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 <utility>
using namespace std;
using P = std::pair<int, int>;


int main() {
    int W, H;
    while(cin >> W >> H, W) {
        vector<vector<char>> tile(W, vector<char>(H));
        vector<vector<int>> ret(W, vector<int>(H));
        queue<P> que;
        for(int i=0; i<H; ++i) {
            for(int j=0; j<W; ++j) {
                cin >> tile[j][i];
                if(tile[j][i] == '@') {
                    que.push(make_pair(j, i));
                    ret[j][i] = 1;
                }
            }
        }
        int dx[4] = {0, 1, 0, -1};
        int dy[4] = {1, 0, -1, 0};
        while(!que.empty()) {
            P p = que.front();
            que.pop();
            for(int i=0; i<4; ++i) {
                int nx = p.first + dx[i];
                int ny = p.second + dy[i];
                if(nx >= 0 && nx < W && ny >= 0 && ny < H && tile[nx][ny] != '#' && ret[nx][ny] == 0) {
                    ret[nx][ny] = ret[p.first][p.second] + 1;
                    que.push(make_pair(nx, ny));
                }
            }
        }
        int ans = 0;
        for(int i=0; i<H; ++i) {
            for(int j=0; j<W; ++j) {
                if(ret[j][i] != 0) {
                    ans += 1;
                }
            }
        }
        cout << ans << endl;
    }
}

C++: #include <stdio.h>
#include <string.h>
#include <vector>
#include <string>
#include <iostream>
using namespace std;
int w, h;
int dxy[4][2] = {{1,0},{0,1},{-1,0},{0,-1}};
char tiles[21][21];
char visited[21][21];

void rec(int y, int x)
{
    visited[y][x] = 1;
    
    for (int i=0; i<4; i++)
    {
        int x1, y1;
        x1 = x + dxy[i][0];
        y1 = y + dxy[i][1];

        if (tiles[y1][x1] == '.' && visited[y1][x1] ==0 && x1 >= 0 && x1 < w && y1 >= 0 && y1 < h)
        {
            
            rec(y1, x1);
        }
    }
}

int main(void)
{
    while (scanf("%d %d", &w, &h) && w && h)
    {
        

        int  x0, y0;

        

        for (int i=0; i<h; i++)
        {
            for (int j=0; j<w; j++)
            {
                cin >> tiles[i][j];

            }
        }
        for (int i=0; i<h; i++)
        {
            for (int j=0; j<w; j++)
            {
                if (tiles[i][j] == '@')
                {
                    y0=i;
                    x0=j;
                }
            }
        }
        memset(visited, 0, sizeof(visited));
        rec(y0, x0);
        //printf("%d %d", x0, y0);
        int cnt=0;
        for (int i=0; i<h; i++)
        {
            for (int j=0; j<w; j++)
            {
                cnt += visited[i][j];
            }
        }
        printf("%d\n", cnt);

    }
    return 0;
}

C++: #include<iostream>
#define REP(i,s,n) for(int i=s ; i < n ; i++)
#define rep(i,n) REP(i,0,n)
using namespace std;

int W,H;
char field[20][20];
int cnt;

void dfs(int x,int y){

  field[y][x]='#';
  
  cnt++;
  
  if( x+1 < W && field[y][x+1] == '.'){
    dfs(x+1,y);
  }
  if(x-1 >= 0 && field[y][x-1] == '.'){
    dfs(x-1,y);
  }
  if(y+1 < H && field[y+1][x] == '.'){
    dfs(x,y+1);
  }
  if(y-1 >=0 && field[y-1][x] == '.'){
    dfs(x,y-1);
  }
  
}
int main(){
  
  int x,y;
  
  while(1){
    cin >>W>>H;
    if(W==0 && H==0)break;
    
    rep(i,H){
      rep(j,W){
	cin >>field[i][j];
	if(field[i][j] == '@'){
	  x=j;
	  y=i;
	}
      }
    }
    cnt=0;
    dfs(x,y);
    cout <<cnt<<endl; 
  }
  
  return 0;
}

C++: #include<iostream>
#include<iomanip>
#include<algorithm>
#include<string>
#include<vector>
#include<deque>
#include<queue>
#include<numeric>
#include<stdio.h>

using std::cout;
using std::cout;
using std::cin;
using std::setw;
using std::setfill;
using std::string;
using std::pair;
using std::make_pair;
using std::vector;
using std::queue;
using std::deque;
using std::priority_queue;
using std::next_permutation;
using std::iota;
using std::sort;
using std::greater;
using std::max;
using std::min;

#define INF 999999999;
#define MOD 1000000007

using ulli = unsigned long long int;

#define EVEL 1

#ifndef EVEL
#define dbug(X) std::cout << #X << ":" <<X<<" " ;
#define dbugf(s) std::cout << s << " ";
#define dbugln std::cout<<"\n";
#else
#define dbug(X) {}
#define dbugf(s) {}
#define dbugln {}
#endif

int W, H;
int M[21][21];
int xd[] = {0, 1, 0, -1};
int yd[] = {-1, 0, 1, 0};
bool Mf[21][21];
queue<int> Q;

int main(){
	cin.tie(0);
	std::ios::sync_with_stdio(false);
    
    while(1){
        cin >> W >> H;
        if(W == 0 && H == 0) break;
        for(int i = 0; i  < W; i++){
            for(int j = 0; j < H; j++){
                M[i][j] = 1;
                Mf[i][j] = false;
            }
        }
        char c;
        for(int i = 0; i < H; i++){
            for(int j = 0; j < W; j++){
                cin >> c;
                if(c == '.') M[j][i] = 0;
                else if(c == '#') M[j][i] = 1;
                else if(c == '@'){
                    M[j][i]=0;
                    Q.push(i*W+j);
                    Mf[j][i] = true;
                }
            }
        }

        int ans = 0;
        while(!Q.empty()){
            const int r = Q.front();Q.pop();
            ans++;
            int x, y;
            for(int i = 0; i < 4; i++){
                x = (r%W) + xd[i];
                y = (r/W) + yd[i];
                if(x < 0 || x >= W || y < 0 || y >= H)continue;
                if(Mf[x][y] || M[x][y]!=0)continue;
                Q.push(x + y*W);
                Mf[x][y] = true;     
            }
        }
        cout << ans << "\n";
    }
    return 0;
}
/*
6 9
....#.
.....#
......
......
......
......
......
#@...#
.#..#.
11 9
.#.........
.#.#######.
.#.#.....#.
.#.#.###.#.
.#.#..@#.#.
.#.#####.#.
.#.......#.
.#########.
...........
11 6
..#..#..#..
..#..#..#..
..#..#..###
..#..#..#@.
..#..#..#..
..#..#..#..
7 7
..#.#..
..#.#..
###.###
...@...
###.###
..#.#..
..#.#..
0 0

*/