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: #!/usr/bin/env python
# conding: utf

from logging import getLogger, StreamHandler, NullHandler, DEBUG, INFO
logger = getLogger(__name__)
#handler = StreamHandler()
handler = NullHandler()
logger.setLevel(DEBUG)
handler.setLevel(DEBUG)
logger.addHandler(handler)
logger.propagate = False

g_field = []
g_count = 0
g_W = -1
g_H = -1

OFFSETS = [(1,0), (-1,0), (0,1), (0,-1)]


if __name__ == "__main__":
    while True:
        g_W, g_H = [int(s) for s in input().strip().split()]

        if g_W == 0 and g_H == 0:
            break
        
        g_field = []
        for _ in range(g_H):
            row = list(input().strip())
            g_field.append(row)
        g_count = 0

        for y in range(g_H):
            for x in range(g_W):
                if g_field[y][x] == "@":
                    start_x = x
                    start_y = y
                    break

        #dfs(start_x, start_y)
        # bfs
        queue = [(start_x, start_y)]
        
        while len(queue) > 0:
            x, y = queue.pop()

            g_count += 1
            g_field[y][x] = "-"

            for dx, dy in OFFSETS:
                logger.debug("diff: {} {}".format(dx, dy))
                new_x = x + dx
                new_y = y + dy

                if new_x < 0 or new_x >= g_W:
                    continue
                if new_y < 0 or new_y >= g_H:
                    continue
                new_tile = g_field[new_y][new_x]
                if new_tile == "#" or new_tile == "-":
                    continue
                if (new_x, new_y) in queue:
                    continue
                
                logger.debug("{}, {}".format(new_x, new_y))
                queue.append((new_x, new_y))

        print(g_count)

Python: from collections import deque

dxy = [(1, 0), (0, 1), (-1, 0), (0, -1)]

while True:
    W, H = map(int, input().split())
    if W == 0 and H == 0:
        break
    
    sx = -1
    sy = -1
    mat = [[None for _ in range(W)] for _ in range(H)]
    for i in range(H):
        s = input()
        for j in range(W):
            if s[j] == "@":
                sx = j
                sy = i
            mat[i][j] = s[j]
    
    ans = 0
    q = deque()
    q.append((sx, sy))
    while q:
        (cx, cy) = q.popleft()
        ans += 1
        for dx, dy in dxy:
            nx = cx + dx
            ny = cy + dy
            if 0 <= nx < W and 0 <= ny < H:
                if mat[ny][nx] == ".":
                    mat[ny][nx] = "#"
                    q.append((nx, ny))
    print(ans)

Python: while 1:
    x, y = list(map(int,input().split()))
    if x == 0:break

    mx = [1,-1,0,0]
    my = [0,0,1,-1]
    m = []
    for i in range(y):
        m.append(list(input()))
        if '@' in m[-1]:
            sx = m[-1].index('@')
            sy = i
    cnt = 0

    def move(cur_x, cur_y):
        global cnt
        if cur_y >= y or cur_y < 0 or cur_x >= x or cur_x < 0:return
        if m[cur_y][cur_x] == '.' or m[cur_y][cur_x] == '@':
            cnt += 1
            m[cur_y][cur_x] = '0'
            for i in range(4):
                move(cur_x + mx[i],cur_y + my[i])
    move(sx,sy)
    print(cnt)