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 <queue>
#include <utility>
using namespace std;

typedef pair<int, int> P;
const int W = 20, H = 20;
char board[H][W];

int bfs(int x, int y, int w, int h){
	int dx[] = {0, -1, 1, 0}, dy[] = {-1, 0, 0, 1}, c = 0;
	queue<P> q;
	q.push(make_pair(x, y));
	board[y][x] = '#';
	while(!q.empty()){
		P p = q.front();
		q.pop();
		++c;
		for(int i = 0;i < 4;++i){
			int nx = p.first + dx[i], ny = p.second + dy[i];
			if(0 <= nx && nx < w && 0 <= ny && ny < h && board[ny][nx] == '.'){
				q.push(make_pair(nx, ny));
				board[ny][nx] = '#';
			}
		}
	}
	return c;
}

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

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

const int MAX_SIZE = 20;
typedef pair<int, int> P;

int calc(char f[][MAX_SIZE], queue<P> q, int W, int H) {
  set<P> s;
  while (q.size()) {
    P p = q.front();
    q.pop();
    s.insert(p);
    int di[] = {-1, 1, 0, 0};
    int dj[] = {0, 0, -1, 1};
    for (int k = 0; k < 4; ++k) {
      int i = p.first  + di[k];
      int j = p.second + dj[k];
      if (i >= 0 && j >= 0 && i < H && j < W && f[i][j] == '.') {
        pair<set<P>::iterator, bool> r = s.insert(P(i, j));
        if (r.second) q.push(*r.first);
      }
    }
  }
  return s.size();
}
int main() {
  int W, H;
  char f[MAX_SIZE][MAX_SIZE];
  char c[MAX_SIZE + 1];
  while (true) {
    queue<P> q;
    cin >> W >> H;
    if (W == 0 && H == 0) break;
    for (int i = 0; i < H; ++i) {
      cin >> c;
      for (int j = 0; j < W; ++j) {
        f[i][j] = c[j];
        if (f[i][j] == '@') {
          q.push(P(i, j));
        }
      }
    }
    cout << calc(f, q, W, H) << endl;
  }
}

C++: //C++14 (Clang 3.8.0)

#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <string>
#define rp(i,a,b) for(int (i)=(int)(a);i<(int)(b);++i)

typedef long long ll;

int mx[]={-1,1,0,0};
int my[]={0,0,-1,1};
using namespace std;
int n=0;
void dfs(int startx,int starty,vector<vector<bool> > &map){
    for(int i=0;i<4;i++){
        int x=startx+mx[i],y=starty+my[i];
        if(x>=0 and y>=0 and y<map.size() and x<map[0].size() and map[y][x]==true){
        map[y][x]=false;
        n++;
        dfs(x,y,map);}
    }
    
}

int main(){
    int x,y;
    while(cin>>x>>y,x|y){
        int startx,starty;
        vector<vector<bool> > map;

        if(x==0 and y==0) return 0;
        for(int i=0;i<y;i++){
            vector<bool> tmp;
            string s;
            cin>>s;
            for(int j=0;j<x;j++){

                if(s[j]=='@') {startx=j;starty=i;tmp.push_back(true);}
                else if(s[j]=='.') tmp.push_back(true);
                else tmp.push_back(false);    
            }
            map.push_back((tmp));
        }
        n=0;
        dfs(startx,starty,map);
        cout<<n<<endl;
        
    }

    return 0;
};

C++: #include<iostream>
#include <functional>
#include<algorithm>
#include <cmath>
#include <string>
#include <map>
using namespace std;

int W,H;
char a[30][30];
void se(int x,int y,int (*re)[30],int *u){
	
	if(x<0||W<=x||y<0||H<=y||a[y][x]=='#')
		return;
	if(re[y][x]==1)
		return;
	
	re[y][x]=1;
		*u+=1;
	
	se(x+1,y,re,u);
	se(x-1,y,re,u);
	se(x,y+1,re,u);
	se(x,y-1,re,u);
}

int main(){
	int x,y;
	int x1,y1;
	while(cin>>W>>H){
		if(W!=0&&H!=0){
			int re[30][30]={0};
			int u=0;
		for(int i=0;i<H;i++){
			for(int j=0;j<W;j++){
			cin>>a[i][j];
				if(a[i][j]=='@'){
				x1=j;
				y1=i;}
			}
		}
			se(x1,y1,re,&u);
				cout<<u<<endl;
		}
		else
			break;
	}

	
}

C++: #include<iostream>
using namespace std;
int w,h,x,y,i,j,k,masu[22][22],checked[22][22],*g,cont;
void check(int xx,int yy){
cont++;
checked[xx][yy]=1;
int m,n;m=1;n=0;
if(checked[xx+m][yy+n]==0 && 
masu[xx+m][yy+n]==0)
check(xx+m,yy+n);
m=0;n=1;
if(checked[xx+m][yy+n]==0 && 
masu[xx+m][yy+n]==0)
check(xx+m,yy+n);
m=-1;n=0;
if(checked[xx+m][yy+n]==0 && 
masu[xx+m][yy+n]==0)
check(xx+m,yy+n);
m=0;n=-1;
if(checked[xx+m][yy+n]==0 && 
masu[xx+m][yy+n]==0)
check(xx+m,yy+n);
}
int main(void){
char a[22];
while(1){
scanf("%d%d",&w,&h);
if(w==0 && h==0)
break;
cont=0;
for(j=0;j<22;j++)
for(i=0;i<22;i++){
masu[i][j]=1;
checked[i][j]=0;
}
for(j=1;j<=h;j++){
scanf("%s",a);
for(i=1;i<=w;i++)
switch(a[i-1]){
case '@':x=i;y=j;
case '.':masu[i][j]=0;break;
}
}
check(x,y);
printf("%d\n",cont);
}
}

C++: #include <bits/stdc++.h>

#define ll long long

using namespace std;


int W,H;

char r[1000][1000];

int sh,sw;
int bnum=0;

int solve(int i,int j){
    int ans=0;

    if(i <0 || H <= i || j < 0 || W <= j)
        return 0;
    if(r[i][j] == '#'){
        return 0;
    }
    ans = 1;
    r[i][j] = '#';
    ans += solve(i-1,j);
    ans += solve(i+1,j);
    ans += solve(i,j-1);
    ans += solve(i,j+1);

    return ans;
}


int main(){

    while(1){
        scanf("%d %d",&W,&H);
        if(W == 0 && H == 0) break;

        for(int i=0;i<H;i++){
            for(int j=0;j<W;j++){
                cin >> r[i][j];
                if(r[i][j] == '@'){
                    sh = i;
                    sw = j;
                    r[i][j] = '.';
                }
            }
        }
        cout << solve(sh,sw) << endl;
    }

}