Solve this interesting problem

Problem Description

Have you learned something about segment tree? If not, don’t worry, I will explain it for you.
Segment Tree is a kind of binary tree, it can be defined as this:
- For each node u in Segment Tree, u has two values:  and .
- If , u is a leaf node. 
- If , u has two children x and y,with ,,,.
Here is an example of segment tree to do range query of sum.



Given two integers L and R, Your task is to find the minimum non-negative n satisfy that: A Segment Tree with root node‘s value  and contains a node u with  and .

 

Input

The input consists of several test cases. 
Each test case contains two integers L and R, as described above.

 

Output

For each test, output one line contains one integer. If there is no such n, just output -1.

 

Sample Input

6 7
10 13
10 11

 

Sample Output

7
-1
12

 

Source

2015 Multi-University Training Contest 3

 

题意：

给定区间[l,r]

用线段树的递归建树方式build(1, n)

问最小的n是多少，使得build(1,n) 中能直接建出区间[l,r]

思路：

注意一下范围，然后推算一下就能发现这个区间距离根节点的距离不超过10

所以从底向上搜就好了