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 <stdio.h>
#include <algorithm>
#include <vector>
#include <queue>
#include <numeric>
#include <string>
#include <string.h>
#include <map>
#include <set>
#include <functional>
#include <complex>
#include <stack>
#include <tuple>
#include <array>
using namespace std;

#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#define ALL(u) begin(u),end(u)
#define PB push_back
#define LE(n,m) ((n) < (m) + EPS)
#define GE(n,m) ((n) + EPS > (m))
#define EQ(n,m) (abs((n)-(m)) < EPS)

typedef long long int ll;
const int INF = (1<<30) - 1;
const double EPS = 1e-9;
const int MOD = 1000000007;

int W, H;
int dw[] = {0, 0, 1, -1};
int dh[] = {1, -1, 0, 0};

int dfs(vector<string> &field, int h, int w){
    field[h][w] = '#';
    
    int res = 1;
    for(int i=0; i<4; i++){
        int nh = h + dh[i];
        int nw = w + dw[i];
        if(0<=nh && nh<H && 0<=nw && nw<W && field[nh][nw]=='.'){
            res += dfs(field, nh, nw);
        }
    }
    return res;
}

//g++ -std=c++0x -msse4.2 -O3
//#include <bits/stdc++.h>
int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    //cout.precision(16);
    //cout.setf(ios::fixed);
    
    while(cin >> W >> H, W|H){
        vector<string> field(H);
        for(auto &l: field)
            cin >> l;
        
        for(int i=0; i<H; i++)
            for(int j=0; j<W; j++) if(field[i][j] == '@')
                cout << dfs(field, i, j) << endl;
    }
    
    
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;char n[100][100];int k[100][100],l[100][100],sum,H,W,i,j,m,s;
#define A(B,C) if(k[B][C]!=1){l[B][C]=1;}
main(){while(true){s=0;memset(l,0,sizeof(l));cin>>W>>H;if(W==0){break;}for(i=1;i<=H;i++){for(j=1;j<=W;j++){cin>>n[i][j];if(n[i][j]=='.'){k[i][j]=0;}else if(n[i][j]=='@'){k[i][j]=2;l[i][j]=1;}else{k[i][j]=1;}}}for(i=0;i<2500;i++){for(j=1;j<=H;j++){for(m=1;m<=W;m++){if(l[j][m]==1){A(j-1,m);A(j+1,m);A(j,m-1);A(j,m+1);}}}}for(j=1;j<=H;j++){for(m=1;m<=W;m++){if(l[j][m]==1){s++;}}}cout<<s<<endl;}}

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

int moved = 0;
int w, h;
char myMap[20][20];

bool safe(int x, int y){
    return x < h && y < w;
}

void move(int x, int y){
    myMap[x][y] = '#';
    moved++;
    if (myMap[x+1][y] == '.' && safe(x+1, y)) move(x+1, y);
    if (myMap[x-1][y] == '.' && safe(x-1, y)) move(x-1, y);
    if (myMap[x][y+1] == '.' && safe(x, y+1)) move(x, y+1);
    if (myMap[x][y-1] == '.' && safe(x, y-1)) move(x, y-1);
    return;
}

int main(){
    while (1){
	cin >> w >> h;
	if (w == 0 && h == 0) break;

	moved = 0;
	int initX, initY;
	
	for (int i = 0; i < h; i++){
	    for (int j = 0; j < w; j++){
		cin >> myMap[i][j];
		if (myMap[i][j] == '@'){
		    initX = i; initY = j;
		}
	    }
	}
	
	move(initX, initY);
	
	cout << moved << endl;
    }
}

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

typedef pair<int, int> P;

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

int sx, sy;
int ans;
int w, h;
vector<string> ma;

void BFS(){
	queue<P> que;
	que.push(P(sy, sx));
	ans++;

	while(que.size()){
		P p = que.front(); que.pop();

		for(int i=0; i<4; i++){
			int ny = p.first + dy[i];
			int nx = p.second + dx[i];

			if(ny >= 0 && ny < h && nx >= 0 && nx < w && ma[ny][nx] == '.'){
				ans++;
				ma[ny][nx] = '#';
				que.push(P(ny, nx));
			}
		}

	}

}

int main(void){
	while(1){
		ma.clear();
		ans = 0;
		cin >> w >> h;
		if(w == 0 && h == 0) break;

		for(int i=0; i<h; i++){
			string tmp;
			cin >> tmp;

			ma.push_back(tmp);
		}

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

			}
		}

		BFS();

		cout << ans << endl;


	}


	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

#define REP(i, n) for(int i = 0;i < (n);i++)
#define FOR(i, a, b) for(int i = (a);i < (b);i++)
#define all(v) v.begin(),v.end()

using int64 = long long;

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

int main(void){
	int32 H, W;
	while(cin >> H >> W && H && W){
		vector<string> f(H+2, string(W+2, '#'));
		int sy, sx;
		REP(i, H){
			cin >> f[i+1];
			REP(j, W+2)
				if(f[i+1][j] == '@'){
					sy = i+1;
					sx = j;
				}
			f[i+1][0] = f[i+1][W+1] = '#';
		}
		bool visited[30][30] = {};
		int cnt = 0;
		
		auto dfs = [&](int y, int x){
			visited[y][x] = 1;
			cnt++;
			REP(i, 4){
				int yy = y+dy[i], xx = x+dx[i];
				if(f[yy][xx] != '#' && !visited[yy][xx]){
					dfs(yy, xx);
				}
			}
		};
		dfs(sy, sx);
		cout << cnt << endl;
	}
}

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

#define rep(i,n) for(int i=0;i<n;i++)
#define REP(n) rep(i,n)
#define all(n) n.begin(),n.end()

const int MAXW = 25, MAXH = 25;
const char B = '.', R = '#';
char tile[MAXW][MAXH];
int dx[] = {-1, 1, 0, 0};
int dy[] = {0, 0, -1, 1};

int solve(int x, int y)
{
    int ans = 1;

    REP(4)if(tile[x + dx[i]][y + dy[i]] == B)
    {
        tile[x + dx[i]][y + dy[i]] = R;
        ans += solve(x + dx[i], y + dy[i]);
    }

    return ans;
}

int main()
{
    int w, h;
    while(cin >> w >> h && ( w || h))
    {
        memset(tile, R, sizeof(tile));

        int x, y; char t;
        rep(j, h)rep(i, w)
        {
            cin >> tile[i + 1][j + 1];
            if(tile[i + 1][j + 1] == '@'){ x = i + 1; y = j + 1; tile[i + 1][j + 1] = R;}
        }
            cout << solve(x, y) << endl;
    }

    return 0;
}