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>

using namespace std;

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

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

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

        if (nx < 0 || ny < 0 || nx >= w || ny >= h) 
            continue;
        else if (map[ny][nx] == '.') {
            dfs(nx, ny);
        }
    }
}

int main()
{
    while (cin >> w >> h, w || h) {
        int sx, sy;

        for (int y = 0; y < h; y++)
            for (int x = 0; x < w; x++) {
                cin >> map[y][x];
                if (map[y][x] == '@') {
                    sx = x;
                    sy = y;
                }
            }

        cnt = 0;
        dfs(sx, sy);
        cout << cnt << endl;
    }

    return 0;
}

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

using namespace std;

int main(){
    int w,h;
    while(cin >> w >> h && w!=0){
        int t[w+2][h+2];
        for(int i=0;i<w+2;i++){
            for(int j=0;j<h+2;j++){
                t[i][j] = 0;
            }
        }

        int ** matrix = new int*[w*h];
        for( int i=0; i<w*h; i++ ) {
            matrix[i] = new int[w*h];
        }

        int start;
        for(int i=0;i<h+2;i++){
            for(int j=0;j<w+2;j++){
                if(i==0 || j==0 || i==w+1 || j==h+1)
                    t[i][j]=0;
                else{
                    char tmp; cin >> tmp;
                    if(tmp=='.'){
                        t[i][j] = 1;
                    }
                    else if(tmp=='#'){
                        t[i][j] = 0;
                    }
                    else{
                        t[i][j] = 1;
                        start = i*w+j;
                    }
                }
            }
        }
        for(int i=0;i<w*h;i++){
            for(int j=0;j<w*h;j++){
                matrix[i][j] = 0;
            }
        }
        for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                if(i==0 || j==0 || i==w || j==h)
                    continue;
                else{
                    int id = i*w+h;
                    if(t[(id-w)/w+1][(id-w)%w+1] == 1)
                        matrix[id][id-w] = 1;
                    if(t[(id+w)/w+1][(id+w)/w+1] == 1)
                        matrix[id][id+w] = 1;
                    if(t[(id-1)/w+1][(id-1)/w+1] == 1)
                        matrix[id][id-1] = 1;
                    if(t[(id+1)/w+1][(id+1)/w+1] == 1)
                        matrix[id][id+1] = 1;
                }
            }
        }
        int count = 1;
        queue<int> Q;
        int came[w*h];
        for(int i=0;i<w*h;i++){
            came[i] = 0;
        }
        Q.push(start);
        came[start] = 1;
        while(Q.size()!=0){
            int cur = Q.front();
            for(int i=0;i<w*h;i++){
                if(matrix[cur][i]==1 && came[i] == 0){
                    Q.push(i);
                    came[i] = 1;
                    count++;
                }
            }
            Q.pop();
        }
        cout << count << endl;
        for( int i=0; i<w*h; i++ ) {
            delete[] matrix[i];
        }
        delete[] matrix;

    }
}

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

int w;
int h;
int ans;
vector <vector<char>> maps;

bool check(int x, int y){
if (0>x || x>=h || 0>y || y>=w){
return false;
}
if (maps[x][y]!='.'){
return false;
}
return true;
}

void bfs(int x, int y){
vector <vector<int>> move = {{1,0},{-1,0},{0,1},{0,-1}};
for (const auto& e: move){
if (check(x+e[0],y+e[1])){
ans+=1;
maps[x+e[0]][y+e[1]]='#';
bfs(x+e[0],y+e[1]);
}
}
}

int main(){
cin >> w >> h;
bool flag = false;
int pos_x;
int pos_y;
while (w!=0 || h!=0){
vector <vector<char>> rawmaps(h,vector<char>(w+1));
ans = 0;
for (int i = 0; i < h; i++){
for (int j = 0; j < w; j++){
cin >> rawmaps[i][j];
if (rawmaps[i][j]=='@'){
pos_x=i;
pos_y=j;
}
}
}
maps = rawmaps;

bfs(pos_x,pos_y);
cout << ans+1 << endl;
cin >> w >> h;
}
}