# util functions

# debug
$debugFlag = ( ARGV[0] != nil )
p "==== DEBUG MODE ====" if $debugFlag

# input
inputs = []
lc = 0
while l = $stdin.gets do
  if true
    inputs << l.chomp.split(" ").map(&:to_s)
  else
    inputs << l.chomp.split(" ").map(&:to_i)
  end
  lc += 1
end

# input check
if $debugFlag
  p "--- INPUT (begin) ---"
  inputs.each do |l_i|
    p l_i
  end
  p "--- INPUT (end) ---"
  print "\n"
end

# body
$ss = inputs[0][0]
if $debugFlag
  p $ss
end

ones = 0
$ss.length.times do |i|
  ones += 1 if $ss[i..i] == "1"
end

puts 2*[ones, $ss.length - ones].min
