function main()
  
  S = split(readline(),"")
  
  c0 = 0
  c1 = 0
  
  for i in 1:length(S)
    
    if S[i] == "0"
      c0 += 1
    else
      c1 += 1
    end
    
  end
  
  println(2 * min(c0,c1))
      
end

main()