from collections import defaultdict, Counter, deque
from itertools import groupby, accumulate, combinations, permutations, product, combinations_with_replacement
from bisect import bisect_left, bisect_right
from operator import itemgetter
import math
from heapq import heapify, heappush, heappop

LMI=lambda:list(map(int, input().split()))
LMS=lambda:list(map(str, input().split()))
MI=lambda:map(int, input().split())
MS=lambda:map(str, input().split())
II=lambda:int(input())
IS=lambda:input().split()
LI=lambda:list(input())

S=input()
cnt=0
while True:
    tmp=S.replace('110','011',1)
    if tmp==S:
        print(cnt)
        exit()
    S=tmp
    cnt+=1