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# Solve this interesting problem

Time Limit
1s
Memory Limit
32768KB
Judge Program
Standard
Ratio(Solve/Submit)
100.00%(2/2)
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: Lu and Ru.
- If Lu=Ru, u is a leaf node.
- If Lu≠Ru, u has two children x and y,with Lx=Lu,Rx=⌊Lu+Ru2⌋,Ly=⌊Lu+Ru2⌋+1,Ry=Ru.
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 Lroot=0 and Rroot=n contains a node u with Lu=L and Ru=R.

Input:

The input consists of several test cases.
Each test case contains two integers L and R, as described above.
0≤L≤R≤109
L/(R−L+1)≤2015

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

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