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 <algorithm>
#include <vector>
#include <iomanip>
#include <math.h>
#include <complex>
#include <queue>
#include <deque>
#include <stack>
#include <map>
#include <unordered_map>
#include <set>
#include <bitset>
#include <functional>
#include <assert.h>
using ll=long long;
#define int ll
#define rng(i,a,b) for(int i=int(a);i<int(b);i++)
#define rep(i,b) rng(i,0,b)
#define gnr(i,a,b) for(int i=int(b)-1;i>=int(a);i--)
#define per(i,b) gnr(i,0,b)
#define eb emplace_back
#define all(x) x.begin(),x.end()
#define SZ(x) ((int)x.size())
#define fi first
#define se second
const int INF = 1e15;
using namespace std;

template<class T> using vc=vector<T>;
template<class T> using vvc=vc<vc<T>>;

using pi=pair<int,int>;
using vi=vc<int>;

template<class T>
inline bool chmax(T &a,T b){if(a<b){a=b;return true;}return false;}
template<class T>
inline bool chmin(T &a,T b){if(a>b){a=b;return true;}return false;}

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

signed main() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout<<fixed<<setprecision(20);

    int N,M;
    while(true){
        cin >> N >> M;
        if(N==0 && M==0) break;
        int x,y;
        char C[M+2][N+2];
        rep(i,M+2) rep(j,N+2) C[i][j]='#';
        rep(i,M){
            rep(j,N){
                cin >> C[i+1][j+1];
                if(C[i+1][j+1]=='@'){
                    x=i+1; y=j+1;
                }
            }
        }
        queue<pi> que;
        que.emplace(x,y);
        int ans=0;
        while(!que.empty()){
            int a=que.front().fi,b=que.front().se;
            que.pop();
            if(C[a][b]=='#') continue;
            ans++;
            C[a][b]='#';
            rep(i,4){
                if(C[a+dx[i]][b+dy[i]]=='.'){
                    que.emplace(a+dx[i],b+dy[i]);
                }
            }
        }
        cout << ans << "\n";
    }
}

C++: #include <iostream>
#include <vector>
#include <unordered_map>
#include <unordered_set>
#include <string>
#include <sstream>

using namespace std;

int count_black (vector<vector<bool>> &is_black, int i, int j, int rows, int columns)
{
	if (i < 0 || rows <= i || j < 0 || columns <= j)
	{
		return 0;
	}
	
	if (is_black[i][j])
	{
		is_black[i][j] = false;
		int sum = 1;
		sum += count_black(is_black, i-1, j, rows, columns);
		sum += count_black(is_black, i+1, j, rows, columns);
		sum += count_black(is_black, i, j-1, rows, columns);
		sum += count_black(is_black, i, j+1, rows, columns);
		return sum;
	}
	else
	{
		return 0;
	}
}

int main ()
{
	while (true)
	{
		int rows, columns;
		cin >> columns >> rows;

		if (0 == rows && 0 == columns) break;

		vector<vector<bool>> is_black(rows);
		int start_i = 0, start_j = 0;
		for (int i=0; i<rows; ++i)
		{
			is_black[i].resize(columns, false);
			for (int j=0; j<columns; ++j)
			{
				char c;
				cin >> c;

				if ('.' == c)
				{
					is_black[i][j] = true;
				}
				else if ('@' == c)
				{
					is_black[i][j] = true;
					start_i = i;
					start_j = j;
				}
			}
		}

		cout << count_black(is_black, start_i, start_j, rows, columns) << endl;
	}
	
	return 0;
}

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

using namespace std;

int W, H;
vector<string> map;
int ans;

int dx[4] = {-1, 0, 1, 0};
int dy[4] = { 0,-1, 0, 1};

void dfs(int x, int y) {
	map[y][x] = '*';
	ans++;
	
	for (int d=0; d<4; d++) {
		if (!(0 <= y + dy[d] && y + dy[d] < H && 0 <= x + dx[d] && x + dx[d] < W)) continue;
		if (map[ y + dy[d] ][ x + dx[d] ] == '.') {
			dfs(x + dx[d], y + dy[d]);
		}
	}
}

int main() {
	while (1) {
		cin >> W >> H; if (W == 0 && H == 0) break;
		map.assign(H, "");
		
		int sx, sy;
		for (int i=0; i<H; i++) {
			cin >> map[i];
			for (int j=0; j<W; j++) if (map[i][j] == '@') { sx = j; sy = i; }
		}
		
		ans = 0;
		dfs(sx, sy);
		cout << ans << endl;
	}
	
	return 0;
}