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 <algorithm>
#include <iostream>
#include <vector>
#include <math.h>
#include <set>
#include <map>
#include <string>
#include <stack>
#include <queue>
#include <iomanip>
#include <numeric>
#include <tuple>
#define _USE_MATH_DEFINES
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> plglg;
typedef tuple<int, int, int> tiii;
typedef tuple<ll, ll, ll> tlglglg;
double pi = 3.141592653589793;
ll mod = 1000000007;
int intmax = 2147483647;
int intmin = -2147483648;
ll llmax = 9223372036854775807;
ll llmin = -9223372036854775807;

struct edge {
    int to, cost;
};

int H, W;
bool marked[100][100];
int sum;

void BFS(pii st) {
    queue<pii> que;
    que.push(st);
    sum++;
    marked[st.first][st.second] = false;
    while (que.size() > 0) {
        pii base = que.front();
        que.pop();
        pii mv[4] = {pii(-1, 0), pii(1, 0), pii(0, -1), pii(0, 1)};
        int basex = base.first;
        int basey = base.second;
        for (int i = 0; i < 4; i++) {
            int x = basex + mv[i].first;
            int y = basey + mv[i].second;
            if (x >= 0 && x < H && y >= 0 && y < W) {
                if (marked[x][y]) {
                    que.push(pii(x, y));
                    sum++;
                    marked[x][y] = false;
                }
            }
        }
    }
}

int main() {
    while (true) {
        cin >> W >> H;
        if (W == 0) {
            break;
        }
        for (int i = 0; i < H; i++) {
            fill(marked[i], marked[i] + W, true);
        }
        pii st;
        for (int i = 0; i < H; i++) {
            string s;
            cin >> s;
            for (int j = 0; j < W; j++) {
                if (s[j] == '#') {
                    marked[i][j] = false;
                } else if (s[j] == '@') {
                    st = pii(i, j);
                }
            }
        }
        sum = 0;
        BFS(st);
        cout << sum << endl;
    }
}

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

int n, m;
char field[100][100];

int dfs(int x, int y){
	field[x][y] = '.';

	for(int dx = -1; dx <= 1; dx++){
		for(int dy = -1; dy <= 1; dy++){
			int nx = x + dx, ny = y + dy;

			if(0 <= nx && nx < n && 0 <= ny && ny < m && field[nx][ny] == 'W') dfs(nx, ny);
		}
	}
	return 0;
}

int main(){
	while(1){
		scanf("%d %d", &n, &m);
		if(n == 0 && m == 0) break;
		for(int i=0; i<100; i++){
			for(int j=0; j<100; j++){
				field[i][j] = '.';
			}
		}
		for(int i=0; i<n; i++){
			scanf("%s", field[i]);
		}
		int ans = 0;
		for(int i=0; i<n; i++){
			for(int j=0; j<m; j++){
				if(field[i][j] == 'W'){
					dfs(i, j);
					ans++;
				}
			}
		}
		printf("%d\n", ans);
	}
	return 0;
}

C++: #include <iostream>
#include <fstream>
#include <algorithm>
#include <vector>
#include <string>
#include <functional>
#include <array>
#include <stack>
#include <queue>
#include <chrono>
#include <numeric>

#pragma warning(disable:4996)
#define REP(i,n) for(int i = 0 ; i < n ; ++i)
typedef long long Int;

using namespace std;

struct point{
	int x;
	int y;
	point(int _x, int _y) :x(_x), y(_y){}
};

char tile[20][20];
stack<point> st;

int main()
{
	cin.tie(0); ios::sync_with_stdio(false); 
	fstream fs("input.txt");
	////////////ここから開始///////////

	
	int x(-1), y(-1);
	while (true){
		REP(i, 20)REP(j, 20)tile[i][j] = '#';
		cin >> x >> y;
		if (x == 0 && y == 0)
			break;
		//fs >> x >> y;
		REP(i, y){
			REP(j, x){
				cin >> tile[i][j];
				//fs >> tile[i][j];
			}
		}

		REP(i, y){
			REP(j, x){
				if (tile[i][j] == '@')
					st.emplace(i, j);
			}
		}

		int dx[]{0, 1, 0, -1};
		int dy[]{1, 0, -1, 0};
		int ans(1);

		while (!st.empty()){
			point now = st.top();
			st.pop();
			REP(i, 4){
				int x1 = now.x + dx[i];
				int y1 = now.y + dy[i];
				if (x1 > 19 || x1 < 0 || y1 > 19 || y1 < 0)
					continue;

				if (tile[x1][y1] == '.'){
					st.emplace(x1, y1);
					tile[x1][y1] = '#';
					++ans;
				}
			}
		}

		fs << ans << endl;
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
int a=0;
int box[22][22]={{}};
int  kazoe(int p,int P,int H,int W){
  int i,I;
  a++;
  box[p][P]=0;
  if(box[p][P+1]==1) kazoe(p,P+1,H,W);
  if(box[p][P-1]==1) kazoe(p,P-1,H,W);
  if(box[p-1][P]==1) kazoe(p-1,P,H,W);
  if(box[p+1][P]==1) kazoe(p+1,P,H,W);
  else {
  return a;
  }
}
int main(){
  int i,I,H,W,p,P;
  
  char v;
  while(1){
  cin>>W>>H;
  for(i=0;i<=H+1;i++){
    for(I=0;I<=W+1;I++){
      box[i][I]=0;
    }
  }
  if(W==0&&H==0) break;
  for(i=1;i<=H;i++){
    for(I=1;I<=W;I++){
      cin>>v;
      if(v=='.') box[i][I]=1;
      if(v=='#') box[i][I]=2;
      if(v=='@') {
	box[i][I]=0;
	p=i;
	P=I;
      }
    }
  }
  cout<<kazoe(p,P,H,W)<<endl;
  a=0;
  }
  return 0;
}