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 <vector>
#include <queue>
#include <string>
#include <algorithm>
#include <utility>
#include <cstring>

using namespace std;

#define REP(var, count) for(int var=0; var<count; var++)

int main(void) {
	while(true) {
		int W, H;
		scanf("%d %d", &W, &H);
		if ( W==0 ) { break; }
		
		char line[26][26];
		
		queue<pair<int, int> > que;
		
		strcpy(line[0], "#######################");
		strcpy(line[H+1], "#######################");
		
		for(int y=1; y<=H; y++) {
			line[y][0] = '#';
			scanf("%s", &line[y][1]);
			
			if ( char* p = strchr(line[y], '@') ) {
				que.push(make_pair(y, p - line[y]));
			}
		}
		
		int ret = 1;
		
		while(que.size()) {
			
			int y = que.front().first;
			int x = que.front().second;
			que.pop();
			
			int m[][2] = { {-1,0},{1,0},{0,1},{0,-1} };
			REP(d, 4) {
				if ( line[y+ m[d][0]] [x+ m[d][1]] == '.' ) {
					line[y+ m[d][0]] [x+ m[d][1]] = '+';
					ret++;
					que.push( make_pair(y+ m[d][0], x+ m[d][1]) );
				}
			}
		}
		
		cout << ret << endl;
	}
}

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

#define rep(i,n) for(int i = 0; i < n; i++)
#define rrep(i,n) for(int i = n - 1; i >= 0; i--)
#define REP(i,k,n) for(int i = k; i < n; i++)

#define vi vector<int>
#define pb push_back
using namespace std;

int dfs(int x,int y);
int dx[] = {0,-1,0,1,0};

int w,h;
string g[21];

int main(){
    while(cin >> w >> h,w) {
        rep(i,h){
            cin >> g[i];
        }
        rep(y,h) {
            rep(x,w){
                if(g[y][x] == '@') {
                    int cnt = dfs(x,y);
                    cout << cnt << endl;
                }
            }
        }
    }
    return 0;

}

int dfs(int x,int y){
    g[y][x] = '#';
    int cnt = 1;
    rep(i,4){
        int nx = x + dx[i], ny = y + dx[i + 1];
        if(nx >= 0 && ny >= 0 && nx < w && ny < h && g[ny][nx] == '.') {
            cnt += dfs(nx,ny);
        }
    }
    return cnt;
}

C++: #include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <algorithm>
#include <cmath>
#include <string>

using namespace std;

typedef vector <int> VI;
typedef vector <VI> VVI;
typedef vector <string> VS;
typedef pair<int,int> PII;
typedef long long LL;
typedef unsigned long long ULL;
typedef pair <int, LL> PIL;
typedef vector <PIL> VPIL;


#define each(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); i++)
#define sort(c) sort((c).begin(),(c).end())

#define range(i,a,b) for(int i=(a); i < (b); i++)
#define rep(i,n) range(i,0,n)

#define MAX_INT 2147483647

typedef struct Coor{
	int x;
	int y;
	Coor(int cx,int cy){
		x = cx;
		y = cy;
	}
} Coor;

Coor dxy[] = {Coor(1,0), Coor(0,1), Coor(-1,0), Coor(0,-1)};
string field[20];
int dfs(Coor here, Coor lb, Coor ub){
	int result = 0;
	if(field[here.y][here.x] != '#'){
		result++;
		field[here.y][here.x] = '#';
		rep(i,4){
			Coor nxt = Coor(here.x + dxy[i].x, here.y + dxy[i].y);
			if (lb.x <= nxt.x && nxt.x < ub.x && lb.y <= nxt.y && nxt.y < ub.y)
				result += dfs(nxt, lb, ub);
		}
	}
	return result;
}
int main(){
	int W, H;
	while (1){
		cin >> W >> H;
		if (!(W and H)) break; 
		rep(i,H) cin >> field[i];
		rep(i,H) 
			rep(j,W) 
			if (field[i][j] == '@') 
				cout << dfs(Coor(j,i),Coor(0,0),Coor(W,H)) << endl; 
	}
	return 0;
}

C++: #include <iostream>
#include <cassert>
#define REP(i,l,n) for(int i=l;i<n;++i)
#define rep(i,n) REP(i,0,n)
#define MAX 20
int ans,baseX,baseY;
int w,h;
char tile[MAX][MAX];
void seek(int x,int y);
using namespace std;
int main() {
  while(1){
    cin >> w;
    cin >> h;
    if(w==0&&h==0) return 0;
    rep(i,h){
      rep(j,w){
        cin >> tile[i][j];
        if(tile[i][j] == '@'){
          baseX = i;
          baseY = j;
        }
      }
      tile[i][w] = '#';
    }
    rep(i,w) tile[h][i] = '#';
    ans = 1;
    seek(baseX,baseY);
    cout << ans << endl;
  }
  return 0;
}

void seek(int x,int y){
  if(tile[x+1][y] == '.'){
    tile[x+1][y] = '#';
    ans++;
    seek(x+1,y);
  }
  if(tile[x][y+1] == '.'){
    tile[x][y+1] = '#';
    ans++;
    seek(x,y+1);
  }
  if(tile[x-1][y] == '.'){
    tile[x-1][y] = '#';
    ans++;
    seek(x-1,y);
  }
  if(tile[x][y-1] == '.'){
    tile[x][y-1] = '#';
    ans++;
    seek(x,y-1);
  }
}

C++: #include<iostream>
#include<vector>
#include<queue>
#include<map>
#include<string>
#define pb push_back
#define INF 1000000000

using namespace std;
typedef pair<int,int> P;
int w,h;
bool inf(int y,int x){
	if(y>=0&&y<h&&x>=0&&x<w)
	return true;
	return false;
}
int main(){
	
	int dx[]={0,0,1,-1},dy[]={1,-1,0,0};
	while(cin>>w>>h,w||h){
		vector<string> s(h);
		vector<vector<int> > d(h,vector<int>(w,INF));
		queue<P> que;
		for(int i=0;i<h;i++){
			cin>>s[i];
			for(int j=0;j<h;j++){
				for(int k=0;k<w;k++){
					if(s[j][k]=='@'){
						d[j][k]=0;
						que.push(P(j,k));
					}
				}
			}
		}
		while(!que.empty()){
			P q=que.front();
			que.pop();
			int x=q.second,y=q.first;
			for(int i=0;i<4;i++){
				int ny=dy[i]+y,nx=dx[i]+x;
				if(inf(ny,nx)&&s[ny][nx]!='#'&&d[ny][nx]>d[y][x]+1){
					d[ny][nx]=d[y][x]+1;
					que.push(P(ny,nx));
				}
			}
			
			
		}
		int cnt=0;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(d[i][j]!=INF)
					cnt++;
			}
		}
		cout<<cnt<<endl;
	}
	return 0;
}

C++: #include<iostream>
#include<utility>
#include<queue>
using namespace std;
char room[21][21];
int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};
typedef pair<int,int> p;


int main(){
	int x,y;
	queue<p> que;
	int w,h;
	cin>>w>>h;
	
	for(int a=0;a<h;a++){
		for(int b=0;b<w;b++){
			cin>>room[a][b];
			if(room[a][b]=='@'){
				x=b,y=a;
				que.push(p(b,a));
				room[a][b]='#';
			}
		}
		
	}
	int cnt=1;
	int nx,ny;
	while(que.size()){
		p ap=que.front();
		que.pop();
		for(int i=-1;i<=1;i++){
			nx=ap.first+dx[i];
			ny=ap.second+dy[i];
			if(0<=nx&&nx<=w&&0<=ny&&ny<=h&&room[ny][nx]!='#'){
				room[ny][nx]='#';
				que.push(p(nx,ny));
				cnt++;
			}
		}
	}
	cout<<cnt;
	return 0;
}