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++: #include <iostream>
#include <algorithm>
#include <vector>
#include <string>

using namespace std;

int dx[] = {1, 0, -1, 0};
int dy[] = {0, 1, 0, -1};

int paint(vector<string> &field, int x, int y) {
    int cnt = 0;

    for(int i = 0; i < 4; ++i) {
        int xx = x + dx[i];
        int yy = y + dy[i];

        if(xx < 0 || field[0].size() <= xx || yy < 0 || field.size() <= yy) continue;

        if(field[yy][xx] == '.') {
            field[yy][xx] = '#';
            cnt += paint(field, xx, yy)+1;
        }
    }
    return cnt;
}

int main() {
    while(true) {
        int W, H;
        cin >> W >> H;

        if(!W && !H) break;

        int sx, sy;
        vector<string> field(H);
        for(int i = 0; i < H; ++i) {
            cin >> field[i];
            for(int x = 0; x < W; ++x) {
                if(field[i][x] == '@') {
                    sx = x;
                    sy = i;
                }
            }
        }

        cout << paint(field, sx, sy)+1 << endl;
    }
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;


/*int f(int x){

  return x * x;
}

int g(int x){
  if(x == 0) return 1;
  return g(x-1) * x;
}
*/

int H, W;

void  solve(int Y, int X, vector<string> &vec,int &count){

   if(Y<0||Y>=H||X<0||X>=W){
    return;
  }
   else if(vec[Y][X]=='#'){
    return;
  }
  count+=1;
  vec[Y][X]='#';
  solve(Y+1,X,vec, count);
  solve(Y-1,X,vec,count);
  solve(Y,X-1,vec,count);
  solve(Y,X+1,vec,count);

}
int main(){
  //vector<vector<int>>vec(N,vector<int>(N));

  while(1){
    // int W,H
    int count;
  cin >> W >>H;
  if(W==0 &&H==0)break;
  count=0;

  vector<string>vec(H);
  int x,y;

  for(int i =0;i<H;i++){
    cin >> vec.at(i);
    for(int j=0;j<W;j++){
      if(vec[i][j]=='@'){
        x = j;
        y = i;
        break;
      }
           }
  }
  solve(y, x,  vec, count);
  cout << count <<endl;
  }


}

C++: #include<cstdio>
#include<algorithm>
#include<queue>
#include<cstring>
using namespace std;
char stage[22][22];
bool vis[22][22];
int dx[] = {0,1,0,-1};
int dy[] = {1,0,-1,0};
class Data{
	public:
	int x;
	int y;
	int cnt;
	Data(){}
	Data(int x,int y,int cnt):x(x),y(y),cnt(cnt){}
};
int main(){
	int w,h;
	while(scanf("%d%d",&w,&h),(w|h)){
		memset(stage,0,sizeof(stage));
		memset(vis,0,sizeof(vis));
		int sx,sy;
		for(int i = 0; i < h; i++){
			scanf("%s",stage[i]);
			for(int j = 0; j < w; j++){
				if(stage[i][j] == '@'){
					sx = j;
					sy = i;
				}
			}
		}
		
		queue < Data > Q;
		int ans = 0;
		Q.push(Data(sx,sy,1));
		
		while( !Q.empty() ){
			Data d = Q.front();Q.pop();
			if(vis[d.y][d.x]++)continue;
			ans += d.cnt;
			for(int i = 0; i < 4; i++){
				if(dx[i]+d.x < 0 || dx[i]+d.x >= w || dy[i]+d.y < 0 || dy[i]+d.y >= h || stage[dy[i]+d.y][dx[i]+d.x] == '#')continue;
				Q.push(Data(dx[i]+d.x,dy[i]+d.y,d.cnt));
			}
		}
		printf("%d\n",ans);
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>

using namespace std;

const int dm[] = {-1, 0, 1, 0}, dn[] = {0, -1, 0, 1};
int h, w, count;
vector< vector<int> > field, finished;

bool valid(int m, int n) {
    return 0 <= m && m < h && 0 <= n && n < w && field[m][n] == 1;
}

void dfs(int m, int n) {
    finished[m][n] = 1;
    count++;

    for(int i = 0; i < 4; i++) {
        if(valid(m + dm[i], n + dn[i]) && !finished[m + dm[i]][n + dn[i]]) {
            dfs(m + dm[i], n + dn[i]);
        }
    }
}

int main() {
    while(1) {
        cin >> w >> h;
        if (w == 0 && h == 0) {
            break;
        }

        field.resize(h); finished.resize(h);
        int m, n;
        for (int i = 0; i < h; i++) {
            string input;
            cin >> input;

            field[i].resize(w); finished[i].resize(w);

            for (int j = 0; j < w; j++) {
                finished[i][j] = 0;

                if(input[j] == '@') {
                    m = i;
                    n = j;
                }
                if(input[j] == '#') {
                    field[i][j] = 0;
                } else {
                    field[i][j] = 1;
                }
            }
        }

        count = 0;
        dfs(m, n);

        cout << count << endl;
    }

    return 0;
}