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;
    vector<string> v;
    while(true){
        cnt = 0;
        string tmp;
        v.clear();
        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 <algorithm>
#include <cctype>
#include <cstdio>
#include <iostream>
#include <locale>
#include <numeric>
#include <queue>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>

using namespace std;

int solve(int w, int h) {
    vector<string> s(h);
    queue<int> queX;
    queue<int> queY;
    for (int i = 0; i < h; i++) {
        cin >> s[i];
    }
    int x, y;
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            if (s[i][j] == '@') {
                queX.push(i);
                queY.push(j);
                s[i][j] = '#';
                break;
            }
        }
    }
    int xmove[4] = {0, 0, -1, 1};
    int ymove[4] = {1, -1, 0, 0};
    int ans      = 0;
    while (!queX.empty() && !queY.empty()) {
        x = queX.front();
        y = queY.front();
        queX.pop();
        queY.pop();
        ans++;

        for (int i = 0; i < 4; i++) {
            if (x + xmove[i] >= 0 && x + xmove[i] < h && y + ymove[i] >= 0 && y + ymove[i] < w) {
                if (s[x + xmove[i]][y + ymove[i]] == '.') {
                    queX.push(x + xmove[i]);
                    queY.push(y + ymove[i]);
                    s[x + xmove[i]][y + ymove[i]] = '#';
                }
            }
        }
    }
    return ans;
}

int main() {
    int w, h;
    vector<int> ans;
    while (1) {
        cin >> w >> h;
        if (!w && !h)
            break;
        ans.push_back(solve(w, h));
    }
    for (int a : ans) cout << a << endl;
}

C++: #include<iostream>
#include<vector>
#include<string>
#include<queue>
#include<map>
#define INF 1000000000
using namespace std;
typedef pair<int,int> P;
int dx[]={0,0,1,-1},dy[]={1,-1,0,0};
int h,w,n;
vector<string> s;
bool inf(int y,int x){return(y>=0&&y<h&&x>=0&&x<w)?1:0;}
int bfs(int sy,int sx){
	int ret=0;
	queue<P> que;
	que.push(P(sy,sx));
	vector<vector<int> > d(h,vector<int>(w,INF));
	d[sy][sx]=0;
	while(!que.empty()){
		P t=que.front();
		que.pop();
		int y=t.first,x=t.second;
		
		
		for(int i=0;i<4;i++){
			int ny=dy[i]+y,nx=dx[i]+x;
			if(inf(ny,nx)&&d[ny][nx]>d[y][x]+1&&s[ny][nx]!='#'){
				que.push(P(ny,nx));
				d[ny][nx]=d[y][x]+1;
			}
		}
	}
	for(int i=0;i<h;i++){
		for(int j=0;j<w;j++){
			if(d[i][j]!=INF)
				ret++;
		}
	}
	return ret;
}
int main(){
	while(cin>>w>>h,h||w||n){
		s=vector<string>(h);
		for(int i=0;i<h;i++){
			cin>>s[i];
		}
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(s[i][j]=='@'){
					cout<<bfs(i,j)<<endl;
				}
			}
		}
	}
	return 0;
}