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>
using namespace std;

int W, H;
char board[30][30];
bool visited[30][30];

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

void dfs(int x, int y) {
   for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= H || ny < 0 || ny >= W) continue;
      if (board[nx][ny] == '#') continue;
      if (visited[nx][ny]) continue;
      visited[nx][ny] = true;
      dfs(nx, ny);
   }
}

int main() {
   while (cin >> W >> H, W || H) {
      memset(visited, false, sizeof(visited));
      int sx = 0, sy = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            cin >> board[i][j];
            if (board[i][j] == '@') {
               sx = i, sy = j;
            }
         }
      }

      visited[sx][sy] = true;
      dfs(sx, sy);

      int cnt = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (visited[i][j]) cnt++;
         }
      }

      cout << cnt << endl;
   }

   return 0;
}

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

const int dx[] = {1, -1, 0, 0};
const int dy[] = {0, 0, 1, -1};
int W, H;
char board[30][30];

int dfs(int y, int x) {
   board[y][x] = '#';
   int ret = 1;
   for (int i = 0; i < 4; i++) {
      int ny = y + dy[i], nx = x + dx[i];
      if (ny >= 0 && ny < H && nx >= 0 && nx < W && board[ny][nx] == '.') {
         ret += dfs(y, x);
      }
   }
   return ret;
}

int main() {
   while (cin >> W >> H, W) {
      for (int i = 0; i < H; i++) {
         cin >> board[i];
      }

      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (board[i][j] == '@') {
               cout << dfs(i, j) << endl;
            }
         }
      }
   }

   return 0;
}

C++: #include <iostream>
#include <climits>
#include <set>
#include <string>
#include <algorithm>
#include <vector>
#include <queue>
#define MAX 1000000007


using namespace std;
#define ll long long
#define dbg if(0)
#define ISRANGE(val,lo,hi) ((lo<=val)&&(val<hi))



int main(){
    ll ans;
    char t[21][21];//w,h
    ll dx[4]={-1,1,0,0};
    ll dy[4]={0,0,-1,1};
    ll dbg_cnt=0;
    while(true){
        ans=0;
        ll w,h,px,py;
        cin >>w>>h;
        if(w==0&&h==0)break;
        for(int y=0;y<h;y++){
            for(int x=0;x<w;x++){
                cin >>t[x][y];
                if(t[x][y]=='@'){
                    px=x,py=y;
                    t[x][y]='.';
                }
            }
        }

        queue< pair<ll,ll> > kouho;//x,y
        kouho.push(make_pair(px,py));
        t[px][py]='#';
        dbg{
            cout <<"kokomadeiketa "<<dbg_cnt<<endl;
            dbg_cnt++;
            if(dbg_cnt>5)break;
        }
        ans++;
        while(!kouho.empty()){
            pair<ll,ll> now=kouho.front();//x,y
            kouho.pop();
            if(t[now.first][now.second]=='.'){
                t[now.first][now.second]='#';
                ans++;
            }
            for(int i=0;i<4;i++){
                ll qx=now.first+dx[i],qy=now.second+dy[i];
                if(ISRANGE(qx,0,w) && ISRANGE(qy,0,h)){
                    if(t[qx][qy]=='.'){
                        kouho.push(make_pair(qx,qy));
                    }
                }
            }
            dbg{
                for(int y=0;y<h;y++){
                    for(int x=0;x<w;x++){
                        cout << t[x][y];
                    }
                    cout <<endl;
                }
            }
        }

        cout <<ans<<endl;
    }

    return 0;

}

C++: #include<iostream>
#include<string>
using namespace std;
int w,h;
string map[40];
int vec[4][2]={{0,1},{0,-1},{-1,0},{1,0}};
int move(int x,int y){
	int ans=1;
	map[x][y]='#';
	for(int i=0;i<4;i++){
		if((x+vec[i][0]>=0)&&(y+vec[i][1]>=0)&&(x+vec[i][0]<h)&&(y+vec[i][1]<w)){
			if(map[x+vec[i][0]][y+vec[i][1]]=='.'){
				ans+=move(x+vec[i][0],y+vec[i][1]);
			}
		}
	}
	return ans;
}

int main(){
	while(cin>>w>>h,w){
		int fx,fy;
		for(int i=0;i<h;i++){
			cin>>map[i];
		}
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(map[i][j]=='@'){
					fx=i;
					fy=j;
					break;
				}
			}
		}
		int ans=move(fx,fy);
		cout<<ans<<endl;
		
	}
}

C++: #define _overload3(_1,_2,_3,name,...) name
#define _rep(i,n) repi(i,0,n)
#define repi(i,a,b) for(int i=int(a);i<int(b);++i)
#define rep(...) _overload3(__VA_ARGS__,repi,_rep,)(__VA_ARGS__)
#include "bits/stdc++.h"
using namespace std;
typedef long long ll;

int main(int argc, char* argv[]){
    int dx[] = {0, 1, 0, -1};
    int dy[] = {1, 0, -1, 0};
    int w, h;
    int cnt = 0;
    int sx, sy;
    vector<string> v;
    while(true){
        cnt = 0;
        string tmp;
        v.clear();
        cin >> w >> h;
        if(w == 0 && h == 0) break;
        rep(i, h){
            cin >> tmp;
            v.push_back(tmp);
            if(tmp.find('@') != string::npos){
                sx = i;
                sy = tmp.find('@');
            }
        }
        
        queue<pair<int, int> > q;
        q.push({sx, sy});
        while(!q.empty()){
            pair<int, int> p = q.front();
            q.pop();
            // cout << "Now : " << p.first << " " << p.second << endl;
            if(p.first >= h
                || p.second >= w
                || p.first  < 0
                || p.second < 0
            )continue;
            if((v[p.first][p.second] == '.' || v[p.first][p.second] == '@')) {
                v[p.first][p.second] = '#';
                cnt++;
                rep(i, 4){
                    q.push({p.first + dx[i], p.second + dy[i]});
                }
            }
        }
        cout << cnt << endl;
    }
}