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++: #define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
bool c[20][20] = {} ;
int cnt,w, h, p[20][20] = {};

int main() {
	int j, f,cx,cy;
	char in[20];
	while (1) {
		for (int i = 0; i < 400; i++) {
			c[i / 20][i % 20] = p[i / 20][i % 20] = 0;
		}
		cnt = 0;
		f = 1;
		scanf("%d %d", &w, &h);
		if (w ==0&& h == 0)
			break;
		scanf("\n", &j);
		for (int i = 0; i < h; i++) {
			scanf("%s", &in);
			for (j = 0; j < w; j++) {
				switch (in[j]) {
				case '#':
					c[j][i] = 0;
					p[j][i] = -1;
					break;
				case '@':
					cx = j;
					cy = i;
					p[j][i] = 1;
				case '.':
					c[j][i] = 1;
				}
			}
		}

		/*for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++)
				printf("%3d", c[j][i]);
			printf("\n");
		}*/

		while (f == 1) {
			f = 0;
			if (w != 1 && h != 1) {
				for (int i = 0; i < w*h; i++) {
					if (p[i / h][i%h] == 0 && (p[i / h + 1][i % h] == 1 || p[i / h - 1][i % h] == 1 || p[i / h][i % h + 1] == 1 || p[i / h][i % h - 1] == 1)) {
						p[i / h][i%h] = 1;
						f = 1;
					}
				}
			}
			if (w == 1 && h != 1) {
				for (int i = 0; i < h; i++) {
					if (p[0][i] == 0 && (p[0][i + 1] == 1 || p[0][i - 1] == 1)) {
						p[0][i] = 1;
						f = 1;
					}
				}
			}

			if (w != 1 && h == 1) {
				for (int i = 0; i < w; i++) {
					if (p[i][0] == 0 && (p[i + 1][0] == 1 || p[i - 1][0] == 1)) {
						p[i][0] = 1;
						f = 1;
					}
				}
			}


			/*for (int i = 0; i < h; i++) {
				for (int j = 0; j < w; j++)
					printf("%3d", p[j][i]);
				printf("\n");
			}
			printf("\n");*/
		}

		for (int i = 0; i < w*h; i++) {
			if (p[i / h][i%h] == 1)
				cnt++;
		}
		printf("%d\n", cnt);
	}
	return 0;
}

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

#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)

int w,h;
char f[32][32];

int fill(int x, int y){
	if( x<0 || y<0 || x>=w || y>=h || f[y][x]=='#' )return 0;
	f[y][x] = '#';
	return 1 + fill(x-1,y) + fill(x+1,y) + fill(x,y+1) + fill(x,y-1);
}

int main(){
	int x,y;
	while(cin>>w>>h,w|h){
		rep(i,h)cin>>f[i];
		rep(i,h)rep(j,w)if( f[i][j]=='@' ){
			x=j; y=i;
		}
		printf("%d\n",fill(x,y));
	}
	return 0;
}

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

int count;
int w,h;
int x,y;
char map[20][20];

void tannkenn(int x,int y);

int main()
{
	while(cin>>w>>h,w&&h){
	count=0;
	for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                cin >> map[i][j];
				if(map[i][j]=='@'){
					x=i;
					y=j;
				}
			}
	}

	tannkenn(x,y);
	cout<<count<<endl;
	}
	return 0;
}

void tannkenn(int x,int y)
{
	count++;
	map[x][y]='#';

	if(x+1<h && map[x+1][y]=='.')
		tannkenn(x+1,y);
	if(y+1<w && map[x][y+1]=='.')
		tannkenn(x,y+1);
	if(x-1>=0 && map[x-1][y]=='.')
		tannkenn(x-1,y);
	if(y-1>=0 && map[x][y-1]=='.')
		tannkenn(x,y-1);
}

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

void checkboard(void);
void startmove(void);
int checked(int i,int j);

char board[21][21];
int H,W,now[2];

int main(void){
	while(1){
		cin >> H >> W ;
		if (!H && !W) break;
		// count=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				cin >> board[i][j];
				if(board[i][j]=='@'){
					now[0]=i;
					now[1]=j;
				}
			}
		}
		startmove();
		checkboard();
	}

}


void startmove(void){
	int n;
	do{
		n=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				if(board[i][j]=='@')
					n+=checked(i,j);
			}
		}

	}while(n);
}

int checked(int i,int j){
	int a=0;
	if(i>0 && board[i-1][j]=='.'){
		board[i-1][j]='@';
		a=1;
	}
	if(i<W && board[i+1][j]=='.'){
		board[i+1][j]='@';
		a=1;
	}
	if(j>0 && board[i][j-1]=='.'){
		board[i][j-1]='@';
		a=1;
	}
	if(j<H && board[i][j+1]=='.'){
		board[i][j+1]='@';
		a=1;
	}
	// checkboard();
	return a;
}
void checkboard(void){
		int count=0;
		for(int i=0;i<W;i++){
			for(int j=0;j<H;j++){
				if(board[i][j]=='@') count++; 
				// cout << board[i][j];
			}
		}
			cout << count << endl;

}