			}
		}

		cout << serch(posx,posy)+1 << endl;
		for(int j=0;j<H;j++){
			for(int i=0;i<W;i++){
				tile[i][j]='#';
			}
		}

	}

	return 0;
}

int serch(int x,int y){
	//cout << "x=" << x << " " << "y=" << y << endl;
	int cnt=0;
	if(x==0&&y==0){
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
		}
		//cout << "x==y==0" << endl;
	}else if(x==W&&y==0){
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
		}
		//cout << "x==W&&y==0" << endl;
	}else if(x==0&&y==H){
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
		}
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
		}
		//cout << "x==0&&y==H" << endl;
	}else if(x==W&&y==H){
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
		}
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
		}
		//cout << "x==W&&y==H" << endl;
	}else if(x==0){
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
		}
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
		}
		//cout << "x==0" << endl;
	}else if(x==W){
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
		}
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
		}
		//cout << "x==W" << endl;
	}else if(y==0){
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
		}
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
		}
		//cout << "y==0" << endl;
	}else if(y==H){
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
		}
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
		}
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
		}
		//cout << "y==H" << endl;
	}else {
		if(tile[x-1][y]=='.'){
			tile[x-1][y]='@';
			cnt++;
			cnt = cnt + serch(x-1,y);
			//cout << "a---------------a" << endl;
		}
		if(tile[x+1][y]=='.'){
			tile[x+1][y]='@';
			cnt++;
			cnt = cnt + serch(x+1,y);
			//cout << "b---------------b" << endl;
		}
		if(tile[x][y-1]=='.'){
			tile[x][y-1]='@';
			cnt++;
			cnt = cnt + serch(x,y-1);
			//cout << "c---------------c" << endl;
		}
		if(tile[x][y+1]=='.'){
			tile[x][y+1]='@';
			cnt++;
			cnt = cnt + serch(x,y+1);
			//cout << "d---------------d" << endl;
		}
	}
	//cout << endl;
	//for(int j=0;j<H;j++){
	//	for(int i=0;i<W;i++){
	//		cout << tile[i][j];
	//	}
	//	cout << endl;
	//}
	//
	return cnt;
}