from collections import defaultdict,deque
import sys,heapq,bisect,math,itertools,string,queue,copy,time
sys.setrecursionlimit(10**8)
INF = float('inf')
mod = 10**9+7
eps = 10**-7
def inp(): return int(sys.stdin.readline())
def inpl(): return list(map(int, sys.stdin.readline().split()))
def inpl_str(): return list(sys.stdin.readline().split())


N = input() + '000'

ans = 0
tmp0 = 0
cnt0 = 0
cnt1 = 0
for sn in N:
    n = int(sn)
    #print(cnt0,cnt1,tmp0)
    if cnt0 == 0:
        if n == 1:
            cnt1 += 1
        else:
            tmp0 += 1
            cnt0 += 1
    elif cnt0 == 1:
        if n == 1:
            cnt0 = 0
            cnt1 += 1
        else:
            tmp0 += 1
            cnt0 += 1
    elif cnt0 == 2:
        ans += min(tmp0,cnt1)
        tmp0 = 0
        cnt0 += 1
        cnt1 = 0

    if cnt0 >= 3:
        if n == 1:
            cnt0 = 0
            cnt1 += 1

print(ans)