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<string>
#include<stack>
using namespace std;
int w,h;
string map[40];
int vec[4][2]={{0,1},{0,-1},{-1,0},{1,0}};

struct pos{
	pos(int X,int Y){
		x=X,y=Y;
	}
	int x,y;
};

int move(int x,int y){
	int ans=0;
	map[x][y]='#';
	stack<pos> bfs;
	bfs.push(pos(x,y));
	while(!bfs.empty()){
		pos n=bfs.top();bfs.pop();
		ans++;
		for(int i=0;i<4;i++){
			if((x+vec[i][0]>=0)&&(y+vec[i][1]>=0)&&(x+vec[i][0]<h)&&(y+vec[i][1]<w)){
				if(map[x+vec[i][0]][y+vec[i][1]]=='.'){
					map[x+vec[i][0]][y+vec[i][1]]='#';
					bfs.push(pos(x+vec[i][0],y+vec[i][1]));
				}
			}
		}
		
	}
	
	
	return ans;
}

int main(){
	while(cin>>w>>h,w){
		int fx,fy;
		for(int i=0;i<h;i++){
			cin>>map[i];
		}
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(map[i][j]=='@'){
					fx=i;
					fy=j;
					break;
				}
			}
		}
		int ans=move(fx,fy);
		cout<<ans<<endl;
		
	}
}

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

int main() {
    int N, M;
    int dx[] = {1, 0, -1, 0};
    int dy[] = {0, 1, 0, -1};
    while (cin >> M >> N, N || M) {
        vector<string> v(N);
        for (auto& i: v) cin >> i;

        int sx, sy;
        for (int i = 0; i < N; i++) {
            for (int j = 0; j < M; j++) {
                if (v[i][j] == '@') sx = i, sy = j;
            }
        }

        queue<pair<int,int>> que;
        que.push(make_pair(sx, sy));

        vector<vector<int>> flag(N, vector<int>(M));
        int count = 0;
        while (que.size()) {
            auto p = que.front();
            que.pop();

            int x = p.first;
            int y = p.second;

            if (flag[x][y]) continue;

            count++;
            flag[x][y] = 1;

            for (int i = 0; i < 4; i++) {
                int nx = x + dx[i];
                int ny = y + dy[i];

                if (0 <= nx && nx < N && 0 <= ny && ny < M && v[nx][ny] == '.' && !flag[nx][ny]) {
                    que.push(make_pair(nx, ny));
                }
            }
        }

        cout << count << endl;
    }

    return 0;
}

C++: #include <stdio.h>

char m[25][25];
int cou;

void dfs(int i,int j){
  //printf("%d %d\n",i,j);///
  m[i][j]='#';
  if(m[i+1][j]=='.'){
    cou++;    
    dfs(i+1,j);
  }
  if(m[i][j+1]=='.'){
    cou++;
    dfs(i,j+1);  
  }
  if(m[i-1][j]=='.'){
    cou++;
    dfs(i-1,j);
  }
  if(m[i][j-1]=='.'){
    cou++;
    dfs(i,j-1);
  }
}

int main(){
  int a,b,i,j;
  char o;
  while(1){
    cou=0;
    scanf("%d %d%c",&a,&b,&o);
    if(a==0&&b==0)break;
    for(i=1;i<b+1;i++){
      for(j=1;j<=a+1;j++){
	scanf("%c",&m[i][j]);
      }
    }
    for(i=1;i<b+1;i++){
      for(j=1;j<=a+1;j++){
	if(m[i][j]=='@')dfs(i,j);
      }
    }
    printf("%d\n",cou+1);
    for(i=1;i<b+1;i++){
      for(j=1;j<=a+1;j++){
	m[i][j]='0';
      }
    }
  }
return 0;
}

C++: #include "bits/stdc++.h"
#include<unordered_map>
#pragma warning(disable:4996)
using namespace std;

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

struct aa {
	int y;
	int x;
	bool black;
};

int main() {
	while (1) {
		int w, h; cin >> w >> h;
		if (!w)break;
		vector<vector<int>>field(h + 2, vector<int>(w + 2, 0));
		int sx, sy;
		for (int i = 0; i < h; ++i) {
			string st; cin >> st;
			for (int j = 0; j < w; ++j) {
				if (st[j] == '.') {
					field[i + 1][j + 1] = 1;
				}
				else {
					if (st[j] == '#') {
						field[i + 1][j + 1] = 2;
					}
					else {
						sx = j + 1;
						sy = i + 1;
						field[i + 1][j + 1] = 1;
					}
				}
			}
		}
		vector<aa>que;
		que.push_back(aa{ sy, sx, true });
		int ans = 1;
		vector<vector<bool>>used(h + 2, vector<bool>(w + 2, false));
		used[sy][sx] = true;

		while (!que.empty()) {
			aa atop(que.back());
			que.pop_back();
			for (int i = 0; i < 4; ++i) {
				const int ny = atop.y + dy[i];
				const int nx = atop.x + dx[i];
				if (atop.black) {
					if (field[ny][nx] == 2) {
						if (!used[ny][nx]) {
							used[ny][nx] = true;
							ans++;
							que.push_back(aa{ ny,nx,!atop.black });
						}
					}
				}
				else {
					if (field[ny][nx] == 1) {
						if (!used[ny][nx]) {
							used[ny][nx] = true;
							ans++;
							que.push_back(aa{ ny,nx,!atop.black });
						}
					}
				}

			}
		}
		cout << ans << endl;
	}
	return 0;
}