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>

using namespace std;


void search(int h, int w, int tairu[22][22], int check[22][22]){
    if(check[h][w]==1)return;
    if(tairu[h][w]==0)return;
    
    check[h][w]=1;
    search(h+1,w,tairu,check);
    search(h-1,w,tairu,check);
    search(h,w+1,tairu,check);
    search(h,w-1,tairu,check);
}






int main()
{

    int H,W;
    while(cin>>W>>H){
        int tairu[22][22]={0};
        int start_w, start_h;
        for(int i=1; i<=H; i++){
            for(int j=1; j<=W; j++){
                char input;
                cin>>input;
                if(input=='.')tairu[i][j]=1;
                if(input=='@'){
                    tairu[i][j]=1;
                    start_w=j;
                    start_h=i;
                }
            }
        }
        //
        
        int check[22][22]={0};
        search(start_h, start_w, tairu, check);
        int ans=0;
        for(int i=1; i<=H; i++){
            for(int j=1; j<=W; j++){
                ans+=check[i][j];
            }
        }
        cout<<ans<<endl;
        
        
        
    }
}

C++: #include <map>
#include <set>
#include <list>
#include <cstdio>
#include <cmath>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <cctype>
#include <string>
#include <vector>
#include <cstdlib>
#include <cstring>
#include <iomanip>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <functional>

using namespace std;

#define fr first
#define sc second
#define mp make_pair

typedef long long int64;
typedef pair< int, int > iP;
typedef pair< iP, int > iiP;

const int INF = 2 << 28;
const double EPS = 1e-10;

int H, W;
char mas[21][21];

int dfs(int x, int y) {
    static int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
    int ret = 1;
    mas[y][x] = '#';
    for(int i = 0; i < 4; i++) {
        int nx = x + dx[i], ny = y + dy[i];
        if(0 <= nx && nx < W && 0 <= ny && ny < H && mas[ny][nx] == '.') {
            ret += dfs(nx, ny);
        }
    }
    return ret;
}

int main() {

    while(cin >> W >> H, W) {

        int sx, sy;
        for(int i = 0; i < H; i++) {
            for(int j = 0; j < W; j++) {
                cin >> mas[i][j];
                if(mas[i][j] == '@') sx = j, sy = i;
            }
        }
        cout << dfs(sx, sy) << endl;
    }
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <cctype>
#include <algorithm>
#include <string>
#include <queue>
#include <cstring>
using namespace std;

char map[21][21];
int w, h;
int tile = 0;

void dfs(int x, int y){
    if (!(0 <= x && x < w && 0 <= y < h)) return;
    tile++;
    map[y][x] = '#';
    if (map[y-1][x] == '.') dfs(x, y-1);
    if (map[y+1][x] == '.') dfs(x, y+1);
    if (map[y][x-1] == '.') dfs(x-1, y);
    if (map[y][x+1] == '.') dfs(x+1, y);
}

int main(){
    int sx, sy;
    while (1){
	scanf("%d %d", &w, &h);
	if (w == 0 && h == 0) break;
	memset(map, '#', sizeof(map));
	tile = 0;
	for (int i = 0; i < h; i++){
	    cin >> map[i];
	    for (int p = 0; p < w; p++){
		if (map[i][p] == '@'){
		    sx = p;
		    sy = i;
		}
	    }
	}
	dfs(sx, sy);
	printf("%d\n", tile);
    }
    
    return 0;
}

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

using namespace std;

char a[21][21];
int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};
int count;

int W, H;

void dfs(int x, int y)
{
    a[x][y] = 0;
    for (int i = 0; i < 4; i++)
    {
        int nx = x + dx[i];
        int ny = y + dy[i];
        if (0 <= nx && nx < H && 0 <= ny && ny < W && a[nx][ny] == '.')
        {
            count++;
            dfs(nx, ny);
        }
    }
}

int main()
{
    while (cin >> W >> H, W && H)
    {
        count = 1;
        for (int i = 0; i < H; i++)
        {
            cin >> a[i];
        }
        for (int i = 0; i < H; i++)
        {
            for (int j = 0; j < W; j++)
            {
                if (a[i][j] == '@')
                {
                    dfs(i, j);
                    break;
                }
            }
        }
        cout << count << endl;
    }
}

C++: #include <deque>
#include <iostream>
#include <string>
#include <vector>

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

using namespace std;

typedef struct point {
  int x, y;
} point;

int red_black(point size, point sp, vector<vector<char>> &room) {
  deque<point> queue(0);
  queue.push_front(sp);
  int sum = 0;

  while (queue.size() > 0) {
    auto p = queue.front();
    queue.pop_front();

    auto x = p.x;
    auto y = p.y;
    if (x < 0 || y < 0 || x == size.x || y == size.y) {
      continue;
    }

    auto &tile = room[y][x];
    if (tile == '#') {
      continue;
    }

    sum += 1;
    tile = '#';
    queue.push_front(point{x + 1, y});
    queue.push_front(point{x - 1, y});
    queue.push_front(point{x, y + 1});
    queue.push_front(point{x, y - 1});
  }

  return sum;
}

int main() {
  int w, h, sx, sy;
  char c;
  while (true) {
    cin >> w >> h;
    if (w == 0) {
      break;
    }

    vector<vector<char>> room(h, vector<char>(w));
    rep(y, h) {
      rep(x, w) {
        cin >> c;
        if (c == '@') {
          sx = x;
          sy = y;
        }
        room[y][x] = c;
      }
    }

    auto result = red_black(point{w, h}, point{sx, sy}, room);
    cout << result << endl;
  }
}

C++: #include <iostream>
using namespace std;
char S[21][21];
int count = 0;

void a(int i, int j){
	if(S[i][j] == '.'){
		count++;
		S[i][j] = '#';
		a(i - 1, j);
		a(i + 1, j);
		a(i, j + 1);
		a(i, j - 1);
	}
}


int main(){
	while(true){
		memset(S,'#',sizeof(S));
		count = 0;
		int x, y;
		cin >> x >> y;
		if(x == 0 && y == 0){
			break;
		}
		for(int i = 0; i < y; i++){
			for(int j = 0; j < x; j++){
				cin >> S[i][j];
			}
		}

		int startx, starty;
		for(int i = 0; i < y; i++){
			for(int j = 0; j < x; j++){
				if(S[i][j] == '@'){
					startx = j;
					starty = i;
					S[i][j] = '.';
					break;
				}
			}
		}
		a(starty, startx);
		cout << count << endl;
	}
}