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<vector>
#include<algorithm>
#include<iomanip>
#include<stack>
#include<queue>
#include<string>
#include<set>
#include<map>
using namespace std;
#define rep(i,n) for(int i=0;(i)<(n);(i)++)
#define FOR(i,m,n) for(int i=(m);(i)<(n);(i)++)
typedef long long ll;

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

int W,H,res;
void dfs(int x,int y,vector<vector<char> >& m){
    if(m[y][x]=='#')return;
    res++;
    m[y][x]='#';
    rep(i,4){
        int nx=x+dx[i],ny=y+dy[i];
        if(nx>=0&&nx<W&&ny>=0&&ny<H){
            dfs(nx,ny,m);
        }
    }
}

int main(){
    while(cin >> W >> H,W|H){
        vector<vector<char> > m(H,vector<char>(W));
        rep(i,H)rep(j,W)cin >> m[i][j];
        res=0;
        rep(i,H)rep(j,W){
            if(m[i][j]=='@')dfs(j,i,m);
        }
        cout << res << endl;
    }
    return 0;
}

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

int solve(int W, int H) {
    vector<vector<char>> field(H + 2, vector<char>(W + 2, '#'));
    vector<vector<int>> cost(H + 2, vector<int>(W + 2));
    queue<pair<int, int>> que;
    for (int i = 1; i <= H; i++) {
        for (int j = 1; j <= W; j++) {
            cin >> field[i][j];
            if (field[i][j] == '@') {
                que.push(make_pair(i, j));
                cost[i][j] = 1;
            }
        }
    }
    int dx[] = {-1, 0, 1, 0};
    int dy[] = {0, 1, 0, -1};
    while (!que.empty()) {
        pair<int, int> p = que.front();
        que.pop();
        for (int i = 0; i < 4; i++) {
            int nx = p.second + dx[i];
            int ny = p.first + dy[i];
            if (field[ny][nx] == '#') continue;
            if (cost[ny][nx]) continue;
            cost[ny][nx] = 1;
            que.push(make_pair(ny, nx));
        }
    }
    int ret = 0;
    for (int i = 0; i < cost.size(); i++) {
        for (int j = 0; j < cost[i].size(); j++) {
            if (cost[i][j]) ret++;
        }
    }
    return ret;
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int W, H;
    while (1) {
        cin >> W >> H;
        if (!W & !H) break;
        cout << solve(W, H) << endl;
    }
}

C++: #include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <numeric>

#define rep(i, n) for(int i = 0; i < (n); i++)
#define pb push_back

using namespace std;

typedef long long ll;
typedef pair<int, int> P;
typedef vector<int> vi;
int dx[4] = {0,1,0,-1};
int dy[4] = {-1, 0, 1, 0};
const int INF = 1 << 29;
const double EPS = 1e-9;
const int MAX_N = 20;
int W, H, ans = 1;
vector<string> field;

int dfs(int x, int y){
		rep(i, 4){
				int tx = x + dx[i], ty = y + dy[i];
				if(0 <= ty && 0 <= tx && ty < H && tx < W && field[ty][tx] == '.' ){
						ans++;
						field[ty][tx] = '#';
						dfs(tx, ty);
				}
		}
		return ans;
}

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

				field.resize(H);
				rep(i,H) cin >> field[i];
				int sx, sy;

				//rep(i,H) cout << field[i] << endl;

				rep(y,H)rep(x,W)if(field[y][x] == '@'){ sx = x; sy = y; }

				//cout << sx << " " <<sy <<endl;
				dfs(sx, sy);
				//rep(i,H) cin >> field[i];
				cout << ans <<endl;
				ans = 1;
		}
}

C++: #include<iostream>

using namespace std;

void dfs(int mx,int my);

	char math[21][21];
	const int dx[]={1,0,-1,0},dy[]={0,1,0,-1};
	int cnt;
	int x,y;
	int sx,sy;
	
int main(){
	
	while(cin>>x>>y,x&&y){
	    cnt=0;
		for(int i=0;i<y;i++){
			for(int j=0;j<x;j++){
				cin>>math[i][j];
				if(math[i][j]=='@'){
					sx=j;
					sy=i;
				}
			}
		}
		dfs(sx,sy);
		cout<<cnt+1<<endl;
	}
}

void dfs(int mx,int my){
	math[my][mx]='T';
	//移動する方向
	for(int i=0;i<4;i++){
		int nx=mx+dx[i],ny=my+dy[i];
		
		if(0<=nx && nx<x && 0<=ny && ny<y &&
		   (math[ny][nx]=='.' ||math[ny][nx]=='@')
		  )
		  {
			cnt++;
			dfs(nx,ny);
		}
	}
	return ;
}