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;

int main()
{
	int w, h;
	while (true) {
		do {
			cin >> w >> h;
		} while (w < 0 || h < 0);
		
		if (w == 0 && h == 0) {
			break;
		}
		unsigned next = 1U;
		char** room = new char*[h];
		unsigned** group = new unsigned*[h];
		int x, y;
		for (int i = 0; i < h; i++) {
			room[i] = new char[w];
			group[i] = new unsigned[w];
			for (int j = 0; j < w; j++) {
				cin >> room[i][j];
				if (room[i][j] == '@') {
					x = j;
					y = i;
				}
				group[i][j] = 0U;
			}
		}

		for (int k = 0; k < 2; k++) {
			for (int i = 0; i < h; i++) {
				for (int j = 0; j < w; j++) {
					if (room[i][j] != '#') {
						for (int a = i - 1; a <= i + 1; a++) {
							for (int b = j - 1; b <= j + 1; b++) {
								if (a < 0 || b < 0 || a >= h || b >= w) {
									continue;
								}
								if ((a == i && b != j) || (a != i && b == j)) {
									group[i][j] |= group[a][b];
								}
								if (group[i][j] == 0U) {
									group[i][j] |= next;
									next <<= 1;
								}
							}
						}
					}
				}
			}
			for (int i = h - 1; i >= 0; i--) {
				for (int j = w - 1; j >= 0; j--) {
					if (room[i][j] != '#') {
						for (int a = i - 1; a <= i + 1; a++) {
							for (int b = j - 1; b <= j + 1; b++) {
								if (a < 0 || b < 0 || a >= h || b >= w) {
									continue;
								}
								if ((a == i && b != j) || (a != i && b == j)) {
									group[i][j] |= group[a][b];
									if (group[i][j] == 0U) {
										group[i][j] |= next;
										next <<= 1;
									}
								}
							}
						}
					}
				}
			}
			for (int j = 0; j < w; j++) {
				for (int i = 0; i < h; i++) {
					if (room[i][j] != '#') {
						for (int a = i - 1; a <= i + 1; a++) {
							for (int b = j - 1; b <= j + 1; b++) {
								if (a < 0 || b < 0 || a >= h || b >= w) {
									continue;
								}
								if ((a == i && b != j) || (a != i && b == j)) {
									group[i][j] |= group[a][b];
									if (group[i][j] == 0U) {
										group[i][j] |= next;
										next <<= 1;
									}
								}
							}
						}
					}
				}
			}
			for (int j = w - 1; j >= 0; j--) {
				for (int i = h - 1; i >= 0; i--) {
					if (room[i][j] != '#') {
						for (int a = i - 1; a <= i + 1; a++) {
							for (int b = j - 1; b <= j + 1; b++) {
								if (a < 0 || b < 0 || a >= h || b >= w) {
									continue;
								}
								if ((a == i && b != j) || (a != i && b == j)) {
									group[i][j] |= group[a][b];
									if (group[i][j] == 0U) {
										group[i][j] |= next;
										next <<= 1;
									}
								}
							}
						}
					}
				}
			}
		}
		
		int count = 0;
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if ((group[i][j] & group[y][x]) != 0U) {
					count++;
				}
			}
		}

		cout << count << '\n';

		for (int i = 0; i < h; i++) {
			delete[] room[i];
			delete[] group[i];
		}
		delete[] room;
		delete[] group;
	}

	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
template <class T> inline bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T> inline bool chmin(T &a, T b) {
    if(a > b) {
        a = b;
        return 1;
    }
    return 0;
}
typedef long long int ll;

#define ALL(v) (v).begin(), (v).end()
#define RALL(v) (v).rbegin(), (v).rend()
#define endl "\n"
const double EPS = 1e-7;
const int INF = 1 << 30;
const ll LLINF = 1LL << 60;
const double PI = acos(-1);
const int MOD = 1000000007;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

//-------------------------------------

using P = pair<int, int>;

int h, w;

void dfs(int x, int y, vector<string> &g, vector<vector<bool>> &seen) {
    if(seen[y][x] || g[y][x] == '#') {
        return;
    }
    seen[y][x] = true;
    for(int i = 0; i < 4; i++) {
        int ny = y + dy[i];
        int nx = x + dx[i];
        if(0 <= nx && nx < w && 0 <= ny && ny < h && g[ny][nx] == '.' &&
           !seen[ny][nx]) {
            dfs(nx, ny, g, seen);
        }
    }
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    while(1) {
        cin >> w >> h;
        if(h == 0 && w == 0) {
            break;
        }
        vector<string> g(h);
        for(int i = 0; i < h; i++) {
            cin >> g[i];
        }
        vector<vector<bool>> seen(h, vector<bool>(w, false));
        int sx, sy;
        for(int i = 0; i < h; i++) {
            for(int j = 0; j < w; j++) {
                if(g[i][j] == '@') {
                    sy = i, sx = j;
                }
            }
        }
        dfs(sx, sy, g, seen);
        int ans = 0;
        for(int i = 0; i < h; i++) {
            ans += count(ALL(seen[i]), true);
        }
        cout << ans << endl;
    }
}

C++: #include <iostream>
#include <queue>
#include <complex>
using namespace std;
typedef complex<int> P;

int main()
{
  int W, H;
  while (cin >> W >> H && !(W == 0 && H == 0)) {
    vector<string> grid(H);
    P start;
    for (int i = 0; i < H; i++) {
      cin >> grid[i];
      for (int j = 0; j < W; j++) {
        if (grid[i][j] == '@') {
          start = P(i, j);
          grid[i][j] = '.';
        }
      }
    }

    queue<P> q;
    q.push(start);
    vector<vector<bool> > visited(H, vector<bool>(W, false));
    visited[start.real()][start.imag()] = true;
    int ans = 0;
    while (!q.empty()) {
      const P p = q.front();
      q.pop();
      ans++;

      static const P dir[] = {P(-1,0), P(1,0), P(0,-1), P(0,1)};
      for (int d = 0; d < 4; d++) {
        const P next = p + dir[d];
        if (0 <= next.real() && next.real() < H && 0 <= next.imag() && next.imag() < W
            && grid[next.real()][next.imag()] == '.' && !visited[next.real()][next.imag()]) {
          visited[next.real()][next.imag()] = true;
          q.push(next);
        }
      }
    }
    cout << ans << endl;
  }
  return 0;
}