Problem:
Score : 100 points

Problem Statement

The window of Takahashi's room has a width of A. There are two curtains hung over the window, each of which has a horizontal length of B. (Vertically, the curtains are long enough to cover the whole window.)

We will close the window so as to minimize the total horizontal length of the uncovered part of the window.
Find the total horizontal length of the uncovered parts of the window then.

Constraints

1 \leq A \leq 100

1 \leq B \leq 100

A and B are integers.

Input

Input is given from Standard Input in the following format:

A B

Output

Print the total horizontal length of the uncovered parts of the window.

Sample Input 1

12 4

Sample Output 1

4

We have a window with a horizontal length of 12, and two curtains, each of length 4, that cover both ends of the window, for example. The uncovered part has a horizontal length of 4.

Sample Input 2

20 15

Sample Output 2

0

If the window is completely covered, print 0.

Sample Input 3

20 30

Sample Output 3

0

Each curtain may be longer than the window.

Solution in Ruby (2.3.3) (judge verdict: Runtime Error)

n = gets.to_i
length = gets.split.map(&:to_i).sort
counts = 0

# 三辺は、a < b < c
# cを最大から繰り返す
(n-1).downto(2) do |c|
  a=0 #aは最小から
  b=c-1 #bは最大から(<c)
  while a<b
#p [l, r, i, (r-l)]
    # 条件通り三角形が作れる場合、a~bまでを加算して、bを下げる
    if length[c] < length[a]+length[b]
      counts += (b-a)
      b -= 1
    # 作れない場合は、aを上げて再チャレンジ
    else
      a += 1
    end
  end
end

puts counts