S = gets.chomp
N = S.length

num_zero = 0
for i in 1..N do
  if S[i-1] == "0" then
    num_zero += 1
  end
end
num_one = N - num_zero
  
if num_one > num_zero then
  puts num_zero * 2
else
  puts num_one * 2
end