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>
#include<string>
#include<vector>
#include<algorithm> 
#include<functional> 
#include<cassert>
#include<numeric> // std::accumulate(vec.begin(),vec.end(),0) sum of element
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<queue>
#include<map>
#include<set>


using namespace std;


typedef long long ll;
typedef std::pair<int, int> P;
typedef vector<int> vi;


#define rep(i,n) for(int i=0;i<n;i++)
#define rep1(i,n) for(int i=1;i<=(int)(n);i++)
#define rrep(i,n) for(int i=n-1;i>=0;i--)
#define all(c) c.begin(),c.end()
#define show(x) cout << #x << " = " << x << endl

#define fi first
#define se second
#define pb push_back



#define DEBUG


// void print_vector(const std::vector<int> &t);
// void unique_vector(std:: vector<int> &t);

template <class X>
void print_vector(const std::vector<X> &t);
template <class X>
void unique_vector(std:: vector<X> &t);


int w, h;

vector<int> xl={-1, 0, 1, 0};
vector<int> yl={0, 1, 0, -1};

int main(){

  vi ans;
  while(true){
    cin >> w >> h;
    if(w == 0 && h == 0) break;
    vector<vector<char>> f(h, vector<char>(w));
    queue<P> ser;
    for(int i = 0; i < h; i++){
      for(int j = 0; j < w; j++) {
	cin >> f[i][j];
	if(f[i][j] == '@')
	  ser.push(make_pair(i, j));
      }
    }
    int c = 0;
    while(!ser.empty()){
      P t = ser.front();
      ser.pop();
      int y = t.fi, x = t.se;
      // show(x); show(y);
      if(f[y][x] == '@') f[y][x] = '#';
      for(int i = 0; i < 4; i++) {
	int dx = x+xl[i], dy=y+yl[i];
	// cout << dx << ' ' << dy << endl;
	// show(dx); show(dy);
	if(dy < 0 || dy >= h || dx < 0 || dx >= w || f[dy][dx] == '#')
	  continue;
	
	if(f[dy][dx] == '.'){
	  c++;
	  ser.push(make_pair(dy, dx));
	  f[dy][dx] = '#';
	}
      }	
      // for(int i = 0; i < h; i++) {
      // 	for(int j = 0; j < w; j++) {
      // 	  cout << f[i][j] << ' ';
      // 	}
      // 	cout << endl;
      // }
      // cout << endl;
    }
    ans.pb(c+1);
  }
  for(auto &i : ans) cout << i << endl;
}

template <class X>
void unique_vector(std::vector<X> &t)
{
  std::sort(t.begin(), t.end());
  t.erase( std::unique(t.begin(), t.end()), t.end() );
}

template <class X>
void print_vector(const std::vector<X> &t){
  for(auto itr=t.begin(); itr != t.end(); itr++){
    std::cout << *itr;
    if(itr != t.end()-1) std::cout << ' ';
  }
  std::cout << std::endl;
}

C++: #if 0
08-162015?????????????????°???????????????????????????
pair???
#endif

#include <iostream>
#include <queue>
#include <string.h>
#include <utility>
using namespace std;

int w,h;
int tot=1; //?????£????????????(?????????????????????)
const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1}; //??????????§??????¨
char P[30][30]; //??§?¨?????????????????????¨???

bool valid(int x,int y){ //??????????????????
	return (x>=0)&&(x<w)&&(y>=0)&&(y<h)&&(P[x][y]!='#');
}

void bfs(int x,int y){ //???????????¢?´¢
	queue<pair<int,int> > q;
	pair<int,int> p;
	p = make_pair(x,y);
	while(!q.empty()){
		p=q.front();
		q.pop();
		for(int i=0;i<4;++i){
			if(valid(p.first+dx[i],p.second+dy[i]))
				tot+=1;
				P[p.first+dx[i]][p.second+dy[i]]='#'; //???????????£?????´???????°???????
				p.first+=dx[i]; p.second+=dy[i];
				q.push(p);
		}
	}
}

int main(){
	int x1,y1;
	while((cin>>w>>h)&&(w!=0&&h!=0)){
		for(int i=0;i<h;++i){
			for(int j=0;j<w;++j){
				cin>>P[j][i];
				if(P[j][i]=='@')
					x1=j; y1=i; 
			}
		}
		bfs(x1,y1); //??????????????°????????£??\????????¢?´¢????§?
		cout << tot << endl;
	}
}

C++: #include <iostream>

using namespace std;

int W, H;
int dx[4] = {0, 1, 0, -1},
    dy[4] = {-1, 0, 1, 0};
char field[22][22];     // 1-origin

int dfs(int y, int x)
{
  if (field[y][x] == '#') {
    return 0;
  }
  
  field[y][x] = '#';
  int res = 1;
  for (int k = 0; k < 4; k++) {
    res += dfs(y + dy[k], x + dx[k]);
  }

  return res;
}

int main()
{
  // pre exec
  for (int i = 0; i < 22; i++)
    for (int j = 0; j < 22; j++) 
      field[i][j] = '#';

  while (cin >> W >> H && (W && H)) {

    // read
    int sx = 1, sy = 1;
    for (int i = 1; i <= H; i++) {
      for (int j = 1; j <= W; j++) {
        cin >> field[i][j];
        if (field[i][j] == '@') {
          sx = j; sy = i;
          field[i][j] = '*';
        }
      }
    }

    // depth first search
    cout << dfs(sy, sx) << endl;
  }
}

C++: #include <iostream>
#include <cstring>

using namespace std;

int used[20][20];
char M[20][20];
int W, H;

bool in(int sx, int sy) {
  return 0<=sx && sx<W && 0<=sy && sy<H;
}

int dfs(int x, int y) {
  int res = 1;
  used[y][x] = 1;
  
  const int dx[]={-1,1,0,0};
  const int dy[]={0,0,-1,1};
  for(int i=0; i<4; i++) {
    int nx = x+dx[i], ny = y+dy[i];
    if(in(nx, ny) && M[ny][nx] == '.' && !used[ny][nx])
      res += dfs(nx, ny);
  }
  return res;
}

int main() {
  
  int stx, sty;
  while(cin >> W >> H, (W|H)) {
    memset(used, 0, sizeof(used));
    for(int i=0; i<H; i++)
      for(int j=0; j<W; j++) {
	cin >> M[i][j];
	if(M[i][j] == '@') {
	  stx = j, sty = i;
	  M[i][j] = '.';
	}
      }
    cout << dfs(stx, sty) << endl;
  }
  
  return 0;
}

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

using namespace std;

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

string tile[20];

int w, h;

int dfs(int x, int y){
	int ret = 1;
	if (tile[x][y] == '#') return 0;
	tile[x][y] = '#';

	for (int i = 0; i < 4; i++){
		if (x + dx[i] < 0 || x + dx[i] >= h) continue;
		if (y + dy[i] < 0 || y + dy[i] >= w) continue;
		ret += dfs(x + dx[i], y + dy[i]);
	}
	return ret;
}

int main(){
	while (cin >> w >> h,w){
		int x, y;
		for (int i = 0; i < h; i++){
			cin >> tile[i];
			for (int j = 0; j < w; j++){
				if (tile[i][j] == '@'){
					x = i;
					y = j;
				}
			}
		}
		cout << dfs(x, y) << endl;
	}
}