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 <queue>

using namespace std;

int main(){
    int w,h;
    while(cin >> w >> h && w!=0){
        // int t[100][100];
        int t[h+2][w+2];
        for(int i=0;i<h+2;i++){
            for(int j=0;j<w+2;j++){
                t[i][j] = 0;
            }
        }

        int start;
        for(int i=0;i<h+2;i++){
            for(int j=0;j<w+2;j++){
                if(i==0 || j==0 || i==h+1 || j==w+1)
                    t[i][j]=0;
                else{
                    char tmp; cin >> tmp;
                    if(tmp=='.'){
                        t[i][j] = 1;
                    }
                    else if(tmp=='#'){
                        t[i][j] = 0;
                    }
                    else{
                        t[i][j] = 1;
                        start = i*w+j;
                    }
                }
            }
        }

        //test
        // for(int i=0;i<h+2;i++){
        //     for(int j = 0;j<w+2;j++){
        //         if(j==0)
        //             cout << t[i][j];
        //         else
        //             cout << " " << t[i][j];
        //     }
        //     cout << endl;
        // }

        int ** matrix = new int*[(w+2)*(h+2)];
        for( int i=0; i<(w+2)*(h+2); i++ ) {
            matrix[i] = new int[(w+2)*(h+2)];
        }
        for(int i=0;i<w*h;i++){
            for(int j=0;j<w*h;j++){
                matrix[i][j] = 0;
            }
        }
        for(int i=0;i<h+2;i++){
            for(int j=0;j<w+2;j++){
                int id = i*(w+2)+j;
                if(i==0 || j==0 || i == h+1 || j == w+1 || t[i][j]==0)
                    continue;
                else{
                    if(t[(id-w-2)/(w+2)][(id-w-2)%(w+2)] == 1){
                        matrix[id][id-w-2] = 1;
                    }
                    if(t[(id+w+2)/(w+2)][(id+w+2)/(w+2)] == 1){
                        matrix[id][id+w+2] = 1;
                    }
                    if(t[(id-1)/(w+2)][(id-1)/(w+2)] == 1){
                        matrix[id][id-1] = 1;
                    }
                    if(t[(id+1)/(w+2)][(id+1)/(w+2)] == 1){
                        matrix[id][id+1] = 1;
                    }
                }
            }
        }

        // test
        // for(int i=0;i<(h+2)*(w+2);i++){
        //     for(int j = 0;j<(h+2)*(w+2);j++){
        //         if(j==0)
        //             cout << matrix[i][j];
        //         else
        //             cout << " " << matrix[i][j];
        //     }
        //     cout << endl;
        // }


        int count = 1;
        queue<int> Q;
        int came[(w+2)*(h+2)];
        for(int i=0;i<(w+2)*(h+2);i++){
            came[i] = 0;
        }
        Q.push(start);
        came[start] = 1;
        while(Q.size()!=0){
            int cur = Q.front();
            for(int i=0;i<(w+2)*(h+2);i++){
                if(matrix[cur][i]==1 && came[i] == 0){
                    Q.push(i);
                    came[i] = 1;
                    count++;
                }
            }
            Q.pop();
        }
        cout << count << endl;
        for( int i=0; i<(w+2)*(h+2); i++ ) {
            delete[] matrix[i];
        }
        delete[] matrix;

    }
}

C++: #include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<iomanip>
#include<math.h>
#include<complex>
#include<queue>
#include<deque>
#include<stack>
#include<map>
#include<set>
#include<bitset>
#include<functional>
#include<assert.h>
#include<numeric>
using namespace std;
#define REP(i,m,n) for(int i=(int)(m) ; i < (int) (n) ; ++i )
#define rep(i,n) REP(i,0,n)
using ll = long long;
const int inf=1e9+7;
const ll longinf=1LL<<60 ;
const ll mod=1e9+7 ;
int d[]={1,0,-1,0,1};

void solve(int w,int h){
    string s[h];
    rep(i,h)cin>>s[i];
    int sx,sy;
    rep(i,h)rep(j,w)if(s[i][j]=='@')sx=i,sy=j;
    queue<pair<int,int>> que;
    que.emplace(sx,sy);
    vector<vector<int>> used(h,vector<int>(w));
    used[sx][sy]=1;
    int ans=1;
    while(que.size()){
        int x=que.front().first,y=que.front().second;
        que.pop();
        rep(i,4){
            int nx=x+d[i],ny=y+d[i+1];
            if(nx<0||nx>=h||ny<0||ny>=w)continue;
            if(used[nx][ny]||s[nx][ny]=='#')continue;
            ++ans;
            used[nx][ny]=1;
            que.emplace(nx,ny);
        }
    }
    cout<<ans<<endl;
}


int main(){
    int n,m;
    while(cin>>n>>m,n!=0)solve(n,m);
    return 0;
}