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 <algorithm>
#include <cstdio>
#include <stack>

using namespace std;

typedef struct _TAG_POSITION {
	int x;
	int y;
} POSITION;

int main(){
	int W, H;
	stack<POSITION> S;
	POSITION p;
	while(cin >> W >> H, W || H){
		char board[H+2][W+2];
		int ret = 0;
		for(int i = 0; i < H+2; i++){
			for(int j = 0; j < W+2; j++){
				(((i == 0) || (i == H + 1)) || ((j == 0) || (j == W + 1))) && (board[i][j] = '#');
				(0 < i && i < H + 1) && (0 < j && j < W + 1) && cin >> board[i][j];
				if(board[i][j] == '@') { S.push({i,j}); ret++; }
			}
		}

		while(!S.empty()){
			p = S.top();
			S.pop();
			if(board[p.x + 1][p.y] == '.') { board[p.x + 1][p.y] = '@'; ret++; S.push({p.x + 1,p.y}); }
			if(board[p.x - 1][p.y] == '.') { board[p.x - 1][p.y] = '@'; ret++; S.push({p.x - 1,p.y}); }
			if(board[p.x][p.y + 1] == '.') { board[p.x][p.y + 1] = '@'; ret++; S.push({p.x,p.y + 1}); }
			if(board[p.x][p.y - 1] == '.') { board[p.x][p.y - 1] = '@'; ret++; S.push({p.x,p.y - 1}); }
		}
		cout << ret << "\n";
	}
	return 0;
}

C++: #include<iostream>
#include<vector>
#include<string>


int main(){
    std::vector<int> row;
    int prev_col, width, height, max = 0, last = 0;
    bool connect_prev_row;
    
    while(true){
        std::cin >> width >> height;
        if( width == 0 && height == 0 ) break;
        row.resize(width+1);
        for( int i = 0; i <= width; i++ ) row[i] = 0;

        std::string line;
        for( int h = 0; h < height; h++ ){
            std::cin >> line;
            connect_prev_row = false;
            for(std::string::size_type i = 0; i < line.size(); i++ ){
                if( line[i] == '.' ){
                    if( connect_prev_row ){
                        if( row[i] > 0 ) row[i+1] = row[i] + 1;
                        else row[i+1] = row[i] - 1;
                    } else {
                        connect_prev_row = true;
                        if( row[i] <= 0 && row[i+1] <= 0 ) row[i+1] = row[i+1] + row[i] - 1;
                        else row[i+1] = std::abs(row[i+1]) + std::abs(row[i]) + 1;
                    }
                } else if( line[i] == '@' ) {
                    if ( connect_prev_row ) row[i+1] = - row[i] + 1;
                    else {
                        connect_prev_row = true;
                        row[i+1] = - row[i+1] - row[i] + 1;
                    }
                } else {
                    row[i+1] = 0;
                    connect_prev_row = false;
                }
                max = max > row[i+1] ? max : row[i+1];
            }
            last = 0;
            for( int i = line.size(); i > 0; i-- ) {
                if( row[i] == 0 ) last = 0;
                else if( row[i] > 0 ){
                    if(  last == 0 ) last = row[i];
                    else row[i] = last;
                } else {
                    if(  last == 0 ) last = row[i];
                    else row[i] = last;
                } 
            }
        }
            
    }
    return 0;
}

C++: #include <iostream>
#include <queue>
#include <string>

using namespace std;
#define NINF -999999
typedef pair<int, int> P;
int field[21][21];
int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, 1, 0, -1};

void solve()
{
	int W, H;
	while(cin >> W >> H)
	{
		if(!W && !H)
		{
			break;
		}
		string tile[21];
		for(int i = 0; i < H; ++i)
		{
			cin >> tile[i];
		}
		queue<P> Que;
		for(int i = 0; i < H; ++i)
		{
			for(int j = 0; j < W; ++j)
			{
				if(tile[i][j] == '@')
				{
					P p;
					p.first = i;
					p.second = j;
					Que.push(p);
					field[i][j] = 0;
				}
				else
				{
					field[i][j] = NINF;
				}
			}
		}
		int count = 1;
		queue<P> NextQue;
		while(!Que.empty())
		{
			for(int i = 0; i < 4; ++i)
			{
				P p;
				p.first = Que.front().first + dy[i];
				p.second = Que.front().second + dx[i];
				
				if(p.first < 0 || p.first >= H || p.second < 0 || p.second >= W)
				{
					continue;
				}
				else if((tile[p.first][p.second] != '#') && (field[p.first][p.second] == NINF))
				{
					field[p.first][p.second] = field[Que.front().first][Que.front().second] + 1;
					NextQue.push(p);
					count++;
				}
			}
			Que.pop();
			if(Que.empty())
			{
				while(!NextQue.empty())
				{
					Que.push(NextQue.front());
					NextQue.pop();
				}
			}
		}
		cout << count << endl;
	}
}

int main()
{
	solve();
	return(0);
}

C++: #include <iostream>
#include <string>
#include <vector>
using namespace std;
int W, H, sx, sy, cnt;
vector<int> dxy;
string field[20];

void fun(int y, int x){
    field[y][x] = '#';
    cnt++;
    for(int i=0; i<4; ++i){
        int ny = y + dxy[i+1],
            nx = x + dxy[i];
        if(ny < 0 || ny >= H ||nx < 0 || nx >= W || field[ny][nx] != '.') continue;
        fun(ny, nx);
    }
}
    
int main(){
    dxy.push_back(1);
    dxy.push_back(0);
    dxy.push_back(-1);
    dxy.push_back(0);
    dxy.push_back(1);
    while(cin >> W >> H, (W||H)){
        cnt = 0;
        string str;
        cin.ignore();
        for(int i=0; i<H; ++i){
            cin >> field[i];
        }

        for(int i=0; i<H; ++i){
            for(int j=0; j<W; ++j){
                if(field[i][j] == '@'){
                    sx = j; sy = i;
                }
            }
        }
        
        fun(sy, sx);
        cout << cnt << endl;
    }
    return 0;
}