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())
	}
}

Python: from collections import deque

move = ((0,1),(1,0),(0,-1),(-1,0))

def inmap(x,y):
    if 0 <= x <= Lx-1 and 0<= y <= Ly-1:
        return True
    return False

while True:
    Lx,Ly = map(int,input().split())
    if Lx == 0:
        break
    curx= -1
    cury = -1
    orimap = [[] for _ in range(Ly)]
    for i in range(Ly):
        for j in input():
            orimap[i].append(j)
    for y in range(len(orimap)):
        for x in range(len(orimap[y])):
            if orimap[y][x] == "@":
                curx,cury = x,y
                orimap[y][x]="#"
                break
        if curx != -1:
            break
    Q = deque()
    Q.append([curx,cury])
    count = 1
    while len(Q)>0:
        curx,cury = Q.popleft()
        for dx,dy in move:
            nextx = curx + dx
            nexty = cury + dy
            if inmap(nextx,nexty):
                if orimap[nexty][nextx] != "#":
                    orimap[nexty][nextx] = "#"
                    Q.append([nextx,nexty])
                    count += 1
    print(count)

Python: #!/usr/bin/env python3
# -*- coding: utf-8 -*-

import array
import collections
import itertools


class DepthFirstSearchWithStack(object):

    def __init__(self, width, height, tiles):
        self.width = width
        self.height = height
        self.tiles = tiles
        self.reached = [array.array(
            "B", (False for y in range(height))) for x in range(width)]
        for x, y in itertools.product(range(width), range(height)):
            if tiles[x][y] == "@":
                self.start_tile = (x, y)
                break

    def search(self, x0, y0):
        stack = collections.deque()
        self.reached[x0][y0] = True
        stack.append((x0, y0))
        while stack:
            (x, y) = stack.pop()
            for i, j in [(x + 1, y), (x - 1, y), (x, y + 1), (x, y - 1)]:
                if i < 0 or self.width <= i or j < 0 or self.height <= j:
                    pass
                elif self.tiles[i][j] == "#":
                    pass
                elif not self.reached[i][j]:
                    self.reached[i][j] = True
                    stack.append((i, j))

    def count_reachable_tiles(self):
        self.search(*self.start_tile)
        reachable_tiles = sum([self.reached[x][y] for x, y in itertools.product(
            range(self.width), range(self.height))])
        return reachable_tiles


if __name__ == "__main__":
    while True:
        width, height = map(int, input().split())
        if width == 0 and height == 0:
            break
        else:
            tiles = [x for x in zip(*[input() for y in range(height)])]
            dfs = DepthFirstSearchWithStack(width, height, tiles)
            print(dfs.count_reachable_tiles())

Python: data, res

def valid(x, y):
    global data
    return 0 <= x < W and 0 <= y < H and data[y][x] != '#'

def dfs(x, y):
    global data, res
    if valid(x, y):
        res += 1
        data[y][x] = '#'
        dx = [1, 0, -1, 0]
        dy = [0, 1, 0, -1]
        for i in range(4):
            dfs(x + dx[i], y + dy[i])
    else:
        return
    
while True:
    global data, res
    W, H = map(int, input().split())
    if W == 0 and H == 0:
        break
    data = [list(input()) for _ in range(H)]
    res = 0
    for i in range(W):
        for j in range(H):
            if data[j][i] == '@':
                dfs(i, j)
                break
    print(res)