from collections import *
from itertools import *
from functools import cache, partial
from pprint import pprint
import sys
from typing import Any, Final

try:
    from icecream import ic
except ImportError:  # Graceful fallback if IceCream isn't installed.
    ic = lambda *a: None if not a else (a[0] if len(a) == 1 else a)  # noqa
debug = partial(print, file=sys.stderr)
dpprint = partial(pprint, stream=sys.stderr)
sys.setrecursionlimit(10**6)
MOD = 998244353

S = list(input())[::-1]
# ic(S)

queue = deque()
queue_zero = deque()
ans = 0
for i, s in enumerate(S):
    queue.append(s)
    if len(queue) < 3:
        continue

    while (
        len(queue) >= 3 and queue[-1] == "1" and queue[-2] == "1" and queue[-3] == "0"
    ):
        ans += 1
        queue.pop()
        queue.pop()
        queue.pop()
        queue.append("1")
        queue.append("1")
        queue_zero.append("0")

    while queue_zero:
        queue.append(queue_zero.popleft())

print(ans)