<https://www.hackerrank.com/challenges/maximizing-xor>

Given two integers, L and R, find the maximal value of A xor B, where A and B satisfy the
following condition:

```
L <= A <= B <= R
```

#Input Format

The input contains two lines; L is present in the first line and R in the second
line.

#Constraints
```
1 <= L <= L <= 10^3
```

#Output Format

The maximal value as mentioned in the problem statement.

#Sample Input
```
10
15
```

#Sample Output
```
7
```

#Explanation

The input tells us that L = 10 and R = 15. All the pairs which comply to above condition are
the following:

```
10 xor 10 = 0
10 xor 11 = 1
10 xor 12 = 6
10 xor 13 = 7
10 xor 14 = 4
10 xor 15 = 5
11 xor 11 = 0
11 xor 12 = 7
11 xor 13 = 6
11 xor 14 = 5
11 xor 15 = 4
12 xor 12 = 0
12 xor 13 = 1
12 xor 14 = 2
12 xor 15 = 3
13 xor 13 = 0
13 xor 14 = 3
13 xor 15 = 2
14 xor 14 = 0
14 xor 15 = 1
15 xor 15 = 0
```

Here two pairs (10, 13) and (11, 12) have maximum xor value 7, and this is the
answer.