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 <algorithm>
#include <vector>

using namespace std;

void saiki(int x,int y,vector<vector<char> > &field,vector<vector<char> > &k){
    if(field[y][x] == '#' || k[y][x] == 'x'){
        return;
    }

    field[y][x] = '#';
    k[y][x] = 'x';
    saiki(x-1,y,field,k);
    saiki(x+1,y,field,k);
    saiki(x,y-1,field,k);
    saiki(x,y+1,field,k);
    field[y][x] = '.';
    return;
}

int solve(int w,int h){
    //char field[w+2][h+2];
    vector<vector<char> > field(h+2,vector<char>(w+2));
    vector<vector<char> > k(h+2,vector<char>(w+2));

    int x,y;
    for(int i=0;i<h+2;i++){
        for(int j=0;j<w+2;j++){
            field[i][j] = '#';
        }
    }

    for(int i=1;i<=h;i++){
        for(int j=1;j<=w;j++){
            cin >> field[i][j];
            if(field[i][j] == '@'){
                y = i;
                x = j;
            }
            k[i][j] = field[i][j];
        }
    }

#ifdef DEB
    cout << "X:" << x << " Y:" << y << endl;
    for(int i=0;i<h+2;i++){
        for(int j=0;j<w+2;j++){
            cout << field[i][j];
        }
        cout << endl;
    }
#endif
    saiki(x,y,field,k);

    int m = 0;
    for(int i=0;i<h+2;i++){
        for(int j=0;j<w+2;j++){
            if(k[i][j] == 'x'){
                m++;
            }
        }
    }
    cout << m << endl;
}

int main(){
    int x,y;
    while(cin >> x >> y){
        if(x==0 && y == 0){
            break;
        }
        solve(x,y);
    }
}

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

#define ll long long
#define pq priority_queue
#define debug(x) cerr << #x << ": " << x << endl;
#define line() cerr << "---------------" << endl;

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

void dfs(int y, int x) {
   ans++;
   b[y][x] = '*';   
   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 && b[ny][nx] == '.') {
         dfs(ny, nx);
      }
   }
}

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

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

   return 0;
}

C++: #include <iostream>
#include <string>
#include <cstdlib>
#include <cmath>
#include <vector>
#include <map>
#include <set>
#include <algorithm>
#include <queue>
#include <stack>
#include <functional>
#include <bitset>
#include <assert.h>
using namespace std;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<char> vc;
typedef vector<string> vs;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef pair<ll,ll> P;
typedef vector<P> vpl;
typedef tuple<ll,ll,ll> tapu;
#define rep(i,n) for(ll i=0; i<(n); i++)
#define REP(i,a,b) for(int i=(a); i<(b); i++)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
const int inf = 1<<30;
const ll linf = 1LL<<62;
const int MAX = 510000;
ll dy[8] = {0,1,0,-1,1,-1,1,-1};
ll dx[8] = {1,0,-1,0,1,-1,-1,1};
const double pi = acos(-1);
const double eps = 1e-7;
template<typename T1,typename T2> inline bool chmin(T1 &a,T2 b){
	if(a>b){
		a = b; return true;
	}
	else return false;
}
template<typename T1,typename T2> inline bool chmax(T1 &a,T2 b){
	if(a<b){
		a = b; return true;
	}
	else return false;
}
template<typename T1,typename T2> inline void print2(T1 a, T2 b){cout << a << " " << b << "\n";}
template<typename T1,typename T2,typename T3> inline void print3(T1 a, T2 b, T3 c){
	cout << a << " " << b << " " << c << "\n";
}
const int mod = 1e9 + 7;
//const int mod = 998244353;

bool in(ll y, ll x, ll h, ll w){
	return 0 <= y && y < h && 0 <= x && x < w;
}

int main(){
	while(1){
		ll w,h; cin >> w >> h;
		if(!w && !h) break;
		vector<vc> a(h,vc(w));
		ll sy,sx;
		rep(i,h) rep(j,w) cin >> a[i][j];
		rep(i,h) rep(j,w) if(a[i][j]=='@') sy = i, sx = j;
		vvl d(h,vl(w,-1));
		d[sy][sx] = 1;
		queue<P> q;
		q.emplace(sy,sx);
		while(!q.empty()){
			P p = q.front(); q.pop();
			rep(i,4){
				ll ny = p.first + dy[i];
				ll nx = p.second + dx[i];
				if(!in(ny,nx,h,w) || d[ny][nx] > -1) continue;
				d[ny][nx] = a[ny][nx]=='.' ? 1 : 0;
				if(d[ny][nx]) q.emplace(ny,nx);
			}
		}
		ll ans = 0;
		rep(i,h) rep(j,w) if(d[i][j]!=-1) ans += d[i][j];
		cout << ans << endl;
	}
}

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

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




int main() {
	int W,H;
	int q[20][20];
	char p[20][20];
	queue<pair<int,int> > M;
	pair<int,int> X;
	while(cin >> W >> H){
		
		if (W == 0) return 0;
		int count = 0;
		for (int i = 1; i <= H; i++){
			for (int j = 1; j <= W; j++){
				cin >> p[i][j];
				if (p[i][j] == '@'){
					M.push(make_pair(i,j));
					count = 1;
					q[i][j] = 1;
				}
			}
		}
		while(!M.empty()){
			W = M.front().first;
			H = M.front().second;
			M.pop();
			for (int i = 0; i <= 3; i++){		
				if (p[W+dx[i]][H+dy[i]] == '.' && q[W+dx[i]][H+dy[i]] == 0){
					M.push(make_pair(W + dx[i], H + dy[i]));
					q[W + dx[i]][H + dy[i]] = 1;
					count += 1;
				}
			}
		}
		cout << count << endl;
	}
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <queue>
#include <utility>
using namespace std;
int xd[]={1,0,0,-1};
int yd[]={0,1,-1,0};
int main()
{
	int W,H;
	while(cin>>W>>H,W!=0){
		vector<string> board(H);
		queue<pair<int,int>> q;
		for(int i=0;i<H;i++)
			cin>>board[i];
		for(int i=0;i<H;i++){
			string::iterator
			p=find(board[i].begin(),board[i].end(),'@');
			if(p!=board[i].end()){
				q.push(pair<int,int>(i,p-board[i].begin()));
				break;
			}
		}
		int count=0;
		while(!q.empty()){
			pair<int,int> v=q.front();
			q.pop();
			if(board[v.first][v.second]=='#')continue;
			board[v.first][v.second]='#';
			count++;
			for(int i=0;i<4;i++){
				int newx=xd[i]+v.first;
				int newy=yd[i]+v.second;
				if(0<=newx&&newx<H&&0<=newy&&newy<W
						&&board[newx][newy]=='.')
					q.push(pair<int,int>(newx,newy));

			}
		}
		cout<<count<<endl;
	}
	return 0;
}