l = raw_input().split()
s = l[0]

def get_removals(s):
    ret = []
    for i in range(len(s) - 1):
        if int(s[i]) + int(s[i + 1]) == 1:
            ret.append(i)
    return ret

def remove(s, index):
    if index == 0:
        return s[2:]
    if index == (len(s) - 1):
        return s[:index-1]
    return  s[0:index] + s[index + 2:]

num_removals = []

def process(s, count):
    removals = get_removals(s)
    if len(removals) == 0:
        num_removals.append(count)
        return
    count += 2
    max_removes = []
    ss = []
    for removal in removals:
        s1 = remove(s, removal)
        max_removes.append(len(get_removals(s1)))
        ss.append(s1)
    max_remove = max(max_removes)
    for s1 in set(ss):
        if len(get_removals(s1)) == max_remove:
            process(s1, count)

process(s, 0)
print(max(num_removals))
