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 <string>
#include <vector>
#include <queue>
using namespace std;

int main() {
	int w, h, sx, sy;
	while (cin >> w >> h , w != 0 || h != 0) {
		vector<string> field = vector<string>(h);

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

		int ans = 0;
		auto visited = vector<vector<bool>>(h, vector<bool>(w, false));
		queue<int> q = queue<int>();
		q.push(sy * 100 + sx);
		while (!q.empty()) {
			int x = q.front() % 100, y = q.front() / 100;
			q.pop();
			if (visited[y][x]) continue;
			ans++;
			visited[y][x] = true;
			int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
			for (int i = 0; i < 4; i++) {
				int nx = x + dx[i], ny = y + dy[i];
				if (nx < 0 || w <= nx || ny < 0 || h <= ny) continue;
				if (field[ny][nx] == '.' && !visited[ny][nx])
					q.push(ny * 100 + nx);
			}
		}

		cout << ans << endl;
	}
}

C++: #include <cstdio>
#include <string>
#include <iostream>
using namespace std;

const dx[] = {-1, 0, 1, 0};
const dy[] = {0, -1, 0, 1};
char tile[40][40];
int h, w;

int search(int nx, int ny)
{
	int sum = 0;
	if (ny == 0 || nx == 0 || ny == h + 1 || nx == w + 1)return (sum);
	if (tile[nx][ny] == '.' || tile[nx][ny] == '@'){
		sum++;
	}
	
	tile[nx][ny] = '#';
	
	for (int i = 0; i < 4; i++){
		if (tile[nx + dx[i]][ny + dy[i]] != '#'){
			sum += search(nx + dx[i], ny + dy[i]);
		}
	}
	return (sum);
}
		
int main()
{
	while (1){
		
		scanf("%d %d", &w, &h);
		
		if (h == 0 && w == 0)return (0);
		int mx, my;
		for (int i = 1; i <= h; i++){
			for (int j = 1; j <= w; j++){
				scanf(" %c", &tile[j][i]);
				if (tile[j][i] == '@'){
					mx = j;
					my = i;
				}
			}
		}
		printf("%d\n", search(mx, my));
	}

}

C++: #include<iostream>
#include<cstdio>
#include<vector>
#include<algorithm>
#include<queue>
using namespace std;
typedef long long ll;
//Global
int x, y;
const int MAX_N = 20;
char maze[MAX_N][MAX_N];
const int INF = 1000000;
int X, Y;//"@"???????????????
int memo[MAX_N][MAX_N];
int dx[4] = { 1,0,-1,0 };
int dy[4] = { 0,1,0,-1 };
int counter;

void bsd(int a, int b) {
	maze[b][a] = '#';
	memo[b][a] = INF;
	counter++;
	for (int i = 0; i < 4; i++) {
		int nx = a + dx[i];
		int ny = b + dy[i];
		if (memo[ny][nx] == 0 && maze[ny][nx] == '.' && 0 <= nx && nx < x && 0 <= ny && ny < y) {
			bsd(nx, ny);
		}
	}

}



//solve
//??±???????????¢?´¢??§"@"??¨???????????£?????????"."????????°????±???????
void solve()
{
	//memo????????????
	for (int i = 0; i < y; i++) {
		for (int j = 0; j < x; j++) {
			memo[i][j] = 0;
		}
	}

	//"@"??¨??£??\????????????  ??????  INF??§??????  ??????  ????????????????????£?????????????????????"."??§??????
	counter = 0;//"@"??????
	bsd(X, Y);


	cout << counter << endl;

}







//main
int main()
{
	while (true) {
		cin >> x >> y;
		if (x == 0 && y == 0)break;
		//"W"??¨"."??¨"@"?????\???
		for (int i = 0; i < y; i++) {
			for (int j = 0; j < x; j++) {
				cin >> maze[i][j];
				if (maze[i][j] == '@') {
					X = j; Y = i;//@???????????????????¨????
				}
			}
		}
		solve();
	}
    return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;

#define MOD 1000000007
#define REP(i, N) for (int i = 0; i < N; ++i)
#define REP1(i, N) for (int i = 1; i <= N; ++i)
#define RREP(i, N) for (int i = N - 1; i >= 0; --i)
#define ALL(a) a.begin(), a.end()

int w, h;

char c[20][20];
bool visit[20][20];

int dy[4] = {-1, 0, 1, 0};
int dx[4] = {0, 1, 0, -1};

void dfs(int y, int x) {
  visit[y][x] = true;
  REP(i, 4) {
    int ny = y + dy[i];
    int nx = x + dx[i];
    if (0 <= ny && ny < h && 0 <= nx && nx < w && c[ny][nx] == '.' &&
        !visit[ny][nx])
      dfs(ny, nx);
  }
}

int main() {
  while (cin >> w >> h) {
    if (w == 0 && h == 0) break;
    REP(i, h) REP(j, w) cin >> c[i][j];
    REP(i, h) REP(j, w) visit[i][j] = false;
    REP(i, h) REP(j, w) {
      if (c[i][j] == '@') dfs(i, j);
    }
    int ans = 0;
    REP(i, h) REP(j, w) {
      if (visit[i][j]) ans++;
    }
    cout << ans << endl;
  }
  return 0;
}

C++: #include <iostream>
#include <vector>
#include <queue>
#include <string>
#include <utility>
#include <unordered_set>

using namespace std;

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

int main(){
    int w, h;
    while(cin >> w >> h && w > 0 && h > 0){
        string tmp;
        pair<int, int> init;
        vector<vector <int> > grid(25, vector<int>(25, -1));
        vector<vector <int> > visited(25, vector<int>(25, 0));
        for(int i = 0; i < h; i++){
            cin >> tmp;
            for (int j = 0; j < w; j++){
                if(tmp[j] == '.'){
                    grid[i][j] = 0;
                }else if(tmp[j] == '#'){
                    grid[i][j] = -1;
                }else if(tmp[j] == '@'){
                    grid[i][j] = 0;
                    init = make_pair(i, j);
                }
            }
        }
        queue<pair<int, int> > Q;
        Q.push(init);
        visited[init.first][init.second] = 1; 
        while(!Q.empty()){
            pair<int, int> cur = Q.front(); 
            Q.pop();
            int cur_x = cur.first;
            int cur_y = cur.second;
            for(int i = 0; i < 4; i++){
                pair<int, int> dst = make_pair(cur_x + dx[i], cur_y + dy[i]);
                int x = dst.first;
                int y = dst.second;
                if(0 <= x && 0 <= y && grid[x][y] == 0 && visited[x][y] == 0){
                    Q.push(dst);
                    visited[x][y] = 1;
                }
            }
        }
        int ans = 0;
        for(int i = 0; i < h; i++){
            for (int j = 0; j < w; j++){
                ans += visited[i][j];
            }
        }
        cout << ans << endl << endl << endl;
    }
}