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Solve this interesting problem

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)

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: 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

LR?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

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define prt(k) cout<<#k" = "<<k<<endl;
const ll inf = 1e18;
ll ans;
void dfs(ll L, ll R)
{
        if (R>=ans) return;
        if (L < 0) return;
        if(L==0) {
                ans = R;
                return;
        }
        if (R - L + 1 > L) return;
        dfs(2*L - R - 1,  R);
        dfs(2*L - R - 2, R);
        dfs(L, 2*R - L);
        dfs(L, 2*R - L + 1);
}
ll L, R;
int main()
{
        while (cin>>L>>R)
        {
                ans = inf;
                dfs(L, R);
                if (ans == inf) ans =  -1;
                cout<<ans<<endl;
        }
}

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