[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);
    }else{
    	while(n>>=1)ans<<=1;
	    printf("%I64d
",(ans<<1)-1);  
	}
    return 0;  
}