Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

type Node struct {
	ID    int
	X, Y  int
	Color Color
	Nexts []*Node
}

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])

		// Initialize map
		m := map[int]map[int]*Node{}
		for x := 1; x <= w; x++ {
			m[x] = map[int]*Node{}
			for y := 1; y <= h; y++ {
				m[x][y] = &Node{ID: x + y*w, X: x, Y: y}
			}
		}
		for x := 1; x <= w; x++ {
			for y := 1; y <= h; y++ {
				nexts := []*Node{}
				if n, ok := m[x-1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y-1]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x+1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y+1]; ok {
					nexts = append(nexts, n)
				}
				m[x][y].Nexts = nexts
			}
		}

		// Set colors
		queue := []*Node{}
		touchedIDs := map[int]struct{}{}
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					n := m[i+1][y]
					n.Color = ColorBlack
					if c == '@' {
						queue = append(queue, n)
						touchedIDs[n.ID] = struct{}{}
					}
				case '#':
					m[i+1][y].Color = ColorRed
				}
			}
		}

		// Search
		visitedIDs := map[int]struct{}{}
		for {
			if len(queue) == 0 {
				break
			}
			n := queue[0]
			queue = queue[1:]
			visitedIDs[n.ID] = struct{}{}
			for _, next := range n.Nexts {
				if next.Color == ColorRed {
					continue
				}
				if _, ok := touchedIDs[next.ID]; !ok {
					queue = append(queue, next)
					touchedIDs[n.ID] = struct{}{}
				}
			}
		}
		fmt.Println(len(visitedIDs))
	}
}

C++: #include <iostream>
using namespace std;
char S[21][21];
int count = 0;

void a(int i, int j){
	if(S[i][j] == '.'){
		count++;
		S[i][j] = '#';
		a(i - 1, j);
		a(i + 1, j);
		a(i, j + 1);
		a(i, j - 1);
	}
}


int main(){
	while(true){
		count = 0;
		int x, y;
		cin >> x >> y;
		if(x == 0 && y == 0){
			break;
		}
		for(int i = 0; i < y; i++){
			for(int j = 0; j < x; j++){
				cin >> S[i][j];
			}
		}

		int startx, starty;
		for(int i = 0; i < y; i++){
			for(int j = 0; j < x; j++){
				if(S[i][j] == '@'){
					startx = j;
					starty = i;
					S[i][j] = '.';
					break;
				}
			}
		}
		a(starty, startx);
		cout << count << endl;
	}
}

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

using namespace std;

vector<string> map;
int w,h,area;
int dx[]={0,0,-1,1};
int dy[]={-1,1,0,0};

void f(int x,int y){
	map[y][x]='#';
	for(int i=0;i<4;i++)
		if(0<=x+dx[i]&&x+dx[i]<w&&0<=y+dy[i]&&y+dy[i]<h)
			if(map[y+dy[i]][x+dx[i]]=='.'){
				area++;
				f(x+dx[i],y+dy[i]);
			}
}

int main(){
	ostringstream out;

	string line;

	while(cin >> w >> h, w|h){
		map.clear();
		int x=-1,y;
		area=1;
		for(int iy=0;iy<h;iy++){
			cin >> line;
			map.push_back(line);
			if(x==-1)
				for(int ix=0;ix<line.length();ix++)
					if(line[ix]=='@'){
						x=ix;
						y=iy;
					}
		}

		f(x,y);

		out << area << endl;
	}


	cout << out.str();
	return 0;
}

C++: //include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <queue>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

//conversion
//------------------------------------------
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();}

//math
//-------------------------------------------
template<class T> inline T sqr(T x) {return x*x;}

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1000000007;

//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

int ans;
char c[40][40];
int a,b;


void bfs(int sx, int sy) {
	queue< pair<int,int> > q;
	queue< PII > qp;
	q.push(MP(sx,sy));
	c[sx][sy] = 'x';
	for (int i = 1; i < 1000; i++) {
		while (!q.empty()) {
			PII v = q.front(); //get value
			q.pop();
			int x = v.first, y = v.second;
			if (c[x+1][y] == '.'){ qp.push(MP(x+1,y)); c[x+1][y] = 'x'; ans++; }
			if (c[x-1][y] == '.'){ qp.push(MP(x-1,y)); c[x-1][y] = 'x'; ans++; }
			if (c[x][y+1] == '.'){ qp.push(MP(x,y+1)); c[x][y+1] = 'x'; ans++; }
			if (c[x][y-1] == '.'){ qp.push(MP(x,y-1)); c[x][y-1] = 'x'; ans++; }

		}
		while (!qp.empty()) {
			q.push(qp.front());
			qp.pop();
		}
		//		for (int h = 0; h < b+2; h++) {
		//			for (int w = 0; w < a+2; w++) {
		//				cout << c[w][h];
		//			}
		//			cout << endl;
		//		}
		//		cout << endl;

	}
}

int main(int argc, char const* argv[])
{
	while (true) {
		int sx,sy;
		cin >> a >> b;
		if (a == 0 && b == 0) {
			break;
		}
		for (int h = 0; h < b+2; h++) {
			for (int w = 0; w < a+2; w++) {
				if (h == 0 || w == 0 || h == b+1 || w == a+1) {
					c[w][h] = '#';
				}
				else {
					cin >> c[w][h];
					if (c[w][h] == '@') {
						sx = w;
						sy = h;
					}
				}
			}
		}

		ans = 1;
		bfs(sx,sy);
		cout << ans << endl;
	}
	return 0;

}

C++: #include<bits/stdc++.h>
using namespace std;
int main(){
  int w,h,a[20][20],str1,str2,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;
}