"""
    S：長さNの文字列
    if S[i] = '0':
        color = 'red'
    elif S[i] = '1':
        color = 'blue'
    
    I can remove set of boxes which put together red and blue box.

    output : How many can I remove box?
    sに存在する0と1は必ずどこかで隣り合う。
    1と0の組み合わせは、どの順番で取り出してもmin(num(0), num(1))

"""
s = input()
count1 = s.count('1')
count0 = s.count('0')

if count0 >= count1:
    print(count0*2)
else:
    print(count1*2)
