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 <string>
using namespace std;

int explorer(vector<string> *map,int i, int j){
	if(map->at(i)[j]=='#') return 0;
	
	int cnt=1;
	vector<int>vi;
    vi.reserve(4);
	vector<int>vj;
    vj.reserve(4);
	
	map->at(i)[j]='#';
	
	if(i!=0&&map->at(i-1)[j]=='.'){
		vi.push_back(i-1);
		vj.push_back(j);
	}if(i+1!=map->size()&&map->at(i+1)[j]=='.'){
		vi.push_back(i+1);
		vj.push_back(j);		
	}if(j!=0&&map->at(i)[j-1]=='.'){
		vi.push_back(i);
		vj.push_back(j-1);		
	}if(j+1!=map->at(i).size()&&map->at(i)[j+1]=='.'){
		vi.push_back(i);
		vj.push_back(j+1);		
	}

	while(!vi.empty()){
		cnt += explorer(map,vi.back(),vj.back());
		vi.pop_back();
		vj.pop_back();
	}

	return cnt;
}

int main() {
	int w,h;
	
	while(cin >> w >> h,w){
	vector<string> map;
	int psx=0,psy=0;
	map.resize(h);
	for(int i=0;i<h;i++){
		cin >> map[i];
		
		if(psx==0){
			if(map[i].find('@')!=-1){
				psx=map[i].find('@');
				psy=i;
			}
		}
	}
	
	cout << explorer(&map,psy,psx) << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <string>
#include <math.h>
#include <algorithm>
#include <vector> 
using namespace std;

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

string map[20];
int W, H;

int dfs(int y, int x){
  
  int count = 1;
  int i;
  for(i = 0;i < 4;i++){
    int next_x = x + dx[i] ,next_y = y + dy[i];
    if(0 <= next_x && next_x < W && 0 <= next_y && next_y < H && map[next_y][next_x] == '.'){
      map[next_y][next_x] = 'a';
      count += dfs(next_y, next_x);
    }
  }
  return count;
}


int main(){
  int i, j, x, y;
  while(1){
    cin >> W >> H;
    if(W == 0 && H == 0) break;
    bool start = false;
    for(i = 0;i < H;i++){
      cin >> map[i];
      
    }

    for(i = 0;i < H;i++){
      for(j = 0;j < W;j++){
        if(map[i][j] == '@'){
          x = j;
          y = i;
          start = true;
          break;
        }
      }
      if(start) break;
    }

    cout << dfs(y, x) << endl;

    
  }

  return 0;
}

C++: #include <iostream>
using namespace std;
#define MAX 21

string str[MAX];
int n,m,cnt,dx[4] = {1,0,-1,0},dy[4] = {0,1,0,-1};

void dfs(int x,int y){
  str[x][y] = '#';

  for(int i = 0 ; i < 4 ; i++){
    int nx = x + dx[i] , ny = y + dy[i];
    if(nx < 0 || nx > m || ny < 0 || ny > n) return; 
    if(str[nx][ny] == '.'){
      cnt++;
      dfs(nx,ny);
    }
  }

  return;
}

int main(){
  while(cin >> n >> m , n + m){
    for(int i = 0 ; i < m ; i++)
      cin >> str[i];
    
    int x,y;
    for(int i = 0 ; i < n ; i++){
      for(int j = 0 ; j < m ; j++){
	if(str[j][i] == '@'){
	  x = j;
	  y = i;
	  break;
	}
      }
    }
    cnt = 1;
    dfs(x,y);
    cout << cnt << endl;

  }

  return 0;
}

C++: #define B(a,x,b) ((a)<=(x)&&(x)<(b))

char T[999];
int x,y,cx,cy,i,c;

void R(a,b){
	if(B(0,a,x)&&B(0,b,y)&&T[b*20+a]){
		T[b*20+a]=0;i++;R(a-1,b);R(a+1,b);R(a,b-1);R(a,b+1);
	}
}

main(){
	for(;scanf("%d%d",&x,&y),x;){
		for(i=0;i<x*y;i++){
			int _x=i%x, _y=i/x;
			for(;(c=getchar())!='\n';);
			switch(c){
				case '@': cx=_x;cy=_y;
				case '.': T[_y*20+_x]=1; break;
				case '#': T[_y*20+_x]=0; break;
			}
		}
		i=0;
		R(cx,cy);
		printf("%d\n",i);
	}
	return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <limits.h>
#include <stdlib.h>
#include <string>
#include <vector>

#define el endl
#define fd fixed
#define INF INT_MAX/2-1
#define pb push_back

using namespace std;

bool tile[20][20];
bool visited[20][20];

void visit(int x, int y) {
  visited[y][x] = true;
  if (x > 0 && tile[y][x-1] && !visited[y][x-1]) visit(x-1, y);
  if (x < 19 && tile[y][x+1] && !visited[y][x+1]) visit(x+1, y);
  if (y > 0 && tile[y-1][x] && !visited[y-1][x]) visit(x, y-1);
  if (y < 19 && tile[y+1][x] && !visited[y+1][x]) visit(x, y+1);
}

int main() {
  int w, h, x, y, count;
  string tmp;
  while (cin >> w >> h, w | h) {
    for (int i = 0; i < 20; i++) {
      for (int j = 0; j < 20; j++) {
        visited[i][j] = false;
        tile[i][j] = false;
      }
    }
    for (int i = 0; i < h; i++) {
      cin >> tmp;
      for (int j = 0; j < w; j++) {
        if (tmp[j] == '.') {
          tile[i][j] = true;
        } else if (tmp[j] == '@') {
          x = j;
          y = i;
          tile[i][j] = true;
        }
      }
    }
    visit(x, y);
    count = 0;
    for (int i = 0; i < 20; i++) {
      for (int j = 0; j < 20; j++) {
        if (visited[i][j]) count++;
      }
    }
    cout << count << el;
  }
}