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 <string>

using namespace std;

/*??°?????????????????°*/
int W, H, ans;
int tile[20][20];

/*??¢??°??????????????????*/
void DPS(int x, int y);

int main() {
	int sx = 0, sy = 0;
	string str;

	while (cin >> W >> H, W, H) {
		/*??\???*/
		for (int i = 0; i < H; i++) {
			cin >> str;
			for (int j = 0; j < W; j++) {
				tile[i][j] = (str[j] == '#') ? 0 : 1;
				
				if (str[j] == '@') {
					sx = j;
					sy = i;
				}
			}
		}

		ans = 0;
		DPS(sx, sy);

		cout << ans << endl;
	}
}

void DPS(int x, int y) {
	if (tile[y][x]) {
		tile[y][x] = 0;
		ans++;
		
		if (x - 1 >= 0)	DPS(x - 1, y);
		if (x + 1 < W)	DPS(x + 1, y);
		if (y - 1 >= 0)	DPS(x, y - 1);
		if (y + 1 < H)	DPS(x, y + 1);
	}

	return;
}

C++: //#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
bool c[20][20] = {} ;
int cnt,w, h, p[20][20] = {};

int main() {
	int j, f,cx,cy;
	char in[20];
	while (1) {
		for (int i = 0; i < 400; i++) {
			c[i / 20][i % 20] = p[i / 20][i % 20] = 0;
		}
		cnt = 0;
		f = 1;
		scanf("%d %d", &w, &h);
		if (w ==0&& h == 0)
			break;
		scanf("\n", &j);
		for (int i = 0; i < h; i++) {
			scanf("%s", &in);
			for (j = 0; j < w; j++) {
				switch (in[j]) {
				case '#':
					c[j][i] = 0;
					p[j][i] = -1;
					break;
				case '@':
					cx = j;
					cy = i;
					p[j][i] = 1;
				case '.':
					c[j][i] = 1;
				}
			}
		}

		/*for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++)
				printf("%3d", c[j][i]);
			printf("\n");
		}*/

		while (f == 1) {
			f = 0;
			for (int i = 0; i < w*h; i++) {
				if (p[i / h][i%h] == 0 && (p[i / h + 1][i % h] == 1 || p[i / h - 1][i % h] == 1 || p[i / h][i % h + 1] == 1 || p[i / h][i % h - 1] == 1)) {
					p[i / h][i%h] = 1;
					f = 1;
				}
			}
			/*for (int i = 0; i < h; i++) {
				for (int j = 0; j < w; j++)
					printf("%3d", p[j][i]);
				printf("\n");
			}
			printf("\n");*/
		}

		for (int i = 0; i < w*h; i++) {
			if (p[i / h][i%h] == 1)
				cnt++;
		}
		printf("%d\n", cnt);
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <queue>

using namespace std;

#define in(h, w) 0<=h&&h<H&&0<=w&&w<W

int dh[4] = {1, 0, -1, 0};
int dw[4] = {0, 1, 0, -1};

int main(){
    int W, H;
    while (cin >> W >> H and W and H) {
        int h, w;
        vector< vector<char> > map(H, vector<char>(W));
        for (int i = 0; i < H; i++)
            for (int j = 0; j < W; j++) {
                cin >> map[i][j];
                if (map[i][j] == '@') {
                    h = i;
                    w = j;
                }
            }
        int res = 1;
        queue<pair<int, int> > q;
        q.push(make_pair(h, w));
        while (!q.empty()) {
            h = q.front().first;
            w = q.front().second;
            q.pop();
            if (map[h][w] == '.') {
                res++;
                map[h][w] = '#';
            }
            for (int i = 0; i < 4; i++) {
                int th = h+dh[i];
                int tw = w+dw[i];
                if (0<=th&&th<H&&0<=tw&&tw<W) {
                    if (map[th][tw] == '.')
                        q.push(make_pair(th, tw));
                }
            }
        }
        cout << res << endl;
    }
}

C++: #include <iostream>
#include <string>
using namespace std;
typedef pair<int,int> P;
int dx[4]={0,1,0,-1};
int dy[4]={1,0,-1,0};
string s[22];
int cnt;
void dfs(int x, int y){
	cnt++;
	s[x][y]='#';
	for(int i=0;i<4;i++){
		if(s[ x+dx[i] ][ y+dy[i] ]!='#') dfs(x+dx[i],y+dy[i]);
	}
}
int main(){
	while(true){
		int w,h,sx,sy;
		cnt=0;
		cin >> w >> h;
		if(!w) break;
		for(int i=1;i<=h;i++){
			cin >> s[i];
			s[i]="#"+s[i]+"#";
		}
		s[0]="";
		s[h+1]="";
		s[0].append(w+2,'#');
		s[h+1].append(w+2,'#');
		for(int i=1;i<h+1;i++){
			for(int j=1;j<w+1;j++){
				if(s[i][j]=='@'){sx=i;sy=j;}
			}
		}
		dfs(sx,sy);
		cout << cnt << endl;
	}
	return 0;
}

C++: #include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cassert>
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

#define LET(name, value) __typeof(value) name = value
#define REP(i, n) for (int i = 0; i < (int)(n); ++i)
#define FOR(i, a, b) for (int i = (a); i < (int)(b); ++i)
#define FOREQ(i, a, b) for (int i = (a); i <= (int)(b); ++i)
#define ALL(c) (c).begin(), (c).end()
#define FOREACH(i, c) for (LET(i, (c).begin()); i != (c).end(); ++i)

char field[30][30];

int dfs(int y, int x) {
  static int dy[] = {-1, 0, 1, 0};
  static int dx[] = {0, -1, 0, 1};

  if (field[y][x] == '.') {
    field[y][x] = '#';
    int sum = 1;
    REP(k, 4) { sum += dfs(y + dy[k], x + dx[k]); }
    return sum;
  } else {
    return 0;
  }
}

int main() {
  int W, H;
  while (scanf("%d%d", &W, &H), W|H) {
    int sy = -1, sx = -1;
    memset(field, '#', sizeof(field));
    FOREQ(y, 1, H) FOREQ(x, 1, W) {
      scanf(" %c", &field[y][x]);
      if (field[y][x] == '@') {
        sy = y; sx = x;
        field[y][x] = '.';
      }
    }
    printf("%d\n", dfs(sy, sx));
  }
}

C++: #include <iostream>
#include <vector>
#include <queue>
#include <utility>
using namespace std;
int main( void )
{
	const int dx[4] = { 1, 0, -1, 0 };
	const int dy[4] = { 0, -1, 0, 1 };

	int w, h;
	while ( cin>>w>>h && ( w != 0 && h != 0 ) )
	{
		vector < vector <char> > V( h, vector <char>(w, false) );
		queue < pair<int,int> > Q;

		for ( int i = 0; i < h; i++ )
		{
			for ( int j = 0; j < w; j++ )
			{
				cin >> V[i][j];
				if ( V[i][j] == '@' )
					Q.push( make_pair( i, j ) );
			}
		}

		while ( !Q.empty() )
		{
			pair <int,int> now = Q.front();
			Q.pop();

			V[now.first][now.second] = 'v';

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

				if ( ( nx < 0 || nx >= w ) || ( ny < 0 || ny >= h ) )
					continue;

				if ( V[ny][nx] != '.' )
					continue;

				Q.push( make_pair( ny, nx ) );

			}
		}

		int kotae = 0;
		for ( int i = 0; i < h; i++ )
		{
			for ( int j = 0; j < w; j++ )
			{
				if ( V[i][j] == 'v' )
					kotae++;
			}
		}

		cout << kotae << endl;
	}
}