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())

T = []
for i, s in enumerate(S):
    if s == "0":
        T.append(i)
# ic(S)
# ic(T)

ans = 0
U = []
for t in T:
    # ic(U, t)
    if len(U) == 0:
        r = 0
    else:
        r = U[-1] + 1

    cnt = (t - r) // 2
    ans += cnt
    U.append(t - 2 * cnt)

# ic(U)

print(ans)