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>
using namespace std;

char map[21][21];
int count;
int x, y;

void dfs(int nx, int ny){
  map[ny][nx] = '#';
  count++;
  if (nx < x && map[ny][nx+1] == '.'){
    dfs(nx+1, ny);
  }
  if (nx > 0 && map[ny][nx-1] == '.'){
    dfs(nx-1, ny);
  }
  if (ny < y && map[ny+1][nx] == '.'){
    dfs(nx, ny+1);
  }
  if (ny > 0 && map[ny-1][nx] == '.'){
    dfs(nx, ny-1);
  }
  return;
}

int main(){
  while (1){
    cin >> x >> y;
    if (!(x || y)) break;
    int ix, iy;
    count = 0;
    for (int i = 0; i < y; i++){
      for (int j = 0; j < x; j++){
	cin >> map[i][j];
	if (map[i][j] == '@'){ix = j; iy = i;}
      }
    }
    dfs(ix, iy);
    cout << count << endl;
  }
}

C++: #include<iostream>
#include<iomanip>
#include<string>
#include<cstdlib>
#include<cmath>
#include<algorithm>

using namespace std;

int H, W;
int table[20][20];
int cnt;

void calc(int current_x, int current_y) {
	table[current_x][current_y] = 3;

	if(current_x > 0 && table[current_x-1][current_y] == 1)
		calc(current_x-1, current_y);
	if(current_y > 0 && table[current_x][current_y-1] == 1)
		calc(current_x, current_y-1);
	if(current_x < W-1 && table[current_x+1][current_y] == 1)
		calc(current_x+1, current_y);
	if(current_x < H-1 && table[current_x][current_y+1] == 1)
		calc(current_x, current_y+1);

	cnt++;
}


int main() {
	while(1) {
		cnt = 0;
		cin >> W >> H;
		if(H == 0 && W == 0) break;

		char temp;
		int current[2];
		for(int i=0; i<H; i++) {
			for(int j=0; j<W; j++) {
				cin >> temp;
				if(temp=='.') table[i][j] = 1;
				else if(temp=='#') table[i][j] = 2;
				else if(temp=='@') {
					table[i][j] = 0;
					current[0] = i;
					current[1] = j;
				}
			}
		}

		calc(current[0], current[1]);

		cout << cnt << endl;
	}

	return 0;
}

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

int w, h;
char tiles[20][21];

int dx[4] = { 0, 1, 0,-1};
int dy[4] = { 1, 0,-1, 0};

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

int main(){
    while(1){
        cin >> w >> h;
        if(!w && !h) break;
        
        int sx, sy;
        for(int i = 0; i < h; i++){
            cin >> tiles[i];
            for(int j = 0; j < w; j++){
                if(tiles[i][j] == '@'){
                    sx = j; sy = i;
                }
            }
        }
        
        int cnt = 0;
        queue<pair<int, int> > q;
        q.push(make_pair(sx, sy));
        while(!q.empty()){
            cnt++;
            int x = q.front().first, y = q.front().second;
            q.pop();
            
            for(int i = 0; i < 4; i++){
                int nx = x + dx[i], ny = y + dy[i];
                
                if(isRegion(nx, ny) && tiles[ny][nx] == '.'){
                    tiles[ny][nx] = '#';
                    q.push(make_pair(nx, ny));
                }
            }
        }
        cout << cnt << endl;
        
    }
}

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

using namespace std;

int d[] = {-1,1,0,0};

int count(int x,int y, char map[22][22]){
        int c = 0; 
        map[x][y] = '#';
        c++;
        for(int i = 0; i < 4; i++){
                int nx = x+d[i];
                int ny = y+d[3-i];
                if(map[nx][ny] == '.'){
                        c += count(nx,ny,map);
                }  
        }  
        return c; 
}

int main(){
        int w,h;
        cin >> w >> h; 
        while(w){
                char map[22][22];
                memset(map,'#',sizeof(map));
                int x,y;
                for(int i = 1; i <= h; i++){
                        for(int j = 1; j <= w; j++){
                                cin >> map[i][j];
                                if(map[i][j] == '@'){
                                        x = i; y = j; 
                                        map[i][j] = '.';
                                }  
                        }  
                }  
                cout << count(x,y,map) << endl;
                cin >> w >> h; 
        }  
}