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<bits/stdc++.h>
using namespace std;

#define rep(i,n) for(long long i = 0; i < (n); i++)
#define REP(i,s,n) for(long long i = (s); i <= (n); i++)
#define repr(i,n) for(long long i = (n - 1); i >= 0; i--)
#define REPR(i,s,n) for(long long i = (s); i >= (n); i--)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define sumvec(v) accumulate(all(v), 0LL)
#define pb push_back
#define DOUBLE fixed << setprecision(15)
#define OK cerr << "OK\n"
#define OK1 cerr << "OK1\n"
#define OK2 cerr << "OK2\n"
#define sz(s) (long long)s.size()
#define zero(x, n) setw(x) << setfill('0') << n

typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<long long> vll;
typedef vector<double> vd;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<string> vs;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef vector<vd> vvd;
typedef vector<vc> vvc;
typedef vector<vb> vvb;
typedef pair<ll,ll> P;
typedef vector<P> vP;

template<class A, class B>
ostream& operator << (ostream& ost, const pair<A, B> &p) {
    ost << "{" << p.first << ", " << p.second << "} ";
    return ost;
}

template<class T>
ostream& operator << (ostream& ost, const vector<T>&v) {
    ost << "{";
    for (int i = 0; i < (int)v.size(); i++) {
        if(i) ost << " ";
        ost << v[i];
    }
    ost << "} \n";
    return ost;
}

template<class A, class B>
ostream& operator << (ostream& ost, const map<A, B>&v) {
    ost << "{";
    for (auto p : v) {
        ost << "{" << p.first << ", " << p.second << "} ";
    }
    ost << "} ";
    return ost;
}

void debug() { cerr << "\n"; }
template<class T, class... Args>
void debug(const T &x, const Args &... args) {
    cerr << x << " ";
    debug(args...);
}

template<typename T>
istream& operator >> (istream& is, vector<T>& vec) {
    for(T& e : vec) is >> e;
    return is;
}

template<typename A, typename B>
istream& operator >> (istream& is, pair<A, B>& p) {
    is >> p.first >> p.second;
    return is;
}

template<typename T>
string join(vector<T>& vec, string sep=" ") {
    stringstream ss;
    for(int i = 0; i < (int)vec.size(); i++) {
        ss << vec[i] << (i + 1 == (int)vec.size() ? "\n" : sep);
    }
    return ss.str();
}

template<class T>
inline bool chmax(T& a, T b){ if(a < b){ a = b; return true; } return false; }

template<class T>
inline bool chmin(T& a, T b){ if(a > b){ a = b; return true; } return false; }

#define INT(...) int __VA_ARGS__; IN(__VA_ARGS__)
#define LL(...) long long __VA_ARGS__; IN(__VA_ARGS__)
#define CHR(...) char __VA_ARGS__; IN(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__; IN(__VA_ARGS__)
#define STR(...) string __VA_ARGS__; IN(__VA_ARGS__)
#define LD(...) long double __VA_ARGS__; IN(__VA_ARGS__)

void scan(int& a){ cin >> a; }
void scan(long long& a){ cin >> a; }
void scan(char& a){ cin >> a; }
void scan(double& a){ cin >> a; }
void scan(string& a){ cin >> a; }
void scan(long double& a){ cin >> a; }

void IN(){}
template<class Head, class... Tail> void IN(Head& head, Tail&... tail){ scan(head); IN(tail...); }

void YES(bool b) { cout << ((b) ? "YES\n" : "NO\n"); }
void Yes(bool b) { cout << ((b) ? "Yes\n" : "No\n"); }
void yes(bool b) { cout << ((b) ? "yes\n" : "no\n"); }
void Yay(bool b) { cout << ((b) ? "Yay!\n" : ":(\n"); }

const long long INF = ((1LL << 62) - (1LL << 31));
const long double pi = acos(-1.0);
const long long mod=1000000007;
//const long long mod=998244353;

ll powmod(ll a,ll b){ll c=1; while(b>0){ if(b&1){ c=a*c%mod; } a=a*a%mod; b>>=1; } return c;}
ll nCrmod(ll n,ll r){ll x=1,y=1; for(ll i=0;i<r;i++) {x=x*(n-i)%mod; y=y*(i+1)%mod; } return x*powmod(y,mod-2)%mod;}
ll gcd(ll a,ll b){ while(b){ ll c=b; b=a%b; a=c; } return a; }
ll lcm(ll a,ll b){ if(!a || !b) return 0; return a*b/gcd(a,b); }
vll divisor(ll x){ vll v; for(ll i=1;i*i<=x;i++)if(x%i==0){ v.pb(i);if(i*i!=x)v.pb(x/i); } sort(all(v)); return v; };
map<ll,ll> prime_factor(ll n){ map<ll,ll> m; for(ll i=2;i*i<=n;i++){ while(n%i==0){ m[i]++; n/=i; } } if(n!=1) m[n]=1; return m; }

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

int main()
{
    ll h, w;
    while(cin >> w >> h, w && h) {
        vvll a(h, vll(w));
        ll sy, sx;
        rep(i, h) {
            rep(j, w) {
                CHR(c);
                if(c == '#') a[i][j] = -1;
                if(c == '@') sy = i, sx = j;
            }
        }
        queue<P> q;
        q.push({sy, sx});
        a[sy][sx] = 1;
        ll ans = 0;
        while(!q.empty()) {
            ans++;
            ll y = q.front().first;
            ll x = q.front().second;
            q.pop();
            rep(i, 4) {
                ll ny = y + dy[i];
                ll nx = x + dx[i];
                if(ny < 0 || ny >= h || nx < 0 || nx >= w) continue;
                if(!a[ny][nx]) {
                    a[ny][nx] = 1;
                    q.push({ny, nx});
                }
            }
        }
        cout << ans << endl;
    }
    return 0;
}

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

typedef pair<int,int> P;

int w, h, sx, sy;
char t[22][22];

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

void solve(){
  stack<P> open;
  int ans = 0;

  open.push(P(sx, sy));
  t[sy][sx] = '#';

  while(!open.empty()){
    P p = open.top();
    open.pop();
    ans++;

    for(int i = 0; i < 4; i++){
      int nx = p.first + dx[i];
      int ny = p.second + dy[i];

      if(nx < 0 || w <= nx || ny < 0 || h <= ny || t[ny][nx] != '.') continue;
      t[ny][nx] = '#';
      open.push(P(nx, ny));
    }
  }

  cout << ans << endl;
}

int main(){
  while(cin >> w >> h, w || h){
    for(int i = 0; i < h; i++){
      cin >> t[i];
      for(int j = 0; j < w; j++){
        if(t[i][j] == '@'){
          sx = j;
          sy = i;
          t[i][j] = '.';
        }
      }
    }
    solve();
  }
}