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 <bits/stdc++.h>
using namespace std;
typedef unsigned int uint;
typedef long long int lint;
typedef unsigned long long int ulint;

//#define DEBUG

#ifdef DEBUG
#	define dump(i) cout << "[*] " #i " : " << i << endl;
#	define debug(i) i
void print(char map[20][20], int w, int h) {
	for (int i = 0; i < h; ++i) {
		for (int j = 0; j < w; ++j) {
			cout << static_cast<int>(map[i][j]);
		}
		cout << endl;
	}
}
void print(int* arr, int size) {
	for (int i = 0; i < size; ++i) {
		cout << arr[i] << endl;
	}
}
#else
#	define dump(i)
#	define debug(i)
# define print(i, j, k)
# define print(j, k)
#endif
#define rep(i, MAX) for (int i = 0; i < (MAX); ++i)

struct UnionFind
{
	vector<int> par;

	UnionFind(int n): par(n) {
		for (int i = 0; i < n; ++i) {
			par[i] = i;
		}
	}

	int find(int x) {
		if (x == par[x]) return x;
		return par[x] = find(par[x]);
	}

	bool same(int x, int y) {
		return find(x) == find(y);
	}

	void unite(int x, int y) {
		x = find(x);
		y = find(y);
		if (x == y) return;
		par[y] = x;
	}
};

inline void solve() {
	int w, h;
	constexpr int MAX = 20;
	char tile[MAX][MAX];
	dump(MAX);

	while (true) {
		pair<int, int> at;
		cin >> w >> h;
		if (!w && !h) return;
		int table[MAX] = {};
		UnionFind uf = UnionFind(200);

		const int asdf = MAX * MAX;
		dump(asdf);
		rep(i, h) {
			rep(j, w) {
				cin >> tile[i][j];
				if (tile[i][j] == '@') {
					at.first = i;
					at.second = j;
					tile[i][j] = '.';
				}
			}
		}
		int csi = 1;
		if (tile[0][0] == '.') {
			tile[0][0] = csi++;
		} else {
			tile[0][0] = 0;
		}
		table[tile[0][0]]++;
		for (int i = 1; i < w; ++i) {
			if (tile[0][i] == '.') {
				tile[0][i] = tile[0][i-1] != 0 ? tile[0][i-1] : csi++;
			} else {
				tile[0][i] = 0;
			}
			table[tile[0][i]]++;
		}
		rep(i, h) {
			if (i == 0) continue;
			rep(j, w) {
				if (j == 0) {
					if (tile[i][j] == '.') {
						tile[i][j] = tile[i-1][j] != 0 ? tile[i-1][j] : csi++;
					} else {
						tile[i][j] = 0;
					}
				} else {
					if (tile[i][j] == '.') {
						int tmp = tile[i-1][j];
						if (tmp == 0) {
							tmp = tile[i][j-1];
						} else if (tile[i][j-1] != 0) {
							uf.unite(tmp, tile[i][j-1]);
							if (tmp > tile[i][j-1]) {
								tmp = tile[i][j-1];
							}
						}
						tile[i][j] = tmp != 0 ? tmp : csi++;
					} else {
						tile[i][j] = 0;
					}
				}
				table[tile[i][j]]++;
			}
		}
		int ans = 0;
		int iv = tile[at.first][at.second];
		rep(i, csi) {
			if (uf.same(iv, i)) {
				ans += table[i];
			}
		}
		dump("tile");
		print(tile, w, h);
		dump("table");
		print(table, csi);
		dump("ans");
		cout << ans << endl;
	}
}

int main() {
	//*
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	// */
#ifdef DEBUG
	const auto start = chrono::high_resolution_clock::now();
#endif
	solve();
#ifdef DEBUG
	const auto stop = chrono::high_resolution_clock::now();
	cerr << "[*] time: " << chrono::duration_cast<chrono::milliseconds>(stop-start).count()
		<< " [ms]" << endl;
#endif
	return 0;
}

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

int w, h;
string inp[21];

const int MAX=23;
int room[MAX][MAX];
int cnt;

void init(){
    int i, j;
    for(i=0; i<MAX; i++){
        for(j=0; j<MAX; j++){
            room[i][j]=0;
        }
    }
    cnt = 0;
}

void dfs(int x, int y){
    cnt++;
    room[x][y]=0;
    if(room[x+1][y]){
        dfs(x+1,y);
    }
    if(room[x-1][y]){
        dfs(x-1,y);
    }
    if(room[x][y+1]){
        dfs(x,y+1);
    }
    if(room[x][y-1]){
        dfs(x,y-1);
    }
}

int main() {
    ios_base::sync_with_stdio(0);
    
    
    while(1){
        cin >> w >> h;
        
        if(w==0 && h==0)
            break;
        
        int i, j;
        int x=0, y=0;
        for(i=1; i<=h; i++){
            cin >> inp[i];
            for(j=0; j<w; j++){
                switch(inp[i][j]){
                    case '.':
                        room[j+1][i]=1;
                        break;
                    case '#':
                        room[j+1][i]=0;
                        break;
                    case '@':
                        room[j+1][i]=1;
                        x=j+1;
                        y=i;
                        break;
                }
            }
        }
        
        dfs(x, y);
        cout << cnt << endl;
        
        init();
    }
    return 0;
}

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

char c[21][21];
int w, h;
int sy, sx;
int dy[4] = {1, -1, 0, 0};
int dx[4] = {0, 0, 1, -1};
bool f[21][21];
int ans = 0;

void dfs(int y, int x){
    f[y][x] = true;
    ans++;
    for(int i = 0; i < 4; i++){
        int newy = y + dy[i];
        int newx = x + dx[i];
        if(0 <= newy && newy < h && 0 <= newx && newx < w && f[newy][newx] == false && c[newy][newx] != '#'){
            dfs(newy, newx);
        }
    }
    return;
}

int main(){
    while(cin >> w >> h, w, h){
        for(int i = 0; i < h; i++){
            for(int j = 0; j < w; j++){
                cin >> c[i][j];
                if(c[i][j] == '@'){
                    sy = i;
                    sx = j;
                }
                f[i][j] = false;
            }
        }
        ans = 0;
        dfs(sy, sx);
        cout << ans << endl;
    }
}