poj 1979 Red and Black(dfs)

#include<cstdio>
#include<string>
#include<iostream>
#include<cstring>
#include<algorithm>
#include<queue>
#include<stack>
#include<cstdlib>

using namespace std;

char a[22][22];
int n, m, ans;

void dfs ( int s1, int s2 )
{
    if ( s1 <= m - 1 && s2 <= n - 1 && a[s1][s2] == '.' && s1 >= 0 && s2 >= 0 )
    {
        ans++;
        a[s1][s2] = '#';
    }
    else
        return;
    dfs ( s1, s2 + 1 );
    dfs ( s1, s2 - 1 );
    dfs ( s1 + 1, s2 );
    dfs ( s1 - 1, s2 );
}

int main()
{
    while ( cin >> n >> m && ( n + m ) )
    {
        int s1, s2;
        for ( int i = 0; i < m; i++ )
            for ( int j = 0; j < n; j++ )
            {
                cin >> a[i][j];
                if ( a[i][j] == '@' )
                {
                    s1 = i, s2 = j;
                    a[i][j] = '.';
                }
            }
        ans = 0;
        dfs ( s1, s2 );
        printf ( "%d\n", ans );
    }
    return 0;
}