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>
#include <numeric>
#include <set>
#include <map>
#include <string>
#include <stack>
#include <queue>
#include <cmath>
#include <cstdio>
#include <istream>
#include <sstream>
#include <iomanip>
#include <iterator>
#include <climits>
using namespace std;

typedef ostringstream OSS;
typedef istringstream ISS;

typedef vector<int> VI;
typedef vector< VI > VVI;

typedef long long LL;

typedef pair<int, int> PII;
typedef vector<PII> VPII;

#define X first
#define Y second

const char OK = '.';
const char BAD = '#';
const char USER = '@';

int W, H;
int cnt;
vector<string> ts;
vector<vector<bool>> done;

void dfs(int y, int x) {
	if (y < 0 || x < 0 || y >= H || x >= W) return;
	if (done[y][x]) return;
	if (ts[y][x] == BAD) return;
	
	done[y][x] = true;
	cnt++;

	for (int dx = -1; dx <= 1; dx++) {
		for (int dy = -1; dy <= 1; dy++) {
			if (!dy ^ !dx) {
				dfs(y + dy, x + dx);
			}
		}
	}
}

int main(void) {
	while (cin >> W >> H, W) {
		ts = vector<string>(H);
		done = vector<vector<bool>>(H, vector<bool>(W));
		for (auto &line : ts) cin >> line;

		cnt = 0;

		for (int y = 0; y < H; y++) {
			for (int x = 0; x < W; x++) {
				if (ts[y][x] == USER) {
					dfs(y, x);
				}
			}
		}

		cout << cnt << endl;
	}

	return 0;
}

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

char c[21][21];
int w, h;
int sy, sx;
int dy[4] = {1, -1, 0, 0};
int dx[4] = {0, 0, 1, -1};
bool f[21][21];
int ans = 0;

bool contain(int y, int x){
    return (0 <= y && y < h && 0 <= x && x < w);
}

void dfs(int y, int x){
    f[y][x] = true;
    ans++;
    for(int i = 0; i < 4; i++){
        int ny = y + dy[i];
        int nx = x + dx[i];
        if(contain(ny, nx) == true && f[ny][nx] == false && c[ny][nx] != '#'){
            dfs(ny, nx);
        }
    }
    return;
}

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

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

const int di[4] = {
	1, 0, -1, 0
};
const int di2[4] = {
	0, 1, 0, -1
};

const int max_size = 20;
int move(char area[max_size+2][max_size+2], int sh, int sw){
	if(area[sh][sw] == '#') return 0;
	int count = 1;
	area[sh][sw] = '#';
	for(int i = 0; i < 4; i++){
		count += move(area, sh + di[i], sw + di2[i]);
	}
	return count;
}

int main(){
	char area[max_size+2][max_size+2];
	int sh, sw;
	int h, w;
	while(cin >> w >> h){
		if(w == 0 && h == 0) break;
		for(int i = 0;i < max_size+2; i++){
			for(int j = 0; j < max_size+2; j++){
				area[i][j] = '#';
			}
		}
		for(int i = 1; i <= h; i++){
			for(int j = 1; j <= w; j++){
				cin >> area[i][j];
				if(area[i][j] == '@'){
					sh = i; sw = j;
				}
			}
		}
		cout << move(area, sh, sw) << endl;
	}
	return 0;
}

C++: ///
// File:  1130.cpp
// Author: ymiyamoto
//
// Created on Sat Nov 11 02:36:17 2017
//

#include <array>
#include <cstdint>
#include <iostream>
#include <string>
#include <vector>

using namespace std;

void dfs(int32_t H, int32_t W, array<array<char, 20>, 20> &tiles, array<array<bool, 20>, 20> &visited, int32_t i, int32_t j, char index)
{
  visited[i][j] = true;
  tiles[i][j] = index;

  for (int32_t l = -1; l <= 1; l++) {
    if (0 <= j + l && j + l < W) {
      if (tiles[i][j + l] == '.') {
        dfs(H, W, tiles, visited, i, j + l, index);
      }
    }
  }

  for (int32_t k = -1; k <= 1; k++) {
    if (0 <= i + k && i + k < H) {
      if (tiles[i + k][j] == '.') {
        dfs(H, W, tiles, visited, i + k, j, index);
      }
    }
  }
}

int32_t main()
{
  while (true) {
    int32_t W, H;
    cin >> W >> H;
    if (W == 0) break;

    array<array<char, 20>, 20> tiles;
    array<array<bool, 20>, 20> visited;
    uint32_t start_x = 0;
    uint32_t start_y = 0;
    for (int32_t i = 0; i < H; i++) {
      for (int32_t j = 0; j < W; j++) {
        cin >> tiles[i][j];
        if (tiles[i][j] == '@') {
          start_x = i;
          start_y = j;
          tiles[i][j] = '.';
        }
      }
    }

    char index = 0;
    for (int32_t i = 0; i < H; i++) {
      for (int32_t j = 0; j < W; j++) {
        if (tiles[i][j] == '.') {
          dfs(H, W, tiles, visited, i, j, index);
          index++;
        }
      }
    }

    char start_index = tiles[start_x][start_y];
    uint32_t count = 0;
    for (int32_t i = 0; i < H; i++) {
      for (int32_t j = 0; j < W; j++) {
        if (tiles[i][j] == start_index) {
          count++;
        }
      }
    }

    cout << count << endl;
  }

  return 0;
}