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 <stdio.h>
#include <algorithm>
#include <vector>
#include <queue>
#include <numeric>
#include <string>
#include <string.h>
#include <map>
#include <set>
#include <functional>
#include <complex>

using namespace std;

const int INF = (1<<30) - 1;

int dw[] = {0, 0, 1, -1};
int dh[] = {1, -1, 0, 0};

int dfs(vector<string>& field, int h, int w){
    int H = field.size();
    int W = field[0].size();
    
    field[h][w] = 'a';
    int res = 1;
    
    for(int i=0; i<4; i++){
        int nh = h + dh[i];
        int nw = w + dw[i];
        if(nh<0 || H<=nh || nw<0 || W<=nw || field[nh][nw] != '.')
            continue;
        res += dfs(field, nh, nw);
    }
    return res;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int W, H;
    while(cin>>W>>H, W|H){
        vector<string> field(H);
        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] == '@'){
                    cout << dfs(field, i, j) << endl;
                    break;
                }
            }
        }
    }
    
    return 0;
}

C++: #include<iostream>
#include<string>
using namespace std;
int x[21][21],ans=0,w,h;
string d[21];
void f(int i,int j){
	ans++;
	x[i][j]=1;
	if(i+1<h){
		if(x[i+1][j]==0 && d[i+1][j]=='.')
		f(i+1,j);
	}
	if(i-1>=0){
		if(x[i-1][j]==0 && d[i-1][j]=='.')
		f(i-1,j);
	}
	if(j+1<w){
		if(x[i][j+1]==0 && d[i][j+1]=='.')
		f(i,j+1);
	}
	if(j-1>=0){
		if(x[i][j-1]==0 && d[i][j-1]=='.')
		f(i,j-1);
	}
}
int main(){
	while(cin>>w>>h,w){
		int a,b;
		ans=0;
		for(int i=0;i<21;i++){
			for(int j=0;j<21;j++){
				x[i][j]=0;
			}
		}
		for(int i=0;i<h;i++){
			cin>>d[i];
			for(int j=0;j<w;j++){
				if(d[i][j]=='@'){
					a=j;
					b=i;
				}
			}
		}
		f(b,a);
		cout<<ans<<endl;
	}
	return 0;
}

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;
}