Go: package main

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

var strByColor = map[Color]string{
	ColorBlack: "black",
	ColorRed:   "red",
}

func (c Color) String() string {
	return strByColor[c]
}

type Node struct {
	ID      int
	X, Y    int
	Color   Color
	NextIDs map[int]struct{}
}

func (n *Node) String() string {
	d, _ := json.Marshal(n)
	return string(d)
}

type Map struct {
	Width, Height int
	Nodes         map[int]*Node
}

func (m *Map) GetByID(id int) *Node {
	return m.Nodes[id]
}

func (m *Map) Get(x, y int) *Node {
	return m.Nodes[m.getID(x, y)]
}

func (m *Map) SetColor(x, y int, c Color) {
	m.Nodes[m.getID(x, y)].Color = c
}

func (m *Map) getID(x, y int) int {
	return x + y*m.Width
}

func (m *Map) init() {
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			id := m.getID(x, y)
			m.Nodes[id] = &Node{ID: id, X: x, Y: y}
		}
	}
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			nextIDs := map[int]struct{}{}
			if x > 1 {
				nextIDs[m.getID(x-1, y)] = struct{}{}
			}
			if x < m.Width {
				nextIDs[m.getID(x+1, y)] = struct{}{}
			}
			if y > 1 {
				nextIDs[m.getID(x, y-1)] = struct{}{}
			}
			if y < m.Height {
				nextIDs[m.getID(x, y+1)] = struct{}{}
			}
			m.Get(x, y).NextIDs = nextIDs
		}
	}
}

func NewMap(w, h int) *Map {
	m := &Map{Width: w, Height: h, Nodes: map[int]*Node{}}
	m.init()
	return m
}

type Searcher struct {
	Map        *Map
	queue      []*Node
	touchedIDs map[int]struct{}
	visitedIDs map[int]struct{}
}

func NewSearcher(m *Map) *Searcher {
	return &Searcher{
		Map:        m,
		queue:      []*Node{},
		touchedIDs: map[int]struct{}{},
		visitedIDs: map[int]struct{}{},
	}
}

func (s *Searcher) Search(start *Node) {
	s.enqueue(start)

	for {
		if len(s.queue) == 0 {
			break
		}
		for nextID := range s.dequeue().NextIDs {
			n := s.Map.GetByID(nextID)
			if n.Color == ColorRed || s.hasTouched(n) {
				continue
			}
			s.enqueue(n)
		}
	}
}

func (s *Searcher) VisitedNodeCount() int {
	return len(s.visitedIDs)
}

func (s *Searcher) hasTouched(n *Node) bool {
	_, ok := s.touchedIDs[n.ID]
	return ok
}

func (s *Searcher) enqueue(n *Node) {
	s.queue = append(s.queue, n)
	s.touchedIDs[n.ID] = struct{}{}
}

func (s *Searcher) dequeue() *Node {
	n := s.queue[0]
	s.queue = s.queue[1:]
	s.visitedIDs[n.ID] = struct{}{}
	return n
}

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])

		m := NewMap(w, h)

		// Set colors
		var start *Node
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					m.SetColor(i+1, y, ColorBlack)
					if c == '@' {
						start = m.Get(i+1, y)
					}
				case '#':
					m.SetColor(i+1, y, ColorRed)
				}
			}
		}

		// Search
		s := NewSearcher(m)
		s.Search(start)
		fmt.Println(s.VisitedNodeCount())
	}
}

C++: #include<iostream>
#include<algorithm>
#include<memory>
#include<cstdlib>

using namespace std;
int W,H;
void solve(int,int);
char dfs[20][20];
int d_x[]={1,0,-1,0},d_y[]={0,1,0,-1};

int main(){
	
	while(1){
		int ans=0;
		cin>>W>>H;
		if(W==0&&H==0)	break;

		int dx,dy;
		for(int i=0;i<H;i++){
			for(int j=0;j<W;j++){
				cin>>dfs[j][i];
				if(dfs[j][i]=='@'){
					dx=j;
					dy=i;
				}
			}
		}
		solve(dx,dy);
		for(int i=0;i<H;i++)
			for(int j=0;j<W;j++)
				if(dfs[j][i]=='T')
					ans++;
		cout<<ans<<endl;
		memset(dfs,0,sizeof(dfs));
	}
}

void solve(int x,int y){
	dfs[x][y]='T';
	
	for(int j=0;j<4;j++){
		int mx=x+d_x[j],my=y+d_y[j];
		if(mx<0||my<0||mx>W||my>H)	continue;
		if(dfs[mx][my]=='.')
			solve(mx,my);
	}
	return ;
}

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

#define rep(i,j) for(int (i)=0;(i)<(int)(j);++(i))

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

int main() {
    int w, h;
    char f[20][20];
    while (cin >> w >> h && w && h) {
        int c = 1;
        queue<pair<int, int>> q;
        rep(y,h) {
            rep(x,w) {
                cin >> f[y][x];
                if (f[y][x] == '@') {
                    q.push({x, y});
                    f[y][x] = 'x';
                }
            }
        }
        while (q.size()) {
            pair<int, int> p = q.front(); q.pop();
            for (int i = 0; i < 4; ++i) {
                int nx = p.first+dx[i];
                int ny = p.second+dy[i];
                if (0 <= nx && nx < w && 0 <= ny && ny < h && f[ny][nx] == '.') {
                    ++c;
                    q.push({nx, ny});
                    f[ny][nx] = 'x';
                }
            }
        }
        cout << c << endl;
    }
}

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

string t[21];


void dfs(int y,int x){
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(y+dy[k],x+dx[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",&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(sy,sx);
cnt =0;

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

cout <<endl<<cnt;;


}

return 0;
}