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++: /*
 * http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=1130&lang=jp
 */


#include <iostream>
#include <limits>

constexpr size_t MAX_HEIGHT = 20;
const char START_TILE = '@';
const char PASSABLE_TILE = '.';
const char PROHIBITED_TILE = '#';
const char PASSED_TILE = 'P';

bool is_wall(std::string world[MAX_HEIGHT], size_t x, size_t y) {
	char tile = world[y][x];
	return tile == PROHIBITED_TILE || tile == PASSED_TILE;
}

bool valid(std::string world[MAX_HEIGHT], size_t width, size_t height, size_t x, size_t y) {
	return x < width && y < height // x and y is unsigned
		   && !is_wall(world, x, y);
}

int walk(std::string world[MAX_HEIGHT], size_t width, size_t height, size_t x, size_t y) {
	if (!valid(world, width, height, x, y)) {
		return 0; // ????????????????????\?????????????£?or??¢?´¢??????
	}
	int total = 1; // ????????????????????\?????????1?????¨??????????????????
	world[y][x] = PASSED_TILE; // ??????????????¨????????????????????¨?????????????????????????????????
	// ?????????????????§??????????????????valid???????????????walk????????§
	total += walk(world, width, height, x - 1, y);
	total += walk(world, width, height, x + 1, y);
	total += walk(world, width, height, x, y - 1);
	total += walk(world, width, height, x, y + 1);
	return total;
}

void process(size_t width, size_t height) {
	std::string world[MAX_HEIGHT];
	size_t x = std::numeric_limits<size_t>::max(), y = std::numeric_limits<size_t>::max();
	for (size_t i = 0; i < height; ++i) {
		std::cin >> world[i];
		size_t start_index = world[i].find(START_TILE);
		if (start_index != std::string::npos) {
			x = start_index;
			y = i;
		}
	}

	std::cout << walk(world, width, height, x, y) << std::endl;
}

int main() {
	size_t width, height;
	while (true) {
		std::cin >> width >> height;
		if (width == 0 || height == 0) {
			break;
		}
		else {
			process(width, height);
		}
	}
}

C++: #include <bits/stdc++.h>

using namespace std;

class P
{
public:
	int x,y;
	P(int X,int Y)
	{
		x=X;
		y=Y;
	}
};

int main()
{
	stack<P>st;
	int n,m;
	int numX[4]={0,1,0,-1};
	int numY[4]={1,0,-1,0};

	while(cin>>n>>m && n)
	{
		int flag[500][500]={};
		int count=0;
		string in;
		char set[500][500]={};

		for(int i=1;i<=m;i++)
		{
			for(int j=1;j<=n;j++)
			{
				cin >> set[i][j];
			}
		}

		for(int i=1;i<=m;i++)
		{
			for(int j=1;j<=n;j++)
			{
				if(set[i][j]=='@')
				{
					st.push(P(j,i));
					flag[i][j]=1;
				}
			}
		}

		int xx,yy;

		while(!st.empty())
		{
			count++;
			xx=st.top().x;
			yy=st.top().y;
			st.pop();
			for(int i=0;i<4;i++)
			{
				if(set[numY[i]+yy][numX[i]+xx]=='.' && flag[numY[i]+yy][numX[i]+xx]==0)
				{
					st.push(P(numX[i]+xx,numY[i]+yy));
					flag[numY[i]+yy][numX[i]+xx]=1;
				}
			}

		}
		cout << count << endl;
	}

    return 0;
}

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

#define INF 1.1e9
#define LINF 1.1e18
#define FOR(i,a,b) for (int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define BIT(x,n) bitset<n>(x)
#define PI 3.14159265358979323846

typedef long long ll;
typedef pair<int,int> P;
typedef pair<ll,P> PP;

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

int sx,sy,h,w;
string fld[20];
int dx[]={1,-1,0,0},dy[]={0,0,1,-1};

int bfs() {
	bool used[h][w]={};
	int cnt=1;
	queue<P> q;
	used[sy][sx]=true;
	q.push(P(sx,sy));
	while(!q.empty()) {
		P p=q.front();q.pop();
		REP(i,4) {
			int nx=p.fi+dx[i],ny=p.se+dy[i];
			if(nx<0||nx>=w||ny<0||ny>=h||used[ny][nx]||fld[ny][nx]=='#') continue;
			used[ny][nx]=true;
			cnt++;
			q.push(P(nx,ny));
		}
	}
	return cnt;
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	while(true) {
		cin>>w>>h;
		REP(i,h) {
			cin>>fld[i];
			REP(j,w) if(fld[i][j]=='@') sx=j,sy=i;
		}
		cout<<bfs()<<endl;
	}

	return 0;
}

C++: #include<iostream>
#include<string>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
using namespace std;
typedef long long ll;
const ll MOD = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
typedef pair<int, int> P;
typedef unsigned int ui;
int main(){
	int w, h;
	string s;
	int i, j;
	while (cin >> w >> h) {
		if (w == 0)break;
		char tile[22][22]; int used[22][22];
		for (i = 0; i < 22; i++) {
			for (j = 0; j < 22; j++) {
				used[i][j] = 0; tile[i][j] = '#';
			}
		}
		vector<int> q1,q2;
		for (j = 1; j <= h; j++) {
			cin >> s;
			for (i = 1; i <= w; i++) {
				tile[i][j] = s[i - 1];
				if (s[i - 1] == '@') {
					q1.push_back(i); q2.push_back(j);
				}
			}
		}
		while (!q1.empty()) {
			int x = q1[0]; int y = q2[0]; used[x][y]=1;
			q1.erase(q1.begin() + 0); q2.erase(q2.begin() + 0);
			if (used[x - 1][y] == 0 && tile[x - 1][y]=='.') {
				q1.push_back(x - 1); q2.push_back(y);
			}
			if (used[x + 1][y] == 0 && tile[x + 1][y] == '.') {
				q1.push_back(x + 1); q2.push_back(y);
			}
			if (used[x][y-1] == 0 && tile[x][y-1] == '.') {
				q1.push_back(x); q2.push_back(y-1);
			}
			if (used[x][y+1] == 0 && tile[x][y+1] == '.') {
				q1.push_back(x); q2.push_back(y+1);
			}
		}
		int count = 0;
		for (i = 0; i < 22; i++) {
			for (j = 0; j < 22; j++) {
				if (used[i][j] == 1)count++;
			}
		}
		cout << count << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<string.h>
#define rep(i,n) for(int i =0;i<n;i++)
using namespace std;

string t[21];


void dfs(int x,int y){
int dx[4] = {1,-1, 0, 0};
int dy[4] = {0, 0, 1, -1};


	if((x<0||y<0)||t[x][y]!='.') return;
	t[x][y]='@';	
	rep(k,4){
	dfs(x+dx[k],y+dy[k]);
}
return;
}


int main(void){

	int w,h,sx,sy,cnt;
	rep(i,20)rep(j,20){
t[i][j]=='#';	
	}

while(true){
scanf("%d %d/n",&w,&h);
if(!w&&!h) {break;}
rep(i,h){
cin >> t[i];
}
rep(i,h)rep(j,w){

	if(t[i][j]=='@'){	
	sx =i;
	sy =j;

t[i][j]='.';
	}

}
dfs(sx,sy);
cnt =0;

rep(i,h){ 
	rep(j,w)if(t[i][j]=='@') cnt++;	
}

cout <<cnt <<endl;;


}

return 0;
}