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++: #define _overload3(_1,_2,_3,name,...) name
#define _rep(i,n) repi(i,0,n)
#define repi(i,a,b) for(int i=int(a);i<int(b);++i)
#define rep(...) _overload3(__VA_ARGS__,repi,_rep,)(__VA_ARGS__)
#include "bits/stdc++.h"
using namespace std;
typedef long long ll;

int main(int argc, char* argv[]){
    int dx[] = {0, 1, 0, -1};
    int dy[] = {1, 0, -1, 0};
    int w, h;
    int cnt = 0;
    int sx, sy;
    while(true){
        cnt = 0;
        vector<string> v = {};
        string tmp;
        cin >> w >> h;
        if(w == 0 && h == 0) break;
        rep(i, h){
            cin >> tmp;
            v.push_back(tmp);
            if(tmp.find('@') != string::npos){
                sx = i;
                sy = tmp.find('@');
            }
        }
        
        queue<pair<int, int>> q;
        q.push({sx, sy});
        while(!q.empty()){
            pair<int, int> p = q.front();
            q.pop();
            // cout << "Now : " << p.first << " " << p.second << endl;
            if(p.first >= h
                || p.second >= w
                || p.first  < 0
                || p.second < 0
            )continue;
            if((v[p.first][p.second] == '.' || v[p.first][p.second] == '@')) {
                v[p.first][p.second] = '#';
                cnt++;
                rep(i, 4){
                    q.push({p.first + dx[i], p.second + dy[i]});
                }
            }
        }
        cout << cnt << endl;
    }
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <cmath>
#include <utility>

#define ll (long long)
#define REP(i,n) for(int i = 0;i<(int)n;i++)
#define FOR(i,a,b) for(int i = (a);i < (b);i++)
#define RFOR(i,a,b) for(int i = (a);i>(b);i--)
#define COUNTER int i= 0,j = 0,k = 0,l = 0,m = 0, n= 0;
#define RETURN cout << endl;
/*vector<vector<int>>ArrayName = vector<vector<int>>(sizeofx, vector<int>(sizeofy, 0));*/
using namespace std;



int dfs (int x,int y,vector<vector<char>>&v);

int lim_x,lim_y;
int mx[] = {-1,1,0,0};
int my[] = {0,0,-1,1,};

int main (void){
    
    int x,y;
    int sh_x = 0,sh_y = 0;
    
    while(cin >> lim_x >> lim_y,lim_x+lim_y)
        {
            vector<vector<char>>v = vector<vector<char>>(lim_y, vector<char>(lim_x, 0));
            REP(i,lim_y)
            REP(j,lim_x)
            {
                cin >> v[i][j];
                if(v[i][j] == '@')
                    {
                        sh_y = i;
                        sh_x = j;
                    }
            }
            //cout << lim_x << " " << lim_y << endl;
            //cout << sh_x  <<" "<< sh_y << endl;
            cout << dfs(sh_x,sh_y,v) << endl;
            /*v is array name(pointer)*/
      
        }
    return 0;
}

int dfs(int x,int y, vector<vector<char>>&v)
{
    int counter = 1;
    for(int i = 0; i<4;i++){
            int tx = x+mx[i],ty = y+my[i];
            if((tx >= 0 && tx<lim_x) && (ty >= 0 && ty <lim_y) && (v[ty][tx] == '.'))
            {
                v[ty][tx] = '#';
                //cout << v[ty][tx] << endl;
                counter += dfs(tx, ty, v);
                //cout << "come on !" << endl;
            }
                
        }
            
    return counter;
}

C++: #include <iostream>
#include <string>
#include <queue>
using namespace std;
typedef pair<int, int> P;

int w, h;
string s[20];

int getCount(int sy, int sx) {
	const int dy[4] = {-1, 0, 1, 0};
	const int dx[4] = {0, 1, 0, -1};
	queue<P> que;
	bool used[20][20] = {false};
	
	que.push(P(sy, sx));
	used[sy][sx] = true;
	
	while (!que.empty()) {
		P now = que.front(); que.pop();
		int y = now.first;
		int x = now.second;
		for (int i = 0; i < 4; i++) {
			int ny = y + dy[i];
			int nx = x + dx[i];
			if (!(0 <= ny && ny < h && 0 <= nx && nx < w)) continue;
			if (s[ny][nx] == '#' || used[ny][nx]) continue;
			used[ny][nx] = true;
			que.push(P(ny, nx));
		}
	}
	
	int ans = 0;
	for (int i = 0; i < h; i++) {
		for (int j = 0; j < w; j++) {
			ans += used[i][j];
		}
	}
	return ans;
}

void solve() {
	for (int i = 0; i < h; i++) {
		for (int j = 0; j < w; j++) {
			if (s[i][j] == '@') {
				cout << getCount(i, j) << endl;
				return;
			}
		}
	}
}

int main() {
	while (cin >> w >> h) {
		if (!w) break;
		for (int i = 0; i < h; i++) cin >> s[i];
		solve();
	}
	return 0;
}

C++: #include <cstdio>

using namespace std;

#define rep(i,x) for(int i=0;i<(int)(x);++i)

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

char mp[20][22];
int H, W;

void dfs(int y, int x)
{
	if (y < 0 || y >= H || x < 0 || x >= W || mp[y][x] == '#' || mp[y][x] == ' ') return;

	mp[y][x] = ' ';

	rep(i, 4) dfs(y + dy[i], x + dx[i]);
}

int main()
{
	while (scanf("%d %d", &W, &H), H || W) {
		int sy, sx;

		rep(i, H) rep(j, W) {
			scanf("\n%c", &mp[i][j]);
			if (mp[i][j] == '@') sy = i, sx = j, mp[i][j] = '.';
		}

		dfs(sy, sx);

		int cnt = 0;

		rep(i, H) rep(j, W) if (mp[i][j] == ' ') cnt++;

		printf("%d\n", cnt);
	}
}