import math
import copy
import fractions
import numpy as np
from functools import reduce

# ===FUNCTION===

def getInputNum():
    inputNum = int(input())
    return inputNum

def getInputListInt():
    outputoData = []
    inputData = input().split()
    outputData = [int(n) for n in inputData]

    return outputData

def getSomeInputInt(n):
    outputDataList = []
    for i in range(n):
        inputData = int(input())
        outputDataList.append(inputData)

    return outputDataList

def getSomeInputListInt(n):
    inputDataList = []
    outputDataList = []
    for i in range(n):
        inputData = input().split()
        inputDataList = [int(n) for n in inputData]
        outputDataList.append(inputDataList)

    return outputDataList

# ===CODE===

n, = map(str, input().split())
data = list(n)

cntOne = 0
cntZero = 0

for i in range(data):
    if data[i] == "0": 
        cntZero += 1
    else: 
        cntOne += 1

ans(int(2*min(cntOne, cntZero)))