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 "climits"
#include "list"
#include "queue"
#include "stack"
#include "set"
#include "functional"
#include "algorithm"
#include "math.h"
#include "utility"
#include "string"
#include "map"
#include "unordered_map"
#include "iomanip"
#include "random"

using namespace std;
const long long int MOD = 1000000007;

long long int power(long long int x, long long int n, long long int M) {
	long long int tmp = 1;

	if (n > 0) {
		tmp = power(x, n / 2, M);
		if (n % 2 == 0) tmp = (tmp*tmp) % M;
		else tmp = (((tmp*tmp) % M)*x) % M;
	}
	return tmp;
}

long long int N, M, K, Q, W, H, L, R;
long long int ans;



int main() {
	ios::sync_with_stdio(false);
	cin >> W >> H;
	while (H) {
		ans = 0;
		string s[30];
		for (int i = 1; i <= H; i++) {
			cin >> s[i];
			s[i] = '#' + s[i] + '#';
		}
		for (int i = 0; i <= W + 1; i++) {
			s[0].push_back('#');
			s[H + 1].push_back('#');
		}
		int dir[5] = { 1,0,-1,0,1 };
		queue<pair<int, int> >Q;
		for (int i = 1; i <= H; i++) {
			for (int j = 1; j <= W; j++) {
				if (s[i][j] == '@') {
					Q.push({ i,j });
					s[i][j] = '#';
					ans++;
				}
			}
		}
		while (!Q.empty()) {
			//cout << "hi";
			int cy, cx;
			cy = Q.front().first;
			cx = Q.front().second;
			for (int i = 0; i < 4; i++) {
				if (s[cy + dir[i]][cx + dir[i + 1]] == '.') {
					s[cy + dir[i]][cx + dir[i + 1]] = '#';
					ans++;
					Q.push({ cy + dir[i] ,cx + dir[i + 1] });
				}
			}
			Q.pop();
		}
		cout << ans << endl;
		cin >> W >> H;
	}
	return 0;
}

C++: #include <iostream>
#include <fstream>
#include <algorithm>
#include <vector>
#include <string>
#include <functional>
#include <array>
#include <stack>
#include <queue>
#include <chrono>
#include <numeric>

#pragma warning(disable:4996)
#define REP(i,n) for(int i = 0 ; i < n ; ++i)
typedef long long Int;

using namespace std;

struct point{
	int x;
	int y;
	point(int _x, int _y) :x(_x), y(_y){}
};

char tile[20][20];
stack<point> st;

int main()
{
	cin.tie(0); ios::sync_with_stdio(false); 
	//fstream fs("input.txt");
	////////////ここから開始///////////

	
	int x(-1), y(-1);
	while (true){
		REP(i, 20)REP(j, 20)tile[i][j] = '#';
		cin >> x >> y;
		if (x == 0 && y == 0)
			break;
		//fs >> x >> y;
		REP(i, y){
			REP(j, x){
				cin >> tile[i][j];
				//fs >> tile[i][j];
			}
		}

		REP(i, y){
			REP(j, x){
				if (tile[i][j] == '@')
					st.emplace(i, j);
			}
		}

		int dx[]{0, 1, 0, -1};
		int dy[]{1, 0, -1, 0};
		int ans(1);

		while (!st.empty()){
			point now = st.top();
			st.pop();
			REP(i, 4){
				int x1 = now.x + dx[i];
				int y1 = now.y + dy[i];
				if (x1 > 19 || x1 < 0 || y1 > 19 || y1 < 0)
					continue;

				if (tile[x1][y1] == '.'){
					st.emplace(x1, y1);
					tile[x1][y1] = '#';
					++ans;
				}
			}
		}

		cout << ans << endl;
	}
	return 0;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include<bits/stdc++.h>
using namespace std;
#ifndef FAST_IO
 #define IOS ios_base::sync_with_stdio(false);cin.tie(nullptr);
#endif

#define int long long
#define FILE_IO freopen("input.txt","r",stdin)/#define int long long
#define f first
#define s second
#define endl '\n'
#define all(c) c.begin(),c.end()
#define bp __builtin_popcount
#define debug(x) cerr << #x << " = " << x << endl;

const int MOD = 1e9 + 7;
const int INF = 1e18;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
template <typename T>void print(T t){for (auto &it : t) cout << it << " ";}
inline int powerexp(int a, int b, int m = MOD){ int res = 1;while(b){if(b&1)res = res * a % m;a = a * a % m,b >>= 1;}return res % m;}
inline int modinv(int a){ return powerexp(a,MOD-2,MOD);}
inline int lcm(int a, int b){ int g = __gcd(a,b);return a/g*b;}
inline int add(int x, int y){ x += y; if (x >= MOD) x -=  MOD; return x;}
inline int mul(int x, int y){ return (x * 1ll * y) % MOD;}
inline int sub(int x, int y){ x -= y; if (x<0) x += MOD; return x ;}

//-------------------< start here >-------------------------// 

const int N = 20;
int g[N][N];
bool visited[N][N];
int w, h,x ,y, cnt;

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

bool check(int x, int y){
  return (x >= 0 && y >= 0 && x < w && y < h);	
}

void dfs(int c, int r){
	visited[c][r] = true;
	cnt++;
	for (int i = 0; i < 4; i++){
	  int nx = r + dx[i];
	  int ny = c + dy[i];
	  if (check(nx, ny) && g[ny][nx] == 1 && !visited[ny][nx]){
		  dfs(ny, nx);  
		}
	}
}

void solve(){
  while(cin >> w >> h && w > 0 && h > 0){
	  cnt = 0;
	  memset(visited, false, sizeof visited);
	  for (int i = 0; i < h; i++){
		for (int j = 0; j < w; j++){
		  char input;
		  cin >> input;
		  if (input == '.') g[i][j] = 1;
		  else if (input == '#') g[i][j] = 0;
		  else if (input == '@'){
			   x = i;
			   y = j; 
			 }	
		  }  
	  }
	  dfs(x, y);
	  cout << cnt << endl;  
	}	
}

//-----------------------------
//comment from here for TOPCODER 
int32_t main(){
   IOS;
   solve();
   //find();
   #ifdef LOCAL_DEFINE
   cerr << "time elapsed " << (double)clock()/CLOCKS_PER_SEC << endl;
   #endif
}

C++: #include <iostream>

using namespace std;

const int MAX_SIZE = 21;
int w, h;

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

char field[MAX_SIZE][MAX_SIZE];

bool ok(int x, int y) {
	return (x >= 0 && x < w && y >= 0 && y < h) && field[y][x] == '.';
}

int solve(int x, int y) {
	int ret = 0;
	for (int i = 0; i < 4; ++i) {
		int curx = x + dx[i],
			cury = y + dy[i];

		if (ok(curx, cury)) {
			field[cury][curx] = '#';
			ret += solve(curx, cury) + 1;
		}
	}
	return ret;
}

int main() {
	while (true) {
		cin >> w >> h;
		if (w == 0 && h == 0) break;
		int x = -1, y = -1;
		for (int i = 0; i < h; ++i) for (int j = 0; j < w; ++j) {
			cin >> field[i][j];
			if (field[i][j] == '@') {
				field[i][j] = '#';
				x = j;
				y = i;
			}
		}

		cout << solve(x, y) + 1 << endl;

	}

	return 0;
}