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>

using namespace std;

typedef struct {
	int x;
	int y;
} POSITION;

int main() {
	int W, H;
	while (cin >> W >> H, W + H != 0) {
		bool board[W][H];
		vector<POSITION> movable;
		POSITION myPosition;
		int ans = 1;
		for (int y = 0; y < H; y++) {
			string str;
			cin >> str;
			for (int x = 0; x < W; x++) {
				switch (str[x]) {
				case '.': {
					board[x][y] = true;
					break;
				}
				case '@': {
					board[x][y] = false;
					myPosition.x = x;
					myPosition.y = y;
				}	
				case '#': {
					board[x][y] = false;
					break;
				}
				}
			}
		}
		if (myPosition.x - 1 >= 0 && board[myPosition.x - 1][myPosition.y]) {
			movable.push_back( {myPosition.x - 1, myPosition.y} );
			board[myPosition.x - 1][myPosition.y] = false;
			ans++;
		}
		if (myPosition.x + 1 < W && board[myPosition.x + 1][myPosition.y]) {
			movable.push_back( {myPosition.x + 1, myPosition.y} );
			board[myPosition.x + 1][myPosition.y] = false;
			ans++;
		}
		if (myPosition.y - 1 >= 0 && board[myPosition.x][myPosition.y - 1]) {
			movable.push_back( {myPosition.x, myPosition.y - 1} );
			board[myPosition.x][myPosition.y - 1] = false;
			ans++;
		}
		if (myPosition.y + 1 < H && board[myPosition.x][myPosition.y + 1]) {
			movable.push_back( {myPosition.x, myPosition.y + 1} );
			board[myPosition.x][myPosition.y + 1] = false;
			ans++;
		}

		while (movable.size() != 0) {
			POSITION sp = movable[movable.size() - 1];
			movable.pop_back();
			if (sp.x - 1 >= 0 && board[sp.x - 1][sp.y]) {
				movable.push_back( {sp.x - 1, sp.y} );
				board[sp.x - 1][sp.y] = false;
				ans++;
			}
			if (sp.x + 1 < W && board[sp.x + 1][sp.y]) {
				movable.push_back( {sp.x + 1, sp.y} );
				board[sp.x + 1][sp.y] = false;
				ans++;
			}
			if (sp.y - 1 >= 0 && board[sp.x][sp.y - 1]) {
				movable.push_back( {sp.x, sp.y - 1} );
				board[sp.x][sp.y - 1] = false;
				ans++;
			}
			if (sp.y + 1 < H && board[sp.x][sp.y + 1]) {
				movable.push_back( {sp.x, sp.y + 1} );
				board[sp.x][sp.y + 1] = false;
				ans++;
			}
		}
		cout << ans << endl;
	}
	return 0;
}

C++: #if 0
accept?????????
40?????????????????£???

???????????¨??????????????¢?´¢???????????????????????????

???????????????????????????????????°????????????????????????
31-169584????????????????????????????????¨??£??????????????????????????£????????§????????????????????????????????????

#endif


#include <queue>
#include <iostream>
using namespace std;
int w,h;
const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1};
char mat[100][100];

bool valid(int x,int y)
{
    return (x>=0)&&(x<w)&&(y>=0)&&(y<h)&&(mat[x][y]!='#');
}

void bfs(int x,int y) {
  int sum=1;
  queue<pair<int,int> >Q;// ??´??°????????????????????\????????????
  pair<int,int>p;//?????????
  p=make_pair(x,y);
  Q.push(p);
  while (! Q.empty()) {
    pair<int,int> cur = Q.front(); // ??????????´????????????????
    Q.pop();
    for (int i=0;i<4;i++) { //??????????????????????????????4?????????
      if (valid(cur.first+dx[i],cur.second+dy[i])) { //????????????????????????????????£??????
	Q.push(make_pair(cur.first+dx[i],cur.second+dy[i])); // dst ????¨???????????????????
	sum++;
	mat[cur.first+dx[i]][cur.second+dy[i]]='#';//2??????????????¢
      }
    }
    mat[cur.first][cur.second]='#';//2??????????????¢
  }
  cout<<sum<<endl;
}


int main() {
  int x,y;
  while(cin>>w>>h && w!=0){
    for(int j=0;j<h;++j){
        for(int i=0;i<w;++i){
            cin >> mat[i][j];
            if(mat[i][j]=='@'){//???????±???´???
                x=i,y=j;
            }
        }
    }
    bfs(x,y);
  }
}

C++: #include <iostream>
#include <queue>
#include <map>

using namespace std;

typedef pair<int,int> P;
typedef queue<P> Q;

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

#define mp make_pair

int w,h;
char fie[21][21];

void bfs(int x, int y){
    Q q;
    q.push(mp(x,y));
    while(!q.empty()){
        for(int i=0; i<4; i++){
            int xx = q.front().first + dx[i];
            int yy = q.front().second + dy[i];
            if(0 <= xx && xx <= w && 0 <= yy && yy <= h && fie[yy][xx] == '.'){
                fie[yy][xx] = '@';
                q.push(mp(xx,yy));
            }
        }
        q.pop();
    }
}

int main(){
    while(cin >> w >> h , w||h){
        int sx,sy;
        for(int i=0; i<h; i++){
            cin >> fie[i];
            for(int j=0; j<w; j++){
                if(fie[i][j] == '@'){
                    sx = j; sy = i;
                }
            }
        }
        bfs(sx,sy);
        int ans = 0;
        for(int i=0; i<h; i++){
            for(int j=0; j<w; j++){
                if(fie[i][j] == '@') ans++;
            }
        }
        cout << ans << endl;
    }
}