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 <queue>
using namespace std;

class DoubleArray {
    int X,Y;
    int *top;
    public:
    DoubleArray(int *p,int x,int y);
    int *operator[](int i);
    int getx();
    int gety();
};

DoubleArray::DoubleArray(int *p,int x,int y){
    top = p;
    X = x;
    Y = y;
}

int *DoubleArray::operator[](int i){
    return top + Y*i;
}
int DoubleArray::getx(){
    return X;
}
int DoubleArray::gety(){
    return Y;
}

int calc(DoubleArray &map){
    int i,j,a=0;
    int x = map.getx();
    int y = map.gety();
    queue<int> q;
    for(i=0;i<x;i++){
        for(j=0;j<y;j++){
            if(map[i][j]==2){
                q.push(i);
                q.push(j);
                a=1;
                map[i][j]=0;
            }
        }
    }
    while(!q.empty()){
        i=q.front();
        q.pop();
        j=q.front();
        q.pop();
        if(i>0) if(map[i-1][j]==1){
            q.push(i-1);
            q.push(j);
            a++;
            map[i-1][j]=0;
        }
        if(j>0) if(map[i][j-1]==1){
            q.push(i);
            q.push(j-1);
            a++;
            map[i][j-1]=0;
        }
        if(i<x-1) if(map[i+1][j]==1){
            q.push(i+1);
            q.push(j);
            a++;
            map[i+1][j]=0;
        }
        if(j<y-1) if(map[i][j+1]==1){
            q.push(i);
            q.push(j+1);
            a++;
            map[i][j+1]=0;
        }
    }
    return a;
}

int main(){
    int i,j,x,y,a[10000],b = 0;
    string c;
    for(b=0; b<10000; b++){
        cin >> x;
        cin >> y;
        if(x==0&&y==0) break;
        if(x>20) x=20;
        if(y>20) y=20;
        if(x<1||y<1) continue;
        int m[x][y];
        DoubleArray map(&m[0][0],x,y);
        for(j=0;j<y;j++){
            for(i=0;i<x;i++){
                cin >> c;
                if(c=="#") map[i][j]=0;
                if(c==".") map[i][j]=1;
                if(c=="@") map[i][j]=2;
            }
        }
        a[b] = calc(map);
    }
    for(i=0;i<b;i++){
        cout << a[i] << endl;
    }
    return 0;
}

C++: #include<iostream>
using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
char fld[20][20];
bool used[20][20];
int W, H;
 
void rec(int i, int j){
   used[i][j]=1;
   for(pii dd: dxdy){
      int ni=i+dd.first;
      int nj=j+dd.second;
      if((0<=ni&&ni<H&&0<=nj&&nj<W)&&fld[ni][nj]!='#'&&used[ni][nj]==0){
         rec(ni, nj);
      }
   }
}
 
int main(){
 
   while(1){
      cin>> W>> H;
      if(!(H||W)) break;
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
            }
         }
      }
 
      for(int i=0; i<20; i++)for(int j=0; j<20; j++) used[i][j]=0;
      used[sy][sx]=1;
      rec(sy, sx);
      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=used[i][j];
      }
      cout<< ans<< endl;
 
   }
 
   return 0;
}

C++: #include <iostream>
#include <vector>
#include <stack>
#include <string>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
typedef pair<int,int> Pii;

int main(){
    int x[4]={0,1,0,-1};
    int y[4]={1,0,-1,0};
    while(1){
        int w,h;
        cin>>w>>h;
        if(h==0&&w==0) break;
        vector<string> tile(h);
        vector<vector<bool> > checked(h,vector<bool>(w,false));
        int cnt=0,sx,sy;
        rep(i,h) cin>>tile[i];
        rep(i,h) rep(j,w) if(tile[i][j]=='@'){sx=i;sy=j;}
        stack<Pii> stk;
        stk.push(Pii(sx,sy));
        while(!stk.empty()){
            int nx=stk.top().first,ny=stk.top().second;
            stk.pop();
            if(checked[nx][ny] || tile[nx][ny]=='#') continue; 
            checked[nx][ny]=true;
            cnt++;
            rep(k,4){
                int dx=nx+x[k],dy=ny+y[k];
                if(dx>=0 && dx<h && dy>=0 && dy<w) stk.push(Pii(dx,dy));
            }
        }
        cout<<cnt<<endl;
    }
    return 0;
}

C++: #include <iostream>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <functional>
#include <vector>
#include <list>
#include <queue>
#include <deque>
#include <stack>
#include <map>
#include <set>
#include <bitset>
#include <tuple>
#include <cassert>
#include <exception>
#include <iomanip>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll> P;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<string> vs;
typedef vector<P> vp;
#define rep(i,a,n) for(ll i = (a);i < (n);i++)
#define per(i,a,n) for(ll i = (a);i > (n);i--)
#define lep(i,a,n) for(ll i = (a);i <= (n);i++)
#define pel(i,a,n) for(ll i = (a);i >= (n);i--)
#define clr(a,b) memset((a),(b),sizeof(a))
#define pb push_back
#define mp make_pair
#define all(c) (c).begin(),(c).end()
#define sz size()
#define print(X) cout << (X) << endl
static const int INF = 1e+9+7;
ll n,m,l;
string s,t;
int d[200010],dp[1010][1010];
double w[1000],v[1000];
double box[200010];

int movex[4] = {-1,1,0,0};
int movey[4] = {0,0,-1,1};

int main(){
  while(1){
    cin >> m >> n;
    if(!n && !m)break;
    char field[102][102];
    char now;
    int x,y;
    queue<P> que;
    lep(i,0,n+1)lep(j,0,m+1)field[i][j] = '#';
    lep(i,1,n){
      lep(j,1,m){
        cin >> now;
        field[i][j] = now;
        if(now == '@'){
          que.push(P(i,j));
          // cout << i << " " << j << endl;
        }
      }
    }
    ll ans = 1;
    while(!que.empty()){
      P move = que.front(); que.pop();
      rep(i,0,4){
        if(field[move.first + movex[i]][move.second + movey[i]] == '.'){
          ans++;
          que.push(P(move.first + movex[i],move.second + movey[i]));
          field[move.first + movex[i]][move.second + movey[i]] = '#';
        }
      }
    }
    print(ans);
  }
  return 0;
}