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<vector>
#include<algorithm>
#include<string>
#include<cmath>
#include<sstream>
#include<cstdio>
#include<ctime>
#include<utility>
#include<cstdlib>
#include<iterator>
#include<queue>
#include<stack>
using namespace std;
typedef long long llong;
typedef unsigned long long ullong;
typedef unsigned int uint;
#define int long long
const int INF = 9223372036854775807;


namespace G {
}
//vector<vector<char>> map;
//vector<vector<bool>> flag;
int w, h;
int ans = 0;
int X, Y; //@?????????
queue<pair<int, int>> que;

/*
void check() {
	auto xy = que.front();
	que.pop();
	int x = xy.first;
	int y = xy.second;
	if (!flag[x][y]) {
		ans += 1;
		flag[x][y] = true;
	}
	else return;
	
	if ((map[x + 1][y] == '.') && (!flag[x + 1][y])) que.push(pair<int, int>(x + 1, y));
	if ((map[x - 1][y] == '.') && (!flag[x - 1][y])) que.push(pair<int, int>(x - 1, y));
	if ((map[x][y + 1] == '.') && (!flag[x][y + 1])) que.push(pair<int, int>(x, y + 1));
	if ((map[x][y - 1] == '.') && (!flag[x][y - 1])) que.push(pair<int, int>(x, y - 1));
}
*/

void check(vector<vector<char>> &map, vector<vector<bool>> &flag) {
	auto xy = que.front();
	que.pop();
	int x = xy.first;
	int y = xy.second;
	if (!flag[x][y]) {
		ans += 1;
		flag[x][y] = true;
	}
	else return;

	if ((map[x + 1][y] == '.') && (!flag[x + 1][y])) que.push(pair<int, int>(x + 1, y));
	if ((map[x - 1][y] == '.') && (!flag[x - 1][y])) que.push(pair<int, int>(x - 1, y));
	if ((map[x][y + 1] == '.') && (!flag[x][y + 1])) que.push(pair<int, int>(x, y + 1));
	if ((map[x][y - 1] == '.') && (!flag[x][y - 1])) que.push(pair<int, int>(x, y - 1));
}



signed main() {
	cin >> w >> h;
	while ((w != 0) && (h != 0)) {
		vector<vector<char>> map;
		vector<vector<bool>> flag;
		//cout << "w=" << w << " h=" << h << endl << endl;
		map = vector<vector<char>>(h + 2, vector<char>(w + 2,'#'));
		flag = vector<vector<bool>>(h + 2, vector<bool>(w + 2,true));
		for (int i = 1; i < h + 1; ++i) {
			for (int j = 1 ; j < w + 1; ++j) {
				cin >> map[i][j];
				flag[i][j] = false;
				if (map[i][j] == '@') {
					X = j;
					Y = i;
				}
			}
		}

		//cout << "map=" << map.size() << " map1=" << map[0].size() << endl;
		//cout << "X=" << X << " Y=" << Y << endl;

		ans = 0;
		while (!que.empty()) que.pop();
		que.push(pair<int,int>(Y,X));
		while (!que.empty()) {
			check(map,flag);
		}


		/*
		for (auto ngo : map) {
			for (auto ngoo : ngo) {
				cout << ngoo;
			}cout << endl;
		}
		cin >> w >> h;
		*/
		cout << ans << endl;
		cin >> w >> h;
	}
	return 0;
}

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

#define REP(i,a,b) for(int i=int(a);i<int(b);i++)

using namespace std;

typedef long long int lli;
typedef pair<int,int> pii;

int main () {
    int W, H;
    while (cin >> H >> W, W + H) {
        vector<vector<char> > map(W + 2, vector<char>(H + 2, '#'));
        int sx, sy;
        REP (i, 1, W + 1) {
            REP (j, 1, H + 1) {
                cin >> map[i][j];
                if (map[i][j] == '@') {
                    sx = j;
                    sy = i;
                }
            }
        }
        queue<pii> q;
        q.push(make_pair(sx, sy));
        int dx[4] = {0, 1, 0, -1};
        int dy[4] = {1, 0, -1, 0};
        vector<vector<bool> > check(W + 2, vector<bool>(H + 2, false));
        while (q.size()) {
            pii qq = q.front();
            q.pop();
            if (check[qq.second][qq.first]) continue;
            check[qq.second][qq.first] = true;
            REP (k, 0, 4) {
                int xx = qq.first + dx[k];
                int yy = qq.second + dy[k];
                if (map[yy][xx] == '#') continue;
                q.push(make_pair(xx, yy));
            }
        }
        int ans = 0;
        REP (i, 1, W + 1) REP (j, 1, H + 1) ans += check[i][j];
        cout << ans << endl;
    }

    return 0;
}