New Year's Eve

题意：给定两个整数$n$ ,$k$ ,在$1$ ~$n$ 中选择至多$k$ 个整数，使得其异或和最大。求解这个最大值。 感谢@char32_t 提供的翻译

Since Grisha behaved well last year, at New Year's Eve he was visited by Ded Moroz who brought an enormous bag of gifts with him! The bag contains $ n $ sweet candies from the good ol' bakery, each labeled from $ 1 $ to $ n $ corresponding to its tastiness. No two candies have the same tastiness. The choice of candies has a direct effect on Grisha's happiness. One can assume that he should take the tastiest ones — but no, the holiday magic turns things upside down. It is the xor-sum of tastinesses that matters, not the ordinary sum! A xor-sum of a sequence of integers $ a_{1},a_{2},...,a_{m} $ is defined as the bitwise XOR of all its elements: , here  denotes the bitwise XOR operation; more about bitwise XOR can be found [here.](https://en.wikipedia.org/wiki/Bitwise_operation#XOR) Ded Moroz warned Grisha he has more houses to visit, so Grisha can take no more than $ k $ candies from the bag. Help Grisha determine the largest xor-sum (largest xor-sum means maximum happiness!) he can obtain.

The sole string contains two integers $ n $ and $ k $ ( $ 1<=k<=n<=10^{18} $ ).

Output one number — the largest possible xor-sum.

4 3

7

6 6

7

In the first sample case, one optimal answer is $ 1 $ , $ 2 $ and $ 4 $ , giving the xor-sum of $ 7 $ . In the second sample case, one can, for example, take all six candies and obtain the xor-sum of $ 7 $ .