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 <queue>
using namespace std;
int dx[] = { -1, 0, 1, 0 };
int dy[] = { 0, -1, 0, 1 };

int main(){
	int W, H;
	while( cin >> W >> H, W, H ){
		char room[20][20];
		queue< pair<int, int> > xy;
		for( int i = 0; i < H; i++ ){
			for( int j = 0; j < W; j++ ){
				cin >> room[i][j];
				if( room[i][j] == '@' ){
					xy.push(pair<int, int>(i, j));
					room[i][j] = '#';
				}
			}
		}
		//cout << xy.front().first << ' ' << xy.front().second << endl;
		int sum = 1;
		while( !xy.empty() ){
			for( int i = 0; i < 4; i++ ){
				int x = xy.front().second + dx[i];
				int y = xy.front().first + dy[i];
				//cout << room[y][x];
				if( 0 <= x && x < W && 0 <= y && y < H && room[y][x] == '.' ){
					room[y][x] = '*';
					xy.push(pair<int, int>(y, x));
					sum++;
				}
			}
			//cout << endl;
			xy.pop();
		}
		/*for( int i = 0; i < H; i++ ){
			for( int j = 0; j < W; j++ ){
				cout << room[i][j];
			}
			cout << endl;
		}*/
		cout << sum << endl;
	}
}

C++: #include <cstdio>
#include <iostream>

using namespace std;

char map[22][22];
int h,w,cnt,dx[4]={0,0,1,-1},dy[4]={1,-1,0,0};

void search(int x,int y){
	cnt++;
	map[y][x] = '#';
	for(int i = 0; i < 4; i++){
		if(map[y+dy[i]][x+dx[i]] == '.')
			search(x+dx[i],y+dy[i]);
	}
}

int main()
{	
	while(scanf("%d%d",&w,&h) && h != 0 && w != 0){
		for(int i = 1; i <= h; i++){
			scanf("%s",&map[i][1]);
		}
		for(int i = 0; i <= h+1; i++){
			map[i][0] = '#';
			map[i][w+1] = '#';
		}
		for(int j = 0; j <= w+1; j++){
			map[0][j] = '#';
			map[h+1][j] = '#';
		}
		/*
		for(int i = 0; i <= h+1; i++){
			for(int j = 0; j <= w+1; j++){
				printf("%c",map[i][j]);
			}
			printf("\n");
		}
		*/
		cnt  = 0;
		for(int i = 1; i <= h; i++){
			for(int j = 1; j <= w; j++){
				if(map[i][j] == '@')
					search(j,i);
			}
		}
		printf("%d\n",cnt);
	}

	return 0;
}

C++: #include <stdio.h>
#include <iostream>
#include <math.h>
#include <string>
#include <vector>

using namespace std;
#define ll long long
#define pb push_back

// 前に&をつけるとvectorの参照渡しになる
int solve(int x, int y, vector<vector<char> > tile, int H, int W, int count, vector<vector<bool> >& flag) {
    // cout << count << ":" << y << ","<< x << endl;
    count++;
    if (!flag[y][x]) {
        return count;
    } else {
        flag[y][x] = false;
    }
    if (y + 1 < H) {
        if (tile[y + 1][x] == '.' && flag[y + 1][x]) {
            count = solve(x, y + 1, tile, H, W, count, flag);
        }
    }
    if (y - 1 >= 0) {
        if (tile[y - 1][x] == '.' && flag[y - 1][x]) {
            count = solve(x, y - 1, tile, H, W, count, flag);
        }
    }
    if (x + 1 < W) {
        if (tile[y][x + 1] == '.' && flag[y][x + 1]) {
            count = solve(x + 1, y, tile, H, W, count, flag);
        }
    }
    if (x - 1 >= 0) {
        if (tile[y][x - 1] == '.' && flag[y][x - 1]) {
            count = solve(x - 1, y, tile, H, W, count, flag);
        }
    }
    return count;
}

int main() {

    int H, W;
    while(true) {
        cin  >> W >> H;
        if (W==0 && H == 0) break;
        vector<vector<char> > tile(H, vector<char>(W));
        vector<vector<bool> > flag(H, vector<bool>(W, true));
        int x, y, count = 0;

        for (int i = 0; i < H; i++) {
            for (int j = 0; j < W; j++) {
                cin >> tile[i][j];
                if (tile[i][j] == '@') {
                    x = j;
                    y = i;
                }
            }
        }

        count = solve(x, y, tile, H, W, count, flag);
        cout << count << endl;
    }

}

C++: #include <iostream>
#include <vector>
#include <string>
using namespace std;

int explorer(vector<string> *map,int i, int j){
	if(map->at(i)[j]=='#') return 0;
	
	int cnt=1;
	vector<int>vi;
    vi.reserve(4);
	vector<int>vj;
    vj.reserve(4);
	
	map->at(i)[j]='#';
	
	if(i!=0&&map->at(i-1)[j]=='.'){
		vi.push_back(i-1);
		vj.push_back(j);
	}if(i+1!=map->size()&&map->at(i+1)[j]=='.'){
		vi.push_back(i+1);
		vj.push_back(j);		
	}if(j!=0&&map->at(i)[j-1]=='.'){
		vi.push_back(i);
		vj.push_back(j-1);		
	}if(j+1!=map->at(i).size()&&map->at(i)[j+1]=='.'){
		vi.push_back(i);
		vj.push_back(j+1);		
	}

	while(!vi.empty()){
		cnt += explorer(map,vi.back(),vj.back());
		vi.pop_back();
		vj.pop_back();
	}

	return cnt;
}

int main() {
	int w,h;
	
	while(cin >> w >> h,w){
	vector<string> map;
	int psx=0,psy=0;
	map.resize(h);
	for(int i=0;i<h;i++){
		cin >> map[i];
		
		if(psx==0){
			if(map[i].find('@')!=-1){
				psx=map[i].find('@');
				psy=i;
			}
		}
	}
	
	cout << explorer(&map,psy,psx) << endl;
	}
	return 0;
}