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<bits/stdc++.h>
using namespace std;
int main(){
  int w,h,a[21][21],count=0,m;
  char in;
  while(1){
    cin>>w>>h;
    if(w==0 && h==0) break;
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin>>in;
	if(in=='#') a[i][j]=0;
	else if(in=='@') {
	  a[i][j]=2;
	}
	else a[i][j]=1;
      }
    }
    count=0;
    while(1){
      m=0;
      for(int i=0;i<h;i++){
	for(int j=0;j<w;j++){
	  if(a[i][j]==2){
	    m=1;
	    count++;
	    a[i][j]=3;
	    if(i==0){
	      if(j==0){
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	      }
	      else {
		if(a[i][j-1]==1) a[i][j-1]=2;
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	      }
	    }
	    else if(j==0){
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	    }
	    else if(i==h-1){
	      if(j==w-1){
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
	      }
	      else {
		if(a[i][j+1]==1) a[i][j+1]=2;
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
	      }
	    }
	    else if(j==w-1){
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
	    }
	    else {
		if(a[i+1][j]==1) a[i+1][j]=2;
		if(a[i-1][j]==1) a[i-1][j]=2;
		if(a[i][j-1]==1) a[i][j-1]=2;
		if(a[i][j+1]==1) a[i][j+1]=2;
	    }
	  }
	}
      }
      if(m==0) break;
      
    }
    cout<<count<<endl;
  }
    
    
  return 0;
}

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

using namespace std;
typedef pair<int,int> pos;

void sub(int x,int y,vector<string> &field,
		vector<pos> &next,int &count){

	if(x>=0 && x<field[0].size() &&
			y>=0 && y<field.size()
			&& field[y][x]=='.'){
				field[y][x]='#';
				count++;
				next.push_back(make_pair(x,y));
			}
}

int solve(int px,int py,vector<string> field){
	int count=1;
	vector<pos> p;
	p.push_back(make_pair(px,py));
	
	field[py][px]='#';
	while(!p.empty()){
		vector<pos> next;
		for(int i=0;i<p.size();i++){
			int x = p[i].first;
			int y = p[i].second;
			sub(x-1,y,field,next,count);
			sub(x,y-1,field,next,count);
			sub(x+1,y,field,next,count);
			sub(x,y+1,field,next,count);
		}
		p=next;
	}
	return count;
}

int main(){
	while(true){
		int sx,sy;
		cin >> sx >> sy;
		if(sx==0 && sy==0)return 0;
		vector<string> field;
		int x,y;
		for(int i=0;i<sy;i++){
			string str;
			for(int k=0;k<sx;k++){
				char c;
				cin>>c;
				if(c!='.' && c!='#' && c!='@'){cout <<"error"<<endl;}
				str.push_back(c);
				if(c=='@'){
					x=k;
					y=i;
				}
			}
			field.push_back(str);
		}
		cout << solve(x,y,field) << endl;
	}
}

C++: #include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <complex>
using namespace std;

typedef long long ll;
typedef pair<int,int> P;

#define all(c) (c).begin(),(c).end()
#define pb push_back
#define mp make_pair
#define ERASE(v,i) (v).erase(remove(all(v),i),(v).end())
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define each(it,c) for(typeof((c).begin()) it=(c).begin();it!=(c).end();++it)
#define debug(x) cerr<<#x<<" = "<<(x)<<endl;
#define LINE cerr<<"LINE: "<<__LINE__<<endl;

inline int toInt(string s){int v;istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x){ostringstream sout;sout<<x;return sout.str();}
template<class T> void preview(T a,T b){for(T it=a;it!=b;++it)cerr<<*it<<" ";cerr<<endl;}

const int INF = 100000000;
const double PI = acos(-1.0), EPS = 1e-10;

int main() {
	for(int W, H; cin >> W >> H, W || H;) {
		vector<string> field(H);
		rep(i,H) cin >> field[i];
		// First = y, Second = x;
		P s;
		rep(y,H) rep(x,W) {
			if(field[y][x] == '@') {
				s.first = y;
				s.second = x;
			}
		}

		bool reach[20][20];
		memset(reach, 0, sizeof reach);

		int ans = 1;
		reach[s.first][s.second] = true;

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

		queue<P> que;
		que.push(s);
		while(!que.empty()) {
			P here = que.front();
			que.pop();

			rep(i,4) {
				int ny = here.first + dy[i];
				int nx = here.second + dx[i];
				if(0 <= ny && ny < H && 0 <= nx && nx < W && field[ny][nx] == '.' && reach[ny][nx] == false) {
					reach[ny][nx] = true;
					ans++;
					que.push(mp(ny, nx));
				}
			}
		}

		cout << ans << endl;
	}
	return 0;
}