Problem Statement

There are N apple trees in a row. People say that one of them will bear golden apples.
We want to deploy some number of inspectors so that each of these trees will be inspected.
Each inspector will be deployed under one of the trees. For convenience, we will assign numbers from 1 through N to the trees. An inspector deployed under the $i-th tree (1 \leq i \leq N)$ will inspect the trees with numbers between i-D and i+D (inclusive).
Find the minimum number of inspectors that we need to deploy to achieve the objective.

Input

Input is given from Standard Input in the following format:
N D

Output

Print the minimum number of inspectors that we need to deploy to achieve the objective.

Constraints

• All values in input are integers.
• $1 \leq N \leq 20$
• $1 \leq D \leq 20$

Sample Input 1

6 2

Sample Output 1

2

We can achieve the objective by, for example, placing an inspector under Tree 3 and Tree 4.

Sample Input 2

14 3

Sample Output 2

2

Sample Input 3

20 4

Sample Output 3

3

见题面 Input 部分

见题面 Output 部分