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 <sstream>
#include <vector>

struct MapHeader {
	int w;
	int h;
};

MapHeader read_header() {
	std::string line;
	std::getline(std::cin, line);
	std::istringstream line_stream(line);
	MapHeader result;
	line_stream >> result.w >> result.h;
	return result;
}

struct SearchContext {
	MapHeader header;
	std::vector<bool> map;
	size_t count;

	SearchContext(MapHeader header)
		: header(header),
		count(0)
	{
		map.reserve(header.w * header.h);
	}

	size_t index(int x, int y) {
		return (size_t)header.w * y + x;
	}
};

void search(SearchContext& context, int x, int y) {
	if (x < 0 || x >= context.header.w || y < 0 || y >= context.header.h) return;

	size_t index = context.index(x, y);
	if (context.map[index]) return;

	context.map[index] = true;
	context.count++;
	
	search(context, x + 1, y);
	search(context, x - 1, y);
	search(context, x, y + 1);
	search(context, x, y - 1);
}

int main()
{
	while (true) {
		MapHeader header = read_header();
		if (header.w == 0 && header.h == 0) break;

		SearchContext context(header);
		int start_x, start_y;

		std::string line;
		line.reserve(header.w + 2);

		for (int y = 0; y < header.h; y++) {
			line.clear();
			std::getline(std::cin, line);

			for (int x = 0; x < header.w; x++) {
				switch (line[x]) {
				case '.':
					context.map.push_back(false);
					break;
				case '#':
					context.map.push_back(true);
					break;
				case '@':
					start_x = x;
					start_y = y;
					context.map.push_back(false);
					break;
				}
			}
		}

		search(context, start_x, start_y);
		std::cout << context.count << std::endl;
	}

    return 0;
}

C++: #include <iostream>

using namespace std;

int w , h ,c;
char t[26][26];
int dx[] = {1 , -1 , 0 ,0};
int dy[] = {0 , 0 ,1 , -1};
bool input(){
//	cout <<'a';
 	cin >> h >>w;
 	if(w == 0)return false;
 	//cout << 'a';
 	for(int i = 0;i < 25;i++){
		for(int j = 0 ;j < 25 ; j++){
			t[i][j] = 0;
		}
	}
//	cout << 'a';
	for(int i = 0;i < w ;i++){
		for(int j = 0;j < h ;j++){
			cin >> t[i][j];
		}
	}
	
	return true;
}

void dfs(int  x,int y){
	t[x][y] = '&';
	for (int i = 0;i < 4;i++){
		int nx = x + dx[i];
		int ny = y + dy[i];

		if(0 <= nx && nx <= w && 0 <= ny && ny <= h && t[nx][ny] == '.')dfs(nx , ny);
	}
	return;
}

void solve(){
	for(int i = 0;i < w;i++){
		for(int j = 0;j < h;j++){
			if(t[i][j] == '@'){
				dfs(i,j);
			}
		}
	}
	int ans = 0;
	for(int i = 0;i < w;i++){
		for(int j = 0;j < h;j++){
//			cout << t[i][j];
			if(t[i][j] == '&'){
				
				ans++;
			}
		}
//		cout << endl;
	}

	cout << ans << endl;
	return;
}

int main(){
	while(input()){
		solve();
	}

}

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

using namespace std;

struct Tile {
	int row,
		col;
};

int main()
{
	int x, y, row, col, c;
	vector<string> v(20);
	queue<Tile> t;

	while (cin >> x >> y, x != 0 && y != 0) {
		c = 1;

		for (int i = 0; i < y; i++) {
			cin >> v.at(i);
			for (int j = 0; j < x; j++) {
				if (v.at(i).at(j) == '@') {
					t.push({ i,j });
					v.at(i).at(j) = '#';
					break;
				}
			}
		}

		while (!t.empty()) {
			row = t.front().row;
			col = t.front().col;
			t.pop();

			if (row - 1 >= 0 && v.at(row-1).at(col) == '.') {
				v.at(row-1).at(col) = '#';
				t.push({row-1,col});
				c++;
			}

			if (row + 1 < y && v.at(row+1).at(col) == '.') {
				v.at(row+1).at(col) = '#';
				t.push({ row+1,col });
				c++;
			}

			if (col - 1 >= 0 && v.at(row).at(col-1) == '.') {
				v.at(row).at(col-1) = '#';
				t.push({ row,col-1 });
				c++;
			}

			if (col + 1 < x && v.at(row).at(col+1) == '.') {
				v.at(row).at(col+1) = '#';
				t.push({ row,col+1 });
				c++;
			}
		}

		cout << c << endl;
	}
}