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<bits/stdc++.h>
#define rep(i,n)for(int i=0;i<n;i++)
using namespace std;

int w, h;
char a[21][21];
int dx[]{ 1,-1,0,0 }, dy[]{ 0,0,1,-1 };
int cnt;

void dfs(int x, int y) {
	a[x][y] = 0;
	rep(i, 4) {
		int nx = x + dx[i], ny = y + dy[i];
		if (0 <= nx&&nx < h && 0 <= ny&&ny < w&&a[nx][ny] == '.') {
			cnt++; dfs(nx, ny);
		}
	}
}
int main() {
	while (scanf("%d%d", &w, &h), w) {
		cnt = 1;
		rep(i, h)scanf("%s", a[i]);
		rep(i, h)rep(j, w) {
			if (a[i][j] == '@') {
				dfs(i, j);
				break;
			}
		}
		printf("%d\n", cnt);
	}
}

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

string map[21];
int sx,sy,ans,x,y;

void search(int xx,int yy){
	ans++;
	map[yy][xx]='#';
	if(xx<x-1&&map[yy][xx+1]=='.')search(xx+1,yy);
	if(xx>0&&map[yy][xx-1]=='.')search(xx-1,yy);
	if(yy<y-1&&map[yy+1][xx]=='.')search(xx,yy+1);
	if(yy>0&&map[yy-1][xx]=='.')search(xx,yy-1);
}

int main(){
	while(cin>>x>>y,x||y){
		ans=0;
		for(int i=0;i<y;i++)cin>>map[i];
		for(int i=0;i<y;i++){
			for(int j=0;j<x;j++){
				if(map[i][j]=='@')sy=i,sx=j;
			}
		}
		search(sx,sy);
		cout<<ans<<endl;
	}
}

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

#define FOR( i, m, n )  for( int (i) = (m); (i) < (n); (i)++ )
#define REP( i, n )     FOR( i, 0, n )
#define ALL( a )        (a).begin(), (a).end()             

#define F				first
#define S				second

int w, h, cnt = 0;

void solve( vector<vector<char>>& f, pair<int, int>& mp ) {

	if( mp.F - 1 >= 0 && f[mp.S][mp.F - 1] == '.' ) {
		cnt++;
		f[mp.S][mp.F - 1] = '#';
		solve( f, make_pair( mp.F - 1, mp.S ) );
	}
	if( mp.F + 1 < w && f[mp.S][mp.F + 1] == '.' ) {
		f[mp.S][mp.F + 1] = '#';
		cnt++;
		solve( f, make_pair( mp.F + 1, mp.S ) );
	}
	if( mp.S - 1 >= 0 && f[mp.S - 1][mp.F] == '.' ) {
		f[mp.S - 1][mp.F] = '#';
		cnt++;
		solve( f, make_pair( mp.F, mp.S - 1 ) );
	}
	if( mp.S + 1 < h && f[mp.S + 1][mp.F] == '.' ) {
		f[mp.S + 1][mp.F] = '#';
		cnt++;
		solve( f, make_pair( mp.F, mp.S + 1 ) );
	}
}

int main() {

	while( 1 ) {
		cin >> w >> h; if( w == 0 && h == 0 ) break;
		
		pair<int, int> mp;
		vector<vector<char>> f;
		vector<char> tmp( w );
		REP( i, h ) {
			REP( j, w ) {
				cin >> tmp[j];
				if( tmp[j] == '@' ) { 
					mp.first = j;
					mp.second = i; 
					tmp[j] = '#';
					cnt++;
				}
			}
			f.push_back( tmp );
		}
		solve( f, mp );
		cout << cnt << endl;
		cnt = 0;
	}
}

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

#define ll long long
#define vvi vector<vector<int>>
#define vec vector
#define pq priority_queue
#define all(v) (v).begin(), (v).end()
#define rep(i, n) for (int (i) = (0); (i) < (n); ++(i))
#define repp(i, m, n) for (int (i) = (m); (i) < (n); ++(i))
#define debug(x) cerr << #x << ": " << x << endl;
#define debug2(x, y) cerr<<"("<<#x<<", "<<#y<<") = "<<"("<<x<<", "<<y<<")"<<endl;
#define debug3(x, y, z) cerr<<"("<<#x<<", "<<#y<<", "<<#z<<") = "<<"("<<x<<", "<<y<<", "<<z<<")"<<endl;
#define debugB(x, y) cerr<<#x<<": "<<bitset<y>(x) << endl;
#define line() cerr << "---------------" << endl;

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

template<typename T>
void printA(vector<T> &printArray, char between = ' ') {
   int paSize = printArray.size();
   for (int i = 0; i < paSize; i++) { cerr << printArray[i] << between;}
   if (between != '\n') {cerr << endl;}
}

// ------------------------------------------------------------------------------------------

int W, H;
char B[30][30];

int dfs(int y, int x) {
   int ret = 1;
   B[y][x] = '#';
   rep (i, 4) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= W || ny < 0 || ny >= H || B[ny][nx] == '#') continue;
      ret += dfs(ny, nx);
   }
   return ret;
}

void solve() {
   rep (i, H) {
      cin >> B[i];
   }
   rep (i, H) {
      rep (j, W) {
         if (B[i][j] == '@') {
            cout << dfs(i, j) << endl;
            return;
         }
      }
   }
}

int main() {
   while (1) {
      cin >> W >> H;
      if (H*W == 0) break;
      solve();
   }

   return 0;
}

C++: 
#include <cstdio>
#include <iostream>
#include <string>
#include <cstring>
#include <algorithm>
#include <functional>

using namespace std;

bool f[21][21];
char tile[21][21];
int w,h;

int dfs(int x,int y)
{
	if(x < 0 || y < 0 || x >= h || y >= w || f[x][y] || tile[x][y] == '#')return 0;
	
	f[x][y] = true;

	int sum = 0;

	sum += dfs(x-1,y);
	sum += dfs(x+1,y);
	sum += dfs(x,y-1);
	sum += dfs(x,y+1);

	return sum + 1;
}



int main()
{

	
	while(cin >> w >> h && w && h)
	{
		
		memset(tile,0,21*21*sizeof(char));
		memset(f,0,21*21*sizeof(bool));

		int sx,sy;

		for(int i = 0;i < h;i++)
		{
			for(int j = 0;j < w;j++)
			{
				cin >> tile[i][j];
				if(tile[i][j] == '@'){sx = i;sy = j;}
			}
		}

		cout << dfs(sx,sy) << endl;


	}


	return 0;
}