Problem:
Score : 300 points

Problem Statement

There are N cubes stacked vertically on a desk.

You are given a string S of length N. The color of the i-th cube from the bottom is red if the i-th character in S is 0, and blue if that character is 1.

You can perform the following operation any number of times: choose a red cube and a blue cube that are adjacent, and remove them. Here, the cubes that were stacked on the removed cubes will fall down onto the object below them.

At most how many cubes can be removed?

Constraints

1 \leq N \leq 10^5

|S| = N

Each character in S is 0 or 1.

Input

Input is given from Standard Input in the following format:

S

Output

Print the maximum number of cubes that can be removed.

Sample Input 1

0011

Sample Output 1

4

All four cubes can be removed, by performing the operation as follows:

Remove the second and third cubes from the bottom. Then, the fourth cube drops onto the first cube.

Remove the first and second cubes from the bottom.

Sample Input 2

11011010001011

Sample Output 2

12

Sample Input 3

0

Sample Output 3

0

Solution in Ruby (2.3.3) (judge verdict: Time Limit Exceeded)

def naive(tower)
  orig_size = tower.size
  loop do
    new_tower = []
    tmp = nil
    tower.each do |c|
      if tmp.nil?
        tmp = c
        next
      end
      if tmp == c
        new_tower << tmp
        tmp = c
        next
      end
      tmp = nil
    end
    new_tower << tmp if tmp

    break if tower.size == new_tower.size
    tower = new_tower
  end

  orig_size - tower.size
end

def f(s)
  ss = s.size
  loop do
    tmp = s.gsub('01', '').gsub('10', '')
    break if tmp.size == s.size
    s = tmp
  end
  ss - s.size
end

s = gets.chomp
# puts naive(s.chars)
puts f(s)
__END__
8 12 2