from heapq import heappop, heappush, heapify
from bisect import bisect
#from sortedcontainers import SortedList
from collections import deque, defaultdict
from math import floor, ceil, isqrt, comb
from sys import stdin, setrecursionlimit
#setrecursionlimit(10**7)
intin = lambda: int(stdin.readline())
strin = lambda: stdin.readline().rstrip()
listin = lambda: list(map(int, stdin.readline().split()))
tuplein = lambda m: [tuple(map(lambda x: int(x) if x.isdigit() or (len(x) > 1 and x[0] == "-" and x[1:].isdigit()) else x, stdin.readline().split())) for _ in range(m)]
gridin = lambda m: [list(map(int, stdin.readline().split())) for _ in range(m)]
strgridin = lambda h: [stdin.readline().rstrip() for _ in range(h)]
mapin = lambda: map(int, stdin.readline().split())
S = list(strin())
#左にある0から順番に動かす
tmp = -1
#どこから1が連続しているか
ren = False
N = len(S)
ans = 0
for i in range(N):
    if S[i] == "1":
        if not ren:
            tmp = i
            ren = True
    else:
        if ren:
            ans += (i - tmp) // 2
            tmp += (i - tmp) % 2
            tmp += 1
            if tmp > i:
                ren = False
print(ans)