@正负加 的代码，加了注释

#include <bits/stdc++.h>
#define lowbit(n) ((n) & (-(n))) // 带参数宏定义，它是简单的语句替换，要多打括号 
using namespace std;

//         ↓代表要买的水瓶个数 
int n, k, ans;

int main()
{
	cin >> n >> k;
	// _builtin_popcount(n)代表已经有的水瓶数，即二进制n为1的个数 
	while (__builtin_popcount(n) > k)
		ans += lowbit(n), n += lowbit(n); //lowbit(n)代表要新买多少个瓶子，即一个二进制数一位为1的最低位置  
	cout << ans;
	
	return 0;
}

#include <bits/stdc++.h>
using namespace std;
//与运算遵循特定规则：
//两个数值同时为1时，
//结果才为1；否则，结果为0。 
//例如，0与0的与运算结果为0
//，0与1的与运算结果同样为0
//，1与0的与运算结果依然为0
//，而1与1的与运算结果则为1 
//。按位与运算是一种二目运算
//符，其功能在于参与运算的两
//个数值对应的二进位进行相与
//操作。只有当两个二进位均为
//1时，结果位才为1。
long long n, k, num, ans = 0;
int main()
{
	
	cin >> n >> k;
	while(1)
	{
		int count = 0;
		for(int i = 0;i < 64;i++)
		{
			if((n&((long long)1<<i))>0)
			{
				count++;
			}
		}
		if(count<=k)
		{
			cout << num;
			break;
		}
		else 
		{
			num+=n&-n;
			n+=n&-n;
		}
	}
	return 0;
}

#include <bits/stdc++.h>
using namespace std;
int n, k, ans;
//__builtin_popcount( )
//int popcount(int x)
//{
//    int res = 0;
//    while(x)
//	{
//        if(x%2) 
//		{
//			res++;
//		}
//        x/=2;
//    }
//    return res;
//}
int main()
{
	cin >> n >> k;
	
	while(__builtin_popcount(n) > k)
	{
		ans += n&-n;
		n += n&-n;
//		cout << (n&-n) << endl;
//		cout << ans << endl;
//		cout << __builtin_popcount(n) << endl;
//		cout << n << endl;
	}
	cout << ans;
	return 0;
}

// <algorithm> -*- C++ -*-

// Copyright (C) 2001-2014 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/*
 *
 * Copyright (c) 1994
 * Hewlett-Packard Company
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Hewlett-Packard Company makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 *
 *
 * Copyright (c) 1996,1997
 * Silicon Graphics Computer Systems, Inc.
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Silicon Graphics makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 */

/** @file include/algorithm
 *  This is a Standard C++ Library header.
 */

#ifndef _GLIBCXX_ALGORITHM
#define _GLIBCXX_ALGORITHM 1

#pragma GCC system_header

#include <utility> // UK-300.
#include <bits/stl_algobase.h>
#include <bits/stl_algo.h>

#ifdef _GLIBCXX_PARALLEL
# include <parallel/algorithm>
#endif

#endif /* _GLIBCXX_ALGORITHM */