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

#define ll long long
#define vvi vector<vector<int>>
#define vec vector
#define pq priority_queue
#define all(v) (v).begin(), (v).end()
#define rep(i, n) for (int (i) = (0); (i) < (n); ++(i))
#define repp(i, m, n) for (int (i) = (m); (i) < (n); ++(i))
#define debug(x) cerr << #x << ": " << x << endl;
#define debug2(x, y) cerr<<"("<<#x<<", "<<#y<<") = "<<"("<<x<<", "<<y<<")"<<endl;
#define debug3(x, y, z) cerr<<"("<<#x<<", "<<#y<<", "<<#z<<") = "<<"("<<x<<", "<<y<<", "<<z<<")"<<endl;
#define debugB(x, y) cerr<<#x<<": "<<bitset<y>(x) << endl;
#define line() cerr << "---------------" << endl;

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

template<typename T>
void printA(vector<T> &printArray, char between = ' ') {
   int paSize = printArray.size();
   for (int i = 0; i < paSize; i++) { cerr << printArray[i] << between;}
   if (between != '\n') {cerr << endl;}
}

// ------------------------------------------------------------------------------------------

int W, H;
char B[30][30];

int dfs(int y, int x) {
   int ret = 1;
   B[y][x] = '#';
   rep (i, 4) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= W || ny < 0 || ny >= H || B[ny][nx] == '#') continue;
      ret += dfs(ny, nx);
   }
   return ret;
}

void solve() {
   rep (i, H) {
      cin >> B[i];
   }
   rep (i, H) {
      rep (j, W) {
         if (B[i][j] == '@') {
            cout << dfs(i, j) << endl;
            return;
         }
      }
   }
}

int main() {
   while (1) {
      cin >> W >> H;
      if (H*W == 0) break;
      solve();
   }

   return 0;
}

C++: //Shunji Lin 08-144505
#include <iostream>
#include <string>

using namespace std;

int reachable = 0;

void DFS(int grid[20][20], int visited[20][20],  int row_pos, int col_pos, int H, int W);

void DFS_aux(int grid[20][20], int visited[20][20], int row_pos, int col_pos, int H, int W) {
  if (grid[row_pos][col_pos] && !visited[row_pos][col_pos]) {
    DFS(grid, visited, row_pos, col_pos, H, W);
  }
    
}
	 
void DFS(int grid[20][20], int visited[20][20], int row_pos, int col_pos, int H, int W) {
  visited[row_pos][col_pos] = true;
  reachable += 1;
  if (!(row_pos == 0)) {
    DFS_aux(grid, visited, row_pos-1, col_pos, H, W); //up
  }
  if (!(col_pos == 0)) {
    DFS_aux(grid, visited, row_pos, col_pos-1, H, W); //left
  }
  if (!(row_pos == H-1)) {
    DFS_aux(grid, visited, row_pos + 1, col_pos, H, W); //down
  }
  if (!(col_pos == W-1)) {
    DFS_aux(grid, visited, row_pos, col_pos + 1, H , W); //right
  }
}
      
    


int main() {
  while (true) {
    int grid[20][20] = {{false}};
    int visited[20][20] = {{false}};
    reachable = 0;
    int man_row = -1, man_col = -1;
    int W, H;
    cin >> W >> H;
    if (W == 0 && H == 0) break;
    for (int i = 0; i < H; i++) {
      string line;
      cin >> line;
      for (int j = 0; j < W; j++) {
	if (line[j] == '@') {
	  grid[i][j] = true;
	  man_row = i;
	  man_col = j;
	} else if (line[j] == '.') {
	  grid[i][j] = true;
	}
      }
    }
    /*
    //print out grid
    for (int i = 0; i < H; i++) {
      for (int j = 0; j < W; j++) {
	if (grid[i][j] == true) {
	  cout << 'b';
	} else if (grid[i][j] == false) {
	  cout << 'r';
	} 
      }
      cout << endl;
    }
    */
    
    DFS(grid, visited,  man_row, man_col, H, W );
    cout << reachable << endl;
  }
}

C++: #include <bits/stdc++.h>

//#define var auto
#define ll long long

using namespace std;

int ans = 0;
int h, w;
vector<string> grid;
vector<vector<bool>> arrived;
void dfs(int y, int x) {
	if (y < 0 || h <= y || x < 0 || w <= x) return;
	if (grid[y][x] == '#') return;
	if (arrived[y][x])return;
	arrived[y][x] = true;
	ans++;
	dfs(y - 1, x);
	dfs(y, x + 1);
	dfs(y + 1, x);
	dfs(y, x - 1);
}

int main() {
	while (true)
	{
		cin >> w >> h;
		if (w == 0) break;
		grid = vector<string>(h);
		arrived = vector<vector<bool>>(20, vector<bool>(20, false));
		ans = 0;
		for (int i = 0; i < h; i++) {
			cin >> grid[i];
		}
		int sy, sx;
		for (int i = 0; i < h; i++) {;
			for (int j = 0; j < w; j++) {
				if (grid[i][j] == '@') {
					sy = i;
					sx = j;
				}
			}
		}
		dfs(sy, sx);
		cout << ans << endl;
	}
}