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 <vector>
#include <iomanip>
#include <map>
#include <queue>

static const int MOD = 1000000007;
using ll = long long;
using u32 = unsigned;
using namespace std;

template<class T>
constexpr T INF = ::numeric_limits<T>::max() / 32 * 15 + 208;


class UnionFind {

    int n;
public:
    vector<int> uni;
    explicit UnionFind(int n) : uni(static_cast<u32>(n), -1) , n(n){};

    int root(int a){
        if (uni[a] < 0) return a;
        else return (uni[a] = root(uni[a]));
    }

    bool unite(int a, int b) {
        a = root(a);
        b = root(b);
        if(a == b) return false;
        if(uni[a] > uni[b]) swap(a, b);
        uni[a] += uni[b];
        uni[b] = a;
        return true;
    }
};

int main() {
    int h, w;
    while(cin >> w >> h, h|w){
        UnionFind uf(h*w);
        int dx[] {1, -1, 0, 0}, dy[] {0, 0, -1, 1};
        vector<vector<int>> v(h+2, vector<int> (w+2, 0));
        int sy = 0, sx = 0;
        for (int i = 0; i < h; ++i) {
            string s;
            cin >> s;
            for (int j = 0; j < w; ++j) {
                if(s[j] == '@') sy = i, sx = j;
                if(s[j] != '#') v[i+1][j+1] = 1;
            }
        }
        for (int i = 0; i < h; ++i) {
            for (int j = 0; j < w; ++j) {
                if(v[i+1][j+1] == 0) continue;
                for (int k = 0; k < 4; ++k) {
                    if(v[i+1+dy[k]][j+1+dx[k]]){
                        uf.unite(i*w+j, (i+dy[k])*w+(j+dx[k]));
                    }
                }
            }
        }
        cout << -uf.uni[uf.root(sy*w+sx)] << "\n";
    }
    return 0;
}

C++: #include <iostream>
#include <stack>
#include <queue>
#include <fstream>
using namespace std;

int main(){
  std::ifstream in("input.txt");
  std::cin.rdbuf(in.rdbuf());
  int ans[1024] = {0};
  int dataCounter = 0;
  int i = 0;
  while(1){
    int visited[20][20] = {0};
    int data[20][20];
    int x,y;
    int w = 0;
    int h = 0;
    queue<int> xst;
    queue<int> yst;
    cin >> w;
    cin >> h;
    if(w == 0 && h == 0) break;
    for(i=0;i<h;i++){
      int inputCounter = 0;
      char d;
      while(cin >> d){
        if(d == '.')
          data[i][inputCounter] = 0;
        else if(d == '#')
          data[i][inputCounter] = 1;
        else if(d == '@'){
          data[i][inputCounter] = 2;
          x = inputCounter;
          y = i;
        }
        if(inputCounter == w-1)break;
        inputCounter++;
      }
    }
    int j=0;
    xst.push(x);
    yst.push(y);
    while(!xst.empty()){
      x = xst.front();
      y = yst.front();
      xst.pop();
      yst.pop();
      if(x < 0 || w <= x || y < 0 || h <= y)continue;
      if(visited[y][x]==1 || data[y][x] == 1)continue;
      visited[y][x] = 1;
      ans[dataCounter]++;
      xst.push(x+1);
      yst.push(y);
      xst.push(x);
      yst.push(y+1);
      xst.push(x-1);
      yst.push(y);
      xst.push(x);
      yst.push(y-1);
    }

    dataCounter++;
  }
  for(i=0;i<dataCounter;i++){
    cout << ans[i] << endl;
  }
  return 0;
}

C++: #include <iostream>
using namespace std;
int W,H;
int x[4]={0,0,1,-1};
int y[4]={1,-1,0,0};
int i,j;
int count=0;
char room[100][100];

int dfs(int, int);

int main(){
  while(1){
    int sy,sx;
    cin >> W >> H;
    if(W==0&&H==0)break;
    for(i = 0 ; i < H ; i++){
      for(j = 0 ; j < W ; j++){
	cin >> room[i][j];
	if(room[i][j] == '@'){
	  sy = i;
	  sx = j; 
	}
      }
    }
    cout << dfs(sx,sy) << endl;
  }
  return 0;
}

int dfs(int ax,int ay){
  if(ax<0||ay<0)return 0;
  if(ax>=W||ay>=H)return 0;
  if(room[ay][ax]=='#')return 0;
  room[ay][ax]='#';
  int cnt=1;
  int i;
  for(i=0;i<4;i++){
    int nx=ax+x[i];
    int ny=ay+y[i];
    cnt+=dfs(nx,ny);
  }
  return cnt;
}

C++: #include<bits/stdc++.h>
using namespace std;
char n[100][100];int k[100][100],l[100][100],sum,H,W,i,j,m,s;
#define A(B,C) if(k[B][C]!=1){l[B][C]=1;}
int main(){while(true){s=0;memset(l,0,sizeof(l));cin>>W>>H;if(W==0){break;}for(i=1;i<=H;i++){for(j=1;j<=W;j++){cin>>n[i][j];if(n[i][j]=='.'){k[i][j]=0;}else if(n[i][j]=='@'){k[i][j]=2;l[i][j]=1;}else{k[i][j]=1;}}}
for(i=0;i<2500;i++){for(j=1;j<=H;j++){for(m=1;m<=W;m++){if(l[j][m]==1){A(j-1,m);A(j+1,m);A(j,m-1);A(j,m+1);}}}}
for(j=1;j<=H;j++){for(m=1;m<=W;m++){if(l[j][m]==1){s++;}}}cout<<s<<endl;}}

C++: #include <bits/stdc++.h>
using namespace std;
vector<string> s;
vector<vector<bool> > done;
int w,h;
int dy[] = {-1,1,0,0};
int dx[] = {0,0,-1,1};
int ans = 0;

void dfs(int y, int x)
{
		ans++;
        done[y][x] = true;
        for(int i=0;i<4;i++)
        {
                int ny = y + dy[i];
                int nx = x + dx[i];
                if(0<=ny && ny<h && 0<=nx && nx<w && s[ny][nx]=='.' && !done[ny][nx])
                {
                        dfs(ny,nx);
                }
        }
}
 
signed
main()
{
    while(cin >> w >> h and !(w==0 & h==0))
    {
            s = vector<string>(h);
            done = vector<vector<bool> >(h,vector<bool>(w, false));

			int sy,sx;
            for(int y=0;y<h;y++)
			{
					cin >> s[y];
					for(int x=0;x<w;x++)
					{
						if(s[y][x]=='@')
						{
								sy = y;
								sx = x;
						}
					}
			}
 			dfs(sy,sx);
            cout << ans << endl;
			ans = 0;
    }
}