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;
const long double PI = acos(-1);
typedef long long ll;
typedef pair<ll,ll> pll;
int dx[]={1,-1,0,0,1,-1,1,-1};
int dy[]={0,0,1,-1,1,-1,-1,1};
#define rep(i,a,b) for (int64_t i = (a); i < (b); i++)
#define REP(i,n) rep(i,0,n)
#define ALL(a) begin(a),end(a)
#define sz(s) (s).size()
#define pb push_back
#define mp make_pair
#define fi first
#define se second

int w,h,bx,by,sum=0;
char m[21][21];

void dfs(int x,int y)
{
    if(m[y][x]=='#')return;
    if(m[y][x]=='.'){sum++;m[y][x]='#';}
    REP(i,4)
    {
        int xx=x+dx[i];
        int yy=y+dy[i];
        if(xx>=0&&xx<w&&yy>=0&&yy<h)dfs(xx,yy);
    }
}

int main()
{   
    while(cin>>w>>h,w){
    sum=0;
    REP(i,h)REP(j,w)
    {
        cin>>m[i][j];
        if(m[i][j]=='@'){bx=j;by=i;m[i][j]='.';}
    }
    dfs(bx,by);
    cout<<sum<<endl;
    }
}

C++: #include <cstdio>
#include <cctype>
#include <utility>
#include <string>
#include <vector>
using namespace std;

#define gcu getchar_unlocked
#define ii inline int in
ii(int c) {
	int n = 0;
	bool minus = false; if (c == '-') minus = true, c = gcu();
	do {n = 10 * n + (c - '0'), c = gcu();} while (c >= '0');
	//return n; }
	return minus ? -n : n; }
ii() {return in(gcu());}
inline bool scan(int &n) {int c=gcu();return c==EOF?false:(n=in(c),true);}
void scan(string &s) {for (char c; !isspace(c = gcu()); s += c);}
#undef ii
#define pcu putchar_unlocked
template <typename T>
void out(T n) {
	static char buf[20];
	char *p = buf;
	if(n < 0) pcu('-'), n *= -1;
	if (!n) *p++ = '0';
	else while (n) *p++ = n % 10 + '0', n /= 10;
	while (p != buf) pcu(*--p); }
void out(const char *s){while(*s)pcu(*s++);}
void out(char c){pcu(c);}
void out(string &s){for (char c: s) pcu(c);}
template <typename head, typename... tail>
void out(head&& h, tail&&... t){out(h);out(move(t)...);}
//template <typename T>
//void out(vector<T> &v){for(T &x:v)out(' '),out(x);out('\n');}
//void out(vector<T> &v){for(T &x:v)out(&x == &v[0]?"":" "),out(x);out('\n');}
#undef svo

typedef vector<string> maze;
int walk(int x, int y, maze &m) {
	static const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
	if (x < 0 || x >= (int) m[0].size() ||
		y < 0 || y >= (int) m.size() ||
		m[y][x] == '#' || m[y][x] == 'X')
		return 0;
	m[y][x] = 'X';
	int c = 1;
	for (int i = 0; i < 4; i++)
		c += walk(x + dx[i], y + dy[i], m);
	return c;
}

int main() {
	int W, H;
	while(scan(W) && scan(H) && (W || H)) {
		maze m(H);
		int i = 0, x = 0, y = 0;
		for (auto &s: m) {
			for (int c; !isspace(c = gcu()); s += c)
				if (c == '@')
					y = i, x = s.size();
			i++;
		}
		out(walk(x, y, m), '\n');
	}
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <map>
#include <queue>

static const int MOD = 1000000007;
using ll = long long;
using u32 = unsigned;
using namespace std;

template<class T>
constexpr T INF = ::numeric_limits<T>::max() / 32 * 15 + 208;


class UnionFind {

    int n;
public:
    vector<int> uni;
    explicit UnionFind(int n) : uni(static_cast<u32>(n), -1) , n(n){};

    int root(int a){
        if (uni[a] < 0) return a;
        else return (uni[a] = root(uni[a]));
    }

    bool unite(int a, int b) {
        a = root(a);
        b = root(b);
        if(a == b) return false;
        if(uni[a] > uni[b]) swap(a, b);
        uni[a] += uni[b];
        uni[b] = a;
        return true;
    }
};

int main() {
    int h, w;
    while(cin >> w >> h, h|w){
        UnionFind uf(h*w);
        int dx[] {1, -1, 0, 0}, dy[] {0, 0, -1, 1};
        vector<vector<int>> v(h+2, vector<int> (w+2, 0));
        int sy = 0, sx = 0;
        for (int i = 0; i < h; ++i) {
            string s;
            cin >> s;
            for (int j = 0; j < w; ++j) {
                if(s[j] == '@') sy = i, sx = j;
                if(s[j] != '#') v[i+1][j+1] = 1;
            }
        }
        for (int i = 0; i < h; ++i) {
            for (int j = 0; j < w; ++j) {
                if(v[i+1][j+1] == 0) continue;
                for (int k = 0; k < 4; ++k) {
                    if(v[i+1+dy[k]][j+1+dx[k]]){
                        uf.unite(i*w+j, (i+dy[k])*w+(j+dx[k]));
                    }
                }
            }
        }
        cout << -uf.uni[uf.root(sy*w+sx)] << "\n";
    }
    return 0;
}