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;
			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;
				}
			}
			/*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>
#include <vector>
#include <queue>

using namespace std;

#define in(h, w) 0<=h&&h<H&&0<=w&&w<W

int dh[4] = {1, 0, -1, 0};
int dw[4] = {0, 1, 0, -1};

int main(){
    int W, H;
    while (cin >> W >> H and W and H) {
        int h, w;
        vector< vector<char> > map(H, vector<char>(W));
        for (int i = 0; i < H; i++)
            for (int j = 0; j < W; j++) {
                cin >> map[i][j];
                if (map[i][j] == '@') {
                    h = i;
                    w = j;
                }
            }
        int res = 1;
        queue<pair<int, int> > q;
        q.push(make_pair(h, w));
        while (!q.empty()) {
            h = q.front().first;
            w = q.front().second;
            q.pop();
            if (map[h][w] == '.') {
                res++;
                map[h][w] = '#';
            }
            for (int i = 0; i < 4; i++) {
                int th = h+dh[i];
                int tw = w+dw[i];
                if (0<=th&&th<H&&0<=tw&&tw<W) {
                    if (map[th][tw] == '.')
                        q.push(make_pair(th, tw));
                }
            }
        }
        cout << res << endl;
    }
}

C++: #include <iostream>
#include <string>
using namespace std;
typedef pair<int,int> P;
int dx[4]={0,1,0,-1};
int dy[4]={1,0,-1,0};
string s[22];
int cnt;
void dfs(int x, int y){
	cnt++;
	s[x][y]='#';
	for(int i=0;i<4;i++){
		if(s[ x+dx[i] ][ y+dy[i] ]!='#') dfs(x+dx[i],y+dy[i]);
	}
}
int main(){
	while(true){
		int w,h,sx,sy;
		cnt=0;
		cin >> w >> h;
		if(!w) break;
		for(int i=1;i<=h;i++){
			cin >> s[i];
			s[i]="#"+s[i]+"#";
		}
		s[0]="";
		s[h+1]="";
		s[0].append(w+2,'#');
		s[h+1].append(w+2,'#');
		for(int i=1;i<h+1;i++){
			for(int j=1;j<w+1;j++){
				if(s[i][j]=='@'){sx=i;sy=j;}
			}
		}
		dfs(sx,sy);
		cout << cnt << endl;
	}
	return 0;
}

C++: #include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cassert>
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

#define LET(name, value) __typeof(value) name = value
#define REP(i, n) for (int i = 0; i < (int)(n); ++i)
#define FOR(i, a, b) for (int i = (a); i < (int)(b); ++i)
#define FOREQ(i, a, b) for (int i = (a); i <= (int)(b); ++i)
#define ALL(c) (c).begin(), (c).end()
#define FOREACH(i, c) for (LET(i, (c).begin()); i != (c).end(); ++i)

char field[30][30];

int dfs(int y, int x) {
  static int dy[] = {-1, 0, 1, 0};
  static int dx[] = {0, -1, 0, 1};

  if (field[y][x] == '.') {
    field[y][x] = '#';
    int sum = 1;
    REP(k, 4) { sum += dfs(y + dy[k], x + dx[k]); }
    return sum;
  } else {
    return 0;
  }
}

int main() {
  int W, H;
  while (scanf("%d%d", &W, &H), W|H) {
    int sy = -1, sx = -1;
    memset(field, '#', sizeof(field));
    FOREQ(y, 1, H) FOREQ(x, 1, W) {
      scanf(" %c", &field[y][x]);
      if (field[y][x] == '@') {
        sy = y; sx = x;
        field[y][x] = '.';
      }
    }
    printf("%d\n", dfs(sy, sx));
  }
}