[CF912B]New Year's Eve

题意：在1~n中选不超过m个数，求其异或最大值
题解：经过找规律发现如果m为1，输出n，不然输出最小的不超过n的2^k-1

C++ Code:

#include<cstdio>
using namespace std;
long long n,k,ans=1;
int main(){
    scanf("%I64d%I64d",&n,&k);
    if(k==1){
		printf("%I64d\n",n);
    }else{
    	while(n>>=1)ans<<=1;
	    printf("%I64d\n",(ans<<1)-1);
	}
    return 0;
}