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<algorithm>
#include<cmath>
#include<string>
#include<tuple>
#include<utility>
#include<stack>
#include<queue>
#include<deque>
#include<set>
#include<iomanip>
#include<map>
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;
const int INF = 1e9 + 7;
int dx[8] = { 1,0,-1,0,1,1,-1,-1 };
int dy[8] = { 0,1,0,-1,1,-1,1,-1 };

int w, h;
int cnt = 0;
char s[55][55];
void dfs(int y, int x) {

	s[y][x] = '#';
	cnt++;

	for (int i = 0; i < 4; i++) {
		int ny = y + dy[i];
		int nx = x + dx[i];

		if (0 <= ny && ny < h && 0 <= nx && nx < w && s[ny][nx] == '.') {
			dfs(ny, nx);
		}


	}


}



int main() {
	int sy ;
	int	sx ;
	while (cin >> w >> h) {
		if (w == 0)break;

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

		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (s[i][j] == '@') {
					sy = i;
					sx = j;
				}
			}
		}

		dfs(sy, sx);

		cout << cnt << endl;
		cnt = 0;

	}



	return 0;
}

C++: #include <cmath>
#include <iostream>
#include <vector>
#include <queue>
#include <map>
#include <set>
#include <algorithm>
#include <utility>
#include <iomanip>

#define int long long int
#define rep(i, n) for(int i = 0; i < (n); ++i)

using namespace std;

typedef pair<int, int> P;

const int INF = 1e15;
const int MOD = 1e9+7;

signed main(){
    while(true){
        int n, m;
        cin >> n >> m;
        if(n == 0 && m == 0){
            break;
        }

        P st;
        vector<vector<bool>> area(m, vector<bool>(n, false));
        rep(i, m){
            rep(j, n){
                char c;
                cin >> c;

                if(c == '.'){
                    area[i][j] = true;
                }
                if(c == '@'){
                    area[i][j] = true;
                    st = make_pair(i, j);
                }
            }
        }

        vector<int> d = {0, 1, 0, -1};

        vector<vector<bool>> mark(m, vector<bool>(n, false));
        queue<P> que;
        que.push(st);
        int ans = 0;
        while(not que.empty()){
            P p = que.front();
            que.pop();

            int x = p.first;
            int y = p.second;
            if(mark[x][y] || not area[x][y]){
                continue;
            }

            mark[x][y] = true;
            ans++;
            rep(i, 4){
                int nx = x + d[i];
                int ny = y + d[i^1];
                if(0 <= nx && nx < m && 0 <= ny && ny < n){
                    que.push(make_pair(nx, ny));
                }
            }
        }
        cout << ans << endl;
    }

    return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <utility>
#include <cstring>
using namespace std;

#define rep(i,n) for(int i=0; i<n; i++)
typedef long long ll;

int w, h;
string mp[20];

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

int main() {
	while(cin >> w >> h, w||h) {
		int sx, sy;
		rep(i, h) {
			cin >> mp[i];
			rep(j, w) {
				if(mp[i][j] == '@') {
					sx = j;
					sy = i;
				}
			}
		}
		cout << dfs(sx, sy) << 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>
using namespace std;

int w,h,ret;
char mp[20][20];

void rec(int y,int x){
	if(mp[y][x]=='#')return;
	ret++;
	mp[y][x]='#';
	if(x+1<w)rec(y,x+1);
	if(y+1<h)rec(y+1,x);
	if(x-1>=0)rec(y,x-1);
	if(y-1>=0)rec(y-1,x);
}

int main(){
	int x,y;
	while(cin >> w >> h && w && h){
		ret = 0;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				cin >> mp[i][j];
				if(mp[i][j]=='@'){
					y = i;
					x = j;
				}
			}
		}
		rec(y,x);
		cout << ret << endl;
	}
	return 0;
}

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

const int dx[] = {1, 0, -1, 0}, dy[] = {0, -1, 0, 1};
//#define int long long;
signed main(){
  int w, h;
  while(cin >> w >> h, w){
    int cnt{};
    vector<string> v(h);
    for(int i = 0; i < h; ++i){
      cin >> v[i];
    }
    //initialize
    pair<int, int> p;
    queue<pair<int, int>> q;	
    for(int i = 0; i < h; ++i){
      for(int j = 0; j < w; ++j){
	if(v[i][j] == '@'){
	  p = make_pair(i,j);
	  q.push(p);
	}
      }
    }
    while(!q.empty()){
      pair<int, int> p;
      p = q.front();
      q.pop();

      for(int k = 0; k < 4; ++k){
	int x = p.first + dx[k]; int y = p.second + dy[k];
	if(0 > x || h <= x || 0 > y || w <= y)continue;
	if('.' == v[x][y]){
	  v[x][y] = '@';
	  ++cnt;
	  q.push(make_pair(x, y));
	}
      }
    }
    cout << cnt+1 << endl;
  }
  return 0;
}