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 <bits/stdc++.h>
using namespace std;
const int inf = 1001001001;


int w, h;
int a[20][20];
int sw, sh;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
int D[20][20];
int Ans = 1;


int main()
{
    while(cin >> w, w != 0)
    {
        Ans = 1;
        cin >> h;
        
        for(int i = 0; i < 20; i++)
        {
            for(int j = 0; j < 20; j++)
            {
                D[i][j] = inf;
            }
        }
        
        for(int i = 0; i < h; i++)
        {
            for(int j = 0; j < w; j++)
            {
                char x;
                cin >> x;
                if(x == '.')
                {
                    a[j][i] = 1;
                }
                else if(x == '#')
                {
                    a[j][i] = 0;
                }
                else
                {
                    sw = j;
                    sh = i;
                    a[j][i] = 100;  //  ????????§?????????
                }
            }
        }
        queue< pair<int, int> > Q;
        pair<int, int> S;
        S = make_pair(sw, sh);
        Q.push(S);
        D[sw][sh] = 0;
        int k = 0;
        while(! Q.empty())
        {
            pair<int, int> cur = Q.front();
            Q.pop();
            int x = cur.first;
            int y = cur.second;
            for(int i = 0; i < 4; i++)
            {
                if(a[x+dx[i]][y+dy[i]] == 1 && 0 <= x+dx[i] && x+dx[i] < w && 0 <= y+dy[i] && y+dy[i] < h && D[x+dx[i]][y+dy[i]] == inf)    //  ????????????????¢????????????????????????????????????°???
                {
                    D[x+dx[i]][y+dy[i]] = D[x][y] + 1;
                    Ans = Ans + 1;
                    pair<int, int> T;
                    T = make_pair(x+dx[i], y+dy[i]);
                    Q.push(T);
                }
            }
        }
        cout << Ans << endl;
    }
}

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

char map[20][20];
int visited[20][20];

void dfs(int x, int y, int w, int h) {
    visited[x][y] = 1;
    for (int i = -1; i <= 1; i += 2) {
        if (0 <= x+i && x+i < h && visited[x+i][y] == 0 && map[x+i][y] == '.')
            dfs(x+i, y, w, h);
    }
    for (int i = -1; i <= 1; i += 2) {
        if (0 <= y+i && y+i < w && visited[x][y+i] == 0 && map[x][y+i] == '.')
            dfs(x, y+i, w, h);
    }
}

int main(int argc, char *argv[])
{
    int w, h, x, y, sum;
    string line;
    while (cin >> w >> h, w | h) {
        for (int i = 0; i < 20; ++i)
            for (int j = 0; j < 20; ++j)
                visited[i][j] = 0;
        for (int i = 0; i < h; ++i) {
            cin >> line;
            for (int j = 0; j < w; ++j) {
                map[i][j] = line[j];
                if (line[j] == '@') {
                    x = i;
                    y = j;
                }
            }
        }
        dfs(x, y, w, h);
        sum = 0;
        for (int i = 0; i < 20; ++i)
            for (int j = 0; j < 20; ++j)
                sum += visited[i][j];
        cout << sum << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm> 
#include <queue>
#include <functional>
#include <map>
#include <string>
using namespace std;
int w, h;
char field[25][25];
bool used[25][25];
int vx[] = { 1, 0, -1, 0 };
int vy[] = { 0, 1, 0, -1 };

int dfs(int x,int y){
    
    int ret = 1;
    used[x][y] = true;

    for (int i = 0; i < 4; i++){
        int nx, ny;
        nx = x + vx[i]; ny = y + vy[i];
        if (nx != 0 && nx != w + 1 && ny != 0 && ny != h + 1 && field[nx][ny] != '#'&&
            !used[nx][ny]){
            ret += dfs(nx, ny);
        }
    }

    return ret;
}

int main(void){

    while (true){
        cin >> w >> h;
        if (w == 0 && h == 0)break;
        for (int i = 1; i <= w; i++){
            for (int j = 1; j <= h; j++){
                used[i][j] = false;
            }
        }
        int sx, sy;
        for (int i = 1; i <= h; i++){
            for (int j = 1; j <= w; j++){
                cin >> field[j][i];
                if (field[j][i] == '@'){
                    sx = j; sy = i;
                }
            }
        }

        int ans;
        ans = dfs(sx, sy);
        cout << ans << endl;
        
    }
    

}