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 defaultdict
def explore(now,cnt):
    if cnt>W*H:#理論的にはW*H以下で全部回れるので
        a=0 #無駄なコード
    else:
        d[now]=1
        if now[1]+1<W:
            if not data[now[0]][now[1]+1]=="#":
                if not (now[0],now[1]+1) in d.keys():
                    explore((now[0],now[1]+1),cnt+1)
        if now[0]+1<H:
            if not data[now[0]+1][now[1]]=="#":
                if not (now[0]+1,now[1]) in d.keys():
                    explore((now[0]+1,now[1]),cnt+1)
        if now[0]-1>=0:
            if not data[now[0]-1][now[1]]=="#":
                if not (now[0]-1,now[1]) in d.keys():
                    explore((now[0]-1,now[1]),cnt+1)
        if now[1]-1>=0:
            if not data[now[0]][now[1]-1]=="#":
                if not (now[0],now[1]-1) in d.keys():
                    explore((now[0],now[1]-1),cnt+1)
    
while True:
    W,H=map(int,input().split())
    if W==0 and H==0:
        break
    data=[list(input()) for _ in range(H)]
    d=defaultdict()
    for i in range(H):
        for j in range(W):
            if data[i][j]=="@":
                start=(i,j)
                break
    now=start
    cnt=0
    explore(now,cnt)
    print(len(d.keys()))

Python: 
def depth_first_search(x, y):
    """
    ??±???????????¢?´¢?????????
    :param x: ?????¨??°????¨?
    :param y: ?????¨??°??????
    :return: ?????°
    """

    # ????????????????????§?¢??????????
    # ??????x  ,y+1
    # ??????x+1,y
    # ??????x  ,y-1
    # ??????x-1,y
    move_list = [[0,1],[1,0],[0,-1],[-1,0]]
    for i in range(0,4,1):
        next_x = x+move_list[i][0]
        next_y = y+move_list[i][1]
        # tile_map??????????§??????§????????´??????'.'???
        if 0 <= next_x and 0 <= next_y and \
            next_x < W and next_y < H and \
            tile_map[next_y][next_x] != '#':

            # ?§?????????????????????¨??°????§??????§??????????????????"#"??????????????????
            tile_map[y][x] = '#'
            import __main__
            __main__.move_point = __main__.move_point+1
            depth_first_search(next_x, next_y)

def get_start_point(tile_map):
    """
    @????????????????????????
    :param tile_map:
    :return:
    """
    for i, y in enumerate(tile_map):
        for j, x in enumerate(y):
            if x == '@':
                return i, j



if __name__ == "__main__":
    move_point = 0
    while (True):
        # input_1 : tile????????????
        tile_size = input('').split(' ')
        W, H = [int(x) for x in tile_size]
        if W == 0 and H == 0:
            break
        tile_map=[''] * H
        for i in range(0, H, 1):
            # input_2 : tile???????????????????????????
            map_line = list(input(''))
            tile_map[i] = map_line
        start_H, start_W = get_start_point(tile_map)
        depth_first_search(start_W, start_H)
        print(move_point)

Python: W,H=(int(i) for i in input().split())
while W!=0 or H!=0:
    a=[]
    T=[]
    for i in range(H):
        T.append(input())
        a.append([])
        for j in range(W):
            a[i].append(0)
 #   print(T[0][0])
    i=0
    j=0
    while T[i][j]!="@":
        j=0
        while j<W and T[i][j]!="@":
            j+=1
    #        print(j)
        if j>=W:
            j-=1
        if T[i][j]!="@":
            i+=1
  #      print(i)
    s=[[i,j]]
    a[i][j]=1
    c=1
    while s!=[]:
        u=[]
        for l in range(len(s)):
            i=s[l][0]
            j=s[l][1]
            i-=1
            k=0
            while k<2:
            #    print(j)
                if 0<=i<H and a[i][j]==0 and T[i][j]==".":
                    a[i][j]=1
                    u.append([i,j])
                    c+=1
                i+=2
                k+=1
            i-=3
            j-=1
            k=0
            while k<2:
                if 0<=j<W and a[i][j]==0 and T[i][j]==".":
                    a[i][j]=1 
                    u.append([i,j])
                    c+=1
                j+=2
                k+=1
        s=u
    #    print (a)
    print(c)
    W,H=(int(i) for i in input().split())