import copy as copy
s = input()
dic={}

def func(val, n):
    if val in dic:
        return dic[val]

    if not ('1' in val):
        return n
    elif not ('0' in val):
        return n
    a=0
    b=0    
    if '01' in val:
        a = func(val.replace('01', '', 1), n+2)
    if '10' in val:
        b = func(val.replace('10', '', 1), n+2)
    
    ans=max(a, b, n)
    dic[val]=ans

    return ans

print(func(s, 0))
